Cplusplus 简明教程

#include <iostream>
using namespace std;

// main() is where program execution begins.
int main() {
   cout << "Hello World"; // prints Hello World
   return 0;
}

$ g++ hello.cpp
$ ./a.out
Hello World

x = y;
y = y + 1;
add(x, y);

{
   cout << "Hello World"; // prints Hello World
   return 0;
}

x = y;
y = y + 1;
add(x, y);

x = y; y = y + 1; add(x, y);

mohd       zara    abc   move_name  a_123
myname50   _temp   j     a23b9      retVal

#include <iostream>
using namespace std;

int main() {
   cout << "Size of char : " << sizeof(char) << endl;
   cout << "Size of int : " << sizeof(int) << endl;
   cout << "Size of short int : " << sizeof(short int) << endl;
   cout << "Size of long int : " << sizeof(long int) << endl;
   cout << "Size of float : " << sizeof(float) << endl;
   cout << "Size of double : " << sizeof(double) << endl;
   cout << "Size of wchar_t : " << sizeof(wchar_t) << endl;

   return 0;
}

Size of char : 1
Size of int : 4
Size of short int : 2
Size of long int : 4
Size of float : 4
Size of double : 8
Size of wchar_t : 4

typedef type newname;

typedef int feet;

feet distance;

enum enum-name { list of names } var-list;

enum color { red, green, blue } c;
c = blue;

enum color { red, green = 5, blue };

type variable_list;

int    i, j, k;
char   c, ch;
float  f, salary;
double d;

type variable_name = value;

extern int d = 3, f = 5;    // declaration of d and f.
int d = 3, f = 5;           // definition and initializing d and f.
byte z = 22;                // definition and initializes z.
char x = 'x';               // the variable x has the value 'x'.

int g = 20;

10 = 20;

#include <iostream>
using namespace std;

int main () {
   // Local variable declaration:
   int a, b;
   int c;

   // actual initialization
   a = 10;
   b = 20;
   c = a + b;

   cout << c;

   return 0;
}

#include <iostream>
using namespace std;

// Global variable declaration:
int g;

int main () {
   // Local variable declaration:
   int a, b;

   // actual initialization
   a = 10;
   b = 20;
   g = a + b;

   cout << g;

   return 0;
}

#include <iostream>
using namespace std;

// Global variable declaration:
int g = 20;

int main () {
   // Local variable declaration:
   int g = 10;

   cout << g;

   return 0;
}

10

212         // Legal
215u        // Legal
0xFeeL      // Legal
078         // Illegal: 8 is not an octal digit
032UU       // Illegal: cannot repeat a suffix

85         // decimal
0213       // octal
0x4b       // hexadecimal
30         // int
30u        // unsigned int
30l        // long
30ul       // unsigned long

3.14159       // Legal
314159E-5L    // Legal
510E          // Illegal: incomplete exponent
210f          // Illegal: no decimal or exponent
.e55          // Illegal: missing integer or fraction

#include <iostream>
using namespace std;

int main() {
   cout << "Hello\tWorld\n\n";
   return 0;
}

Hello   World

"hello, dear"

"hello, \

dear"

"hello, " "d" "ear"

#define identifier value

#include <iostream>
using namespace std;

#define LENGTH 10
#define WIDTH  5
#define NEWLINE '\n'

int main() {
   int area;

   area = LENGTH * WIDTH;
   cout << area;
   cout << NEWLINE;
   return 0;
}

50

const type variable = value;

#include <iostream>
using namespace std;

int main() {
   const int  LENGTH = 10;
   const int  WIDTH  = 5;
   const char NEWLINE = '\n';
   int area;

   area = LENGTH * WIDTH;
   cout << area;
   cout << NEWLINE;
   return 0;
}

50

{
   int mount;
   auto int month;
}

{
   register int  miles;
}

#include <iostream>

// Function declaration
void func(void);

static int count = 10; /* Global variable */

main() {
   while(count--) {
      func();
   }

   return 0;
}

// Function definition
void func( void ) {
   static int i = 5; // local static variable
   i++;
   std::cout << "i is " << i ;
   std::cout << " and count is " << count << std::endl;
}

i is 6 and count is 9
i is 7 and count is 8
i is 8 and count is 7
i is 9 and count is 6
i is 10 and count is 5
i is 11 and count is 4
i is 12 and count is 3
i is 13 and count is 2
i is 14 and count is 1
i is 15 and count is 0

A&B = 0000 1100

A|B = 0011 1101

A^B = 0011 0001

~A  = 1100 0011

The Bitwise operators supported by C++ language are listed in the following table. Assume variable A holds 60 and variable B holds 13, then −

link:../cplusplus/cpp_bitwise_operators.html[Show Examples]
[%autowidth]
|===

|Operator|Description|Example
|&|Binary AND Operator copies a bit to the result if it exists in both operands.|(A & B) will give 12 which is 0000 1100
|||Binary OR Operator copies a bit if it exists in either operand.|(A | B) will give 61 which is 0011 1101
|^|Binary XOR Operator copies the bit if it is set in one operand but not both.|(A ^ B) will give 49 which is 0011 0001
|~|Binary Ones Complement Operator is unary and has the effect of 'flipping' bits.|(~A ) will give -61 which is 1100 0011 in 2's complement form due to a signed binary number.
|<<|Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand.|A << 2 will give 240 which is 1111 0000
|>>|Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand.|A >> 2 will give 15 which is 0000 1111
|===


=== Assignment Operators

There are following assignment operators supported by C++ language −

link:../cplusplus/cpp_assignment_operators.html[Show Examples]
[%autowidth]
|===

|Operator|Description|Example
|=|Simple assignment operator, Assigns values from right side operands to left side operand.|C = A + B will assign value of A + B into C
|+=|Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand.|C += A is equivalent to C = C + A
|-=|Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand.|C -= A is equivalent to C = C - A
|*=|Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand.|C *= A is equivalent to C = C * A
|/=|Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand.|C /= A is equivalent to C = C / A
|%=|Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand.|C %= A is equivalent to C = C % A
|<<=|Left shift AND assignment operator.|C <<= 2 is same as C = C << 2
|>>=|Right shift AND assignment operator.|C >>= 2 is same as C = C >> 2
|&=|Bitwise AND assignment operator.|C &= 2 is same as C = C & 2
|^=|Bitwise exclusive OR and assignment operator.|C ^= 2 is same as C = C ^ 2
||=|Bitwise inclusive OR and assignment operator.|C |= 2 is same as C = C | 2
|===


=== Misc Operators

The following table lists some other operators that C++ supports.
[%autowidth]
|===

|Sr.No|Operator & Description
|1|*sizeof*
link:../cplusplus/cpp_sizeof_operator.html[sizeof operator] returns the size of a variable. For example, sizeof(a), where ‘a’ is integer, and will return 4.

|2|*Condition ? X : Y*
link:../cplusplus/cpp_conditional_operator.html[Conditional operator (?)]. If Condition is true then it returns value of X otherwise returns value of Y.

|3|*,*
link:../cplusplus/cpp_comma_operator.html[Comma operator] causes a sequence of operations to be performed. The value of the entire comma expression is the value of the last expression of the comma-separated list.

|4|*. (dot) and -> (arrow)*
link:../cplusplus/cpp_member_operators.html[Member operators] are used to reference individual members of classes, structures, and unions.

|5|*Cast*
link:../cplusplus/cpp_casting_operators.html[Casting operators] convert one data type to another. For example, int(2.2000) would return 2.

|6|*&*
link:../cplusplus/cpp_pointer_operators.html[Pointer operator &] returns the address of a variable. For example &a; will give actual address of the variable.

|7|***
link:../cplusplus/cpp_pointer_operators.html[Pointer operator *] is pointer to a variable. For example *var; will pointer to a variable var.

|===


=== Operators Precedence in C++

Operator precedence determines the grouping of terms in an expression. This affects how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator −

For example x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has higher precedence than +, so it first gets multiplied with 3*2 and then adds into 7.

Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first.

link:../cplusplus/cpp_operators_precedence.html[Show Examples]
[%autowidth]
|===

|Category |Operator |Associativity
|Postfix |() [] -> . ++ - -  |Left to right
|Unary |+ - ! ~ ++ - - (type)* & sizeof |Right to left
|Multiplicative  |* / % |Left to right
|Additive  |+ - |Left to right
|Shift  |<< >> |Left to right
|Relational  |< <= > >= |Left to right
|Equality  |== != |Left to right
|Bitwise AND |& |Left to right
|Bitwise XOR |^ |Left to right
|Bitwise OR || |Left to right
|Logical AND |&& |Left to right
|Logical OR ||| |Left to right
|Conditional |?: |Right to left
|Assignment |= += -= *= /= %=>>= <<= &= ^= |= |Right to left
|Comma |, |Left to right
|===


== C++ Loop Types

There may be a situation, when you need to execute a block of code several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times and following is the general from of a loop statement in most of the programming languages −

image::https://www.iokays.com/tutorialspoint/cplusplus/_images/loop_architecture.jpg[Loop Architecture]

C++ programming language provides the following type of loops to handle looping requirements.
[%autowidth]
|===

|Sr.No|Loop Type & Description
|1|link:../cplusplus/cpp_while_loop.html[while loop]Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.

|2|link:../cplusplus/cpp_for_loop.html[for loop]Execute a sequence of statements multiple times and abbreviates the code that manages the loop variable.

|3|link:../cplusplus/cpp_do_while_loop.html[do...while loop]Like a ‘while’ statement, except that it tests the condition at the end of the loop body.

|4|link:../cplusplus/cpp_nested_loops.html[nested loops]You can use one or more loop inside any another ‘while’, ‘for’ or ‘do..while’ loop.

|===


=== Loop Control Statements

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

C++ supports the following control statements.
[%autowidth]
|===

|Sr.No|Control Statement & Description
|1|link:../cplusplus/cpp_break_statement.html[break statement]Terminates the *loop* or *switch* statement and transfers execution to the statement immediately following the loop or switch.

|2|link:../cplusplus/cpp_continue_statement.html[continue statement]Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.

|3|link:../cplusplus/cpp_goto_statement.html[goto statement]Transfers control to the labeled statement. Though it is not advised to use goto statement in your program.

|===


=== The Infinite Loop

A loop becomes infinite loop if a condition never becomes false. The *for* loop is traditionally used for this purpose. Since none of the three expressions that form the ‘for’ loop are required, you can make an endless loop by leaving the conditional expression empty.

[source]

When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but C++ programmers more commonly use the ‘for (;;)’ construct to signify an infinite loop.

*NOTE* − You can terminate an infinite loop by pressing Ctrl + C keys.


== C++ decision making statements

Decision making structures require that the programmer specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.

Following is the general form of a typical decision making structure found in most of the programming languages −

image::https://www.iokays.com/tutorialspoint/cplusplus/_images/cpp_decision_making.jpg[C++ decision making]

C++ programming language provides following types of decision making statements.
[%autowidth]
|===

|Sr.No|Statement & Description
|1|link:../cplusplus/cpp_if_statement.html[if statement]An ‘if’ statement consists of a boolean expression followed by one or more statements.

|2|link:../cplusplus/cpp_if_else_statement.html[if...else statement]An ‘if’ statement can be followed by an optional ‘else’ statement, which executes when the boolean expression is false.

|3|link:../cplusplus/cpp_switch_statement.html[switch statement]A ‘switch’ statement allows a variable to be tested for equality against a list of values.

|4|link:../cplusplus/cpp_nested_if.html[nested if statements]You can use one ‘if’ or ‘else if’ statement inside another ‘if’ or ‘else if’ statement(s).

|5|link:../cplusplus/cpp_nested_switch.html[nested switch statements]You can use one ‘switch’ statement inside another ‘switch’ statement(s).

|===


=== The ? : Operator

We have covered link:../cplusplus/cpp_conditional_operator.html[conditional operator “? :”] in previous chapter which can be used to replace *if...else* statements. It has the following general form −

[source]

Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon.

The value of a ‘?’ expression is determined like this: Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ‘?’ expression. If Exp1 is false, then Exp3 is evaluated and its value becomes the value of the expression.


== C++ Functions

A function is a group of statements that together perform a task. Every C++ program has at least one function, which is *main()*, and all the most trivial programs can define additional functions.

You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division usually is such that each function performs a specific task.

A function *declaration* tells the compiler about a function's name, return type, and parameters. A function *definition* provides the actual body of the function.

The C++ standard library provides numerous built-in functions that your program can call. For example, function *strcat()* to concatenate two strings, function *memcpy()* to copy one memory location to another location and many more functions.

A function is known with various names like a method or a sub-routine or a procedure etc.


=== Defining a Function

The general form of a C++ function definition is as follows −

[source]

A C++ function definition consists of a function header and a function body. Here are all the parts of a function −

. *Return Type* − A function may return a value. The *return_type* is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword *void*.


. *Function Name* − This is the actual name of the function. The function name and the parameter list together constitute the function signature.


. *Parameters* − A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.


. *Function Body* − The function body contains a collection of statements that define what the function does.



=== Example

Following is the source code for a function called *max()*. This function takes two parameters num1 and num2 and return the biggest of both −

[source]

=== Function Declarations

A function *declaration* tells the compiler about a function name and how to call the function. The actual body of the function can be defined separately.

A function declaration has the following parts −

[source]

For the above defined function max(), following is the function declaration −

[source]

Parameter names are not important in function declaration only their type is required, so following is also valid declaration −

[source]

Function declaration is required when you define a function in one source file and you call that function in another file. In such case, you should declare the function at the top of the file calling the function.


=== Calling a Function

While creating a C++ function, you give a definition of what the function has to do. To use a function, you will have to call or invoke that function.

When a program calls a function, program control is transferred to the called function. A called function performs defined task and when it’s return statement is executed or when its function-ending closing brace is reached, it returns program control back to the main program.

To call a function, you simply need to pass the required parameters along with function name, and if function returns a value, then you can store returned value. For example −

[source]

I kept max() function along with main() function and compiled the source code. While running final executable, it would produce the following result −

[source]

=== Function Arguments

If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the *formal parameters* of the function.

The formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.

While calling a function, there are two ways that arguments can be passed to a function −
[%autowidth]
|===

|Sr.No|Call Type & Description
|1|link:../cplusplus/cpp_function_call_by_value.html[Call by Value]This method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.

|2|link:../cplusplus/cpp_function_call_by_pointer.html[Call by Pointer]This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.

|3|link:../cplusplus/cpp_function_call_by_reference.html[Call by Reference]This method copies the reference of an argument into the formal parameter. Inside the function, the reference is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.

|===

By default, C++ uses *call by value* to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function and above mentioned example while calling max() function used the same method.


=== Default Values for Parameters

When you define a function, you can specify a default value for each of the last parameters. This value will be used if the corresponding argument is left blank when calling to the function.

This is done by using the assignment operator and assigning values for the arguments in the function definition. If a value for that parameter is not passed when the function is called, the default given value is used, but if a value is specified, this default value is ignored and the passed value is used instead. Consider the following example −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== Numbers in C++

Normally, when we work with Numbers, we use primitive data types such as int, short, long, float and double, etc. The number data types, their possible values and number ranges have been explained while discussing C++ Data Types.


=== Defining Numbers in C++

You have already defined numbers in various examples given in previous chapters. Here is another consolidated example to define various types of numbers in C++ −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Math Operations in C++

In addition to the various functions you can create, C++ also includes some useful functions you can use. These functions are available in standard C and C++ libraries and called *built-in* functions. These are functions that can be included in your program and then use.

C++ has a rich set of mathematical operations, which can be performed on various numbers. Following table lists down some useful built-in mathematical functions available in C++.

To utilize these functions you need to include the math header file *<cmath>*.
[%autowidth]
|===

|Sr.No|Function & Purpose
|1|*double cos(double);*
This function takes an angle (as a double) and returns the cosine.

|2|*double sin(double);*
This function takes an angle (as a double) and returns the sine.

|3|*double tan(double);*
This function takes an angle (as a double) and returns the tangent.

|4|*double log(double);*
This function takes a number and returns the natural log of that number.

|5|*double pow(double, double);*
The first is a number you wish to raise and the second is the power you wish to raise it t

|6|*double hypot(double, double);*
If you pass this function the length of two sides of a right triangle, it will return you the length of the hypotenuse.

|7|*double sqrt(double);*
You pass this function a number and it gives you the square root.

|8|*int abs(int);*
This function returns the absolute value of an integer that is passed to it.

|9|*double fabs(double);*
This function returns the absolute value of any decimal number passed to it.

|10|*double floor(double);*
Finds the integer which is less than or equal to the argument passed to it.

|===

Following is a simple example to show few of the mathematical operations −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Random Numbers in C++

There are many cases where you will wish to generate a random number. There are actually two functions you will need to know about random number generation. The first is *rand()*, this function will only return a pseudo random number. The way to fix this is to first call the *srand()* function.

Following is a simple example to generate few random numbers. This example makes use of *time()* function to get the number of seconds on your system time, to randomly seed the rand() function −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== C++ Arrays

C++ provides a data structure, *the array*, which stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Instead of declaring individual variables, such as number0, number1, ..., and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. A specific element in an array is accessed by an index.

All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.


=== Declaring Arrays

To declare an array in C++, the programmer specifies the type of the elements and the number of elements required by an array as follows −

[source]

This is called a single-dimension array. The *arraySize* must be an integer constant greater than zero and *type* can be any valid C++ data type. For example, to declare a 10-element array called balance of type double, use this statement −

[source]

=== Initializing Arrays

You can initialize C++ array elements either one by one or using a single statement as follows −

[source]

The number of values between braces { } can not be larger than the number of elements that we declare for the array between square brackets [ ]. Following is an example to assign a single element of the array −

If you omit the size of the array, an array just big enough to hold the initialization is created. Therefore, if you write −

[source]

You will create exactly the same array as you did in the previous example.

[source]

The above statement assigns element number 5th in the array a value of 50.0. Array with 4th index will be 5th, i.e., last element because all arrays have 0 as the index of their first element which is also called base index. Following is the pictorial representaion of the same array we discussed above −

image::https://www.iokays.com/tutorialspoint/cplusplus/_images/array_presentation.jpg[Array Presentation]


=== Accessing Array Elements

An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. For example −

[source]

The above statement will take 10th element from the array and assign the value to salary variable. Following is an example, which will use all the above-mentioned three concepts viz. declaration, assignment and accessing arrays −

[source]

This program makes use of *setw()* function to format the output. When the above code is compiled and executed, it produces the following result −

[source]

=== Arrays in C++

Arrays are important to C++ and should need lots of more detail. There are following few important concepts, which should be clear to a C++ programmer −
[%autowidth]
|===

|Sr.No|Concept & Description
|1|link:../cplusplus/cpp_multi_dimensional_arrays.html[Multi-dimensional arrays]C++ supports multidimensional arrays. The simplest form of the multidimensional array is the two-dimensional array.

|2|link:../cplusplus/cpp_pointer_to_an_array.html[Pointer to an array]You can generate a pointer to the first element of an array by simply specifying the array name, without any index.

|3|link:../cplusplus/cpp_passing_arrays_to_functions.html[Passing arrays to functions]You can pass to the function a pointer to an array by specifying the array's name without an index.

|4|link:../cplusplus/cpp_return_arrays_from_functions.html[Return array from functions]C++ allows a function to return an array.

|===


== C++ Strings

C++ provides following two types of string representations −

. The C-style character string.

. The string class type introduced with Standard C++.


=== The C-Style Character String

The C-style character string originated within the C language and continues to be supported within C++. This string is actually a one-dimensional array of characters which is terminated by a *null* character '\0'. Thus a null-terminated string contains the characters that comprise the string followed by a *null*.

The following declaration and initialization create a string consisting of the word "Hello". To hold the null character at the end of the array, the size of the character array containing the string is one more than the number of characters in the word "Hello."

[source]

If you follow the rule of array initialization, then you can write the above statement as follows −

[source]

Following is the memory presentation of above defined string in C/C++ −

image::https://www.iokays.com/tutorialspoint/cplusplus/_images/string_representation.jpg[String Presentation in C/C++]

Actually, you do not place the null character at the end of a string constant. The C++ compiler automatically places the '\0' at the end of the string when it initializes the array. Let us try to print above-mentioned string −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

C++ supports a wide range of functions that manipulate null-terminated strings −
[%autowidth]
|===

|Sr.No|Function & Purpose
|1|*strcpy(s1, s2);*
Copies string s2 into string s1.

|2|*strcat(s1, s2);*
Concatenates string s2 onto the end of string s1.

|3|*strlen(s1);*
Returns the length of string s1.

|4|*strcmp(s1, s2);*
Returns 0 if s1 and s2 are the same; less than 0 if s1<s2; greater than 0 if s1>s2.

|5|*strchr(s1, ch);*
Returns a pointer to the first occurrence of character ch in string s1.

|6|*strstr(s1, s2);*
Returns a pointer to the first occurrence of string s2 in string s1.

|===

Following example makes use of few of the above-mentioned functions −

[source]

When the above code is compiled and executed, it produces result something as follows −

[source]

=== The String Class in C++

The standard C++ library provides a *string* class type that supports all the operations mentioned above, additionally much more functionality. Let us check the following example −

[source]

When the above code is compiled and executed, it produces result something as follows −

[source]

== C++ Pointers

C++ pointers are easy and fun to learn. Some C++ tasks are performed more easily with pointers, and other C++ tasks, such as dynamic memory allocation, cannot be performed without them.

As you know every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator which denotes an address in memory. Consider the following which will print the address of the variables defined −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== What are Pointers?

A *pointer* is a variable whose value is the address of another variable. Like any variable or constant, you must declare a pointer before you can work with it. The general form of a pointer variable declaration is −

[source]

Here, *type* is the pointer's base type; it must be a valid C++ type and *var-name* is the name of the pointer variable. The asterisk you used to declare a pointer is the same asterisk that you use for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Following are the valid pointer declaration −

[source]

The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.


=== Using Pointers in C++

There are few important operations, which we will do with the pointers very frequently. *(a)* We define a pointer variable. *(b)* Assign the address of a variable to a pointer. *(c)* Finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. Following example makes use of these operations −

[source]

When the above code is compiled and executed, it produces result something as follows −

[source]

=== Pointers in C++

Pointers have many but easy concepts and they are very important to C++ programming. There are following few important pointer concepts which should be clear to a C++ programmer −
[%autowidth]
|===

|Sr.No|Concept & Description
|1|link:../cplusplus/cpp_null_pointers.html[Null Pointers]C++ supports null pointer, which is a constant with a value of zero defined in several standard libraries.

|2|link:../cplusplus/cpp_pointer_arithmatic.html[Pointer Arithmetic]There are four arithmetic operators that can be used on pointers: ++, --, +, -

|3|link:../cplusplus/cpp_pointers_vs_arrays.html[Pointers vs Arrays]There is a close relationship between pointers and arrays.

|4|link:../cplusplus/cpp_array_of_pointers.html[Array of Pointers]You can define arrays to hold a number of pointers.

|5|link:../cplusplus/cpp_pointer_to_pointer.html[Pointer to Pointer]C++ allows you to have pointer on a pointer and so on.

|6|link:../cplusplus/cpp_passing_pointers_to_functions.html[Passing Pointers to Functions]Passing an argument by reference or by address both enable the passed argument to be changed in the calling function by the called function.

|7|link:../cplusplus/cpp_return_pointer_from_functions.html[Return Pointer from Functions]C++ allows a function to return a pointer to local variable, static variable and dynamically allocated memory as well.

|===


== C++ References

A reference variable is an alias, that is, another name for an already existing variable. Once a reference is initialized with a variable, either the variable name or the reference name may be used to refer to the variable.


=== References vs Pointers

References are often confused with pointers but three major differences between references and pointers are −

. You cannot have NULL references. You must always be able to assume that a reference is connected to a legitimate piece of storage.


. Once a reference is initialized to an object, it cannot be changed to refer to another object. Pointers can be pointed to another object at any time.


. A reference must be initialized when it is created. Pointers can be initialized at any time.



=== Creating References in C++

Think of a variable name as a label attached to the variable's location in memory. You can then think of a reference as a second label attached to that memory location. Therefore, you can access the contents of the variable through either the original variable name or the reference. For example, suppose we have the following example −

[source]

We can declare reference variables for i as follows.

[source]

Read the & in these declarations as *reference*. Thus, read the first declaration as "r is an integer reference initialized to i" and read the second declaration as "s is a double reference initialized to d.". Following example makes use of references on int and double −

[source]

When the above code is compiled together and executed, it produces the following result −

[source]

References are usually used for function argument lists and function return values. So following are two important subjects related to C++ references which should be clear to a C++ programmer −
[%autowidth]
|===

|Sr.No|Concept & Description
|1|link:../cplusplus/passing_parameters_by_references.html[References as Parameters]C++ supports passing references as function parameter more safely than parameters.

|2|link:../cplusplus/returning_values_by_reference.html[Reference as Return Value]You can return reference from a C++ function like any other data type.

|===


== C++ Date and Time

The C++ standard library does not provide a proper date type. C++ inherits the structs and functions for date and time manipulation from C. To access date and time related functions and structures, you would need to include <ctime> header file in your C++ program.

There are four time-related types: *clock_t, time_t, size_t*, and *tm*. The types - clock_t, size_t and time_t are capable of representing the system time and date as some sort of integer.

The structure type *tm* holds the date and time in the form of a C structure having the following elements −

[source]

Following are the important functions, which we use while working with date and time in C or C++. All these functions are part of standard C and C++ library and you can check their detail using reference to C++ standard library given below.
[%autowidth]
|===

|Sr.No|Function & Purpose
|1|*time_t time(time_t *time);*
This returns the current calendar time of the system in number of seconds elapsed since January 1, 1970. If the system has no time, .1 is returned.

|2|*char *ctime(const time_t *time);*
This returns a pointer to a string of the form day month year hours:minutes:seconds year\n\0.

|3|*struct tm *localtime(const time_t *time);*
This returns a pointer to the *tm* structure representing local time.

|4|*clock_t clock(void);*
This returns a value that approximates the amount of time the calling program has been running. A value of .1 is returned if the time is not available.

|5|*char * asctime ( const struct tm * time );*
This returns a pointer to a string that contains the information stored in the structure pointed to by time converted into the form: day month date hours:minutes:seconds year\n\0

|6|*struct tm *gmtime(const time_t *time);*
This returns a pointer to the time in the form of a tm structure. The time is represented in Coordinated Universal Time (UTC), which is essentially Greenwich Mean Time (GMT).

|7|*time_t mktime(struct tm *time);*
This returns the calendar-time equivalent of the time found in the structure pointed to by time.

|8|*double difftime ( time_t time2, time_t time1 );*
This function calculates the difference in seconds between time1 and time2.

|9|*size_t strftime();*
This function can be used to format date and time in a specific format.

|===


=== Current Date and Time

Suppose you want to retrieve the current system date and time, either as a local time or as a Coordinated Universal Time (UTC). Following is the example to achieve the same −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Format Time using struct tm

The *tm* structure is very important while working with date and time in either C or C++. This structure holds the date and time in the form of a C structure as mentioned above. Most of the time related functions makes use of tm structure. Following is an example which makes use of various date and time related functions and tm structure −

While using structure in this chapter, I'm making an assumption that you have basic understanding on C structure and how to access structure members using arrow -> operator.

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== C++ Basic Input/Output

The C++ standard libraries provide an extensive set of input/output capabilities which we will see in subsequent chapters. This chapter will discuss very basic and most common I/O operations required for C++ programming.

C++ I/O occurs in streams, which are sequences of bytes. If bytes flow from a device like a keyboard, a disk drive, or a network connection etc. to main memory, this is called *input operation* and if bytes flow from main memory to a device like a display screen, a printer, a disk drive, or a network connection, etc., this is called *output operation*.


=== I/O Library Header Files

There are following header files important to C++ programs −
[%autowidth]
|===

|Sr.No|Header File & Function and Description
|1|*<iostream>*
This file defines the *cin, cout, cerr* and *clog* objects, which correspond to the standard input stream, the standard output stream, the un-buffered standard error stream and the buffered standard error stream, respectively.

|2|*<iomanip>*
This file declares services useful for performing formatted I/O with so-called parameterized stream manipulators, such as *setw* and *setprecision*.

|3|*<fstream>*
This file declares services for user-controlled file processing. We will discuss about it in detail in File and Stream related chapter.

|===


=== The Standard Output Stream (cout)

The predefined object *cout* is an instance of *ostream* class. The cout object is said to be "connected to" the standard output device, which usually is the display screen. The *cout* is used in conjunction with the stream insertion operator, which is written as << which are two less than signs as shown in the following example.

[source]

When the above code is compiled and executed, it produces the following result −

[source]

The C++ compiler also determines the data type of variable to be output and selects the appropriate stream insertion operator to display the value. The << operator is overloaded to output data items of built-in types integer, float, double, strings and pointer values.

The insertion operator << may be used more than once in a single statement as shown above and *endl* is used to add a new-line at the end of the line.


=== The Standard Input Stream (cin)

The predefined object *cin* is an instance of *istream* class. The cin object is said to be attached to the standard input device, which usually is the keyboard. The *cin* is used in conjunction with the stream extraction operator, which is written as >> which are two greater than signs as shown in the following example.

[source]

When the above code is compiled and executed, it will prompt you to enter a name. You enter a value and then hit enter to see the following result −

[source]

The C++ compiler also determines the data type of the entered value and selects the appropriate stream extraction operator to extract the value and store it in the given variables.

The stream extraction operator >> may be used more than once in a single statement. To request more than one datum you can use the following −

[source]

This will be equivalent to the following two statements −

[source]

=== The Standard Error Stream (cerr)

The predefined object *cerr* is an instance of *ostream* class. The cerr object is said to be attached to the standard error device, which is also a display screen but the object *cerr* is un-buffered and each stream insertion to cerr causes its output to appear immediately.

The *cerr* is also used in conjunction with the stream insertion operator as shown in the following example.

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== The Standard Log Stream (clog)

The predefined object *clog* is an instance of *ostream* class. The clog object is said to be attached to the standard error device, which is also a display screen but the object *clog* is buffered. This means that each insertion to clog could cause its output to be held in a buffer until the buffer is filled or until the buffer is flushed.

The *clog* is also used in conjunction with the stream insertion operator as shown in the following example.

[source]

When the above code is compiled and executed, it produces the following result −

[source]

You would not be able to see any difference in cout, cerr and clog with these small examples, but while writing and executing big programs the difference becomes obvious. So it is good practice to display error messages using cerr stream and while displaying other log messages then clog should be used.


== C++ Data Structures

C/C++ arrays allow you to define variables that combine several data items of the same kind, but *structure* is another user defined data type which allows you to combine data items of different kinds.

Structures are used to represent a record, suppose you want to keep track of your books in a library. You might want to track the following attributes about each book −

. Title

. Author

. Subject

. Book ID


=== Defining a Structure

To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member, for your program. The format of the struct statement is this −

[source]

The *structure tag* is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. Here is the way you would declare the Book structure −

[source]

=== Accessing Structure Members

To access any member of a structure, we use the *member access operator (.)*. The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use *struct* keyword to define variables of structure type. Following is the example to explain usage of structure −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Structures as Function Arguments

You can pass a structure as a function argument in very similar way as you pass any other variable or pointer. You would access structure variables in the similar way as you have accessed in the above example −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Pointers to Structures

You can define pointers to structures in very similar way as you define pointer to any other variable as follows −

[source]

Now, you can store the address of a structure variable in the above defined pointer variable. To find the address of a structure variable, place the & operator before the structure's name as follows −

[source]

To access the members of a structure using a pointer to that structure, you must use the -> operator as follows −

[source]

Let us re-write above example using structure pointer, hope this will be easy for you to understand the concept −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== The typedef Keyword

There is an easier way to define structs or you could "alias" types you create. For example −

[source]

Now, you can use Books directly to define variables of Books type without using struct keyword. Following is the example −

[source]

You can use *typedef* keyword for non-structs as well as follows −

[source]

x, y and z are all pointers to long ints.


== C++ Classes and Objects

The main purpose of C++ programming is to add object orientation to the C programming language and classes are the central feature of C++ that supports object-oriented programming and are often called user-defined types.

A class is used to specify the form of an object and it combines data representation and methods for manipulating that data into one neat package. The data and functions within a class are called members of the class.


=== C++ Class Definitions

When you define a class, you define a blueprint for a data type. This doesn't actually define any data, but it does define what the class name means, that is, what an object of the class will consist of and what operations can be performed on such an object.

A class definition starts with the keyword *class* followed by the class name; and the class body, enclosed by a pair of curly braces. A class definition must be followed either by a semicolon or a list of declarations. For example, we defined the Box data type using the keyword *class* as follows −

[source]

The keyword *public* determines the access attributes of the members of the class that follows it. A public member can be accessed from outside the class anywhere within the scope of the class object. You can also specify the members of a class as *private* or *protected* which we will discuss in a sub-section.


=== Define C++ Objects

A class provides the blueprints for objects, so basically an object is created from a class. We declare objects of a class with exactly the same sort of declaration that we declare variables of basic types. Following statements declare two objects of class Box −

[source]

Both of the objects Box1 and Box2 will have their own copy of data members.


=== Accessing the Data Members

The public data members of objects of a class can be accessed using the direct member access operator (.). Let us try the following example to make the things clear −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

It is important to note that private and protected members can not be accessed directly using direct member access operator (.). We will learn how private and protected members can be accessed.


=== Classes and Objects in Detail

So far, you have got very basic idea about C++ Classes and Objects. There are further interesting concepts related to C++ Classes and Objects which we will discuss in various sub-sections listed below −
[%autowidth]
|===

|Sr.No|Concept & Description
|1|link:../cplusplus/cpp_class_member_functions.html[Class Member Functions]A member function of a class is a function that has its definition or its prototype within the class definition like any other variable.

|2|link:../cplusplus/cpp_class_access_modifiers.html[Class Access Modifiers]A class member can be defined as public, private or protected. By default members would be assumed as private.

|3|link:../cplusplus/cpp_constructor_destructor.html[Constructor & Destructor]A class constructor is a special function in a class that is called when a new object of the class is created. A destructor is also a special function which is called when created object is deleted.

|4|link:../cplusplus/cpp_copy_constructor.html[Copy Constructor]The copy constructor is a constructor which creates an object by initializing it with an object of the same class, which has been created previously.

|5|link:../cplusplus/cpp_friend_functions.html[Friend Functions]A *friend* function is permitted full access to private and protected members of a class.

|6|link:../cplusplus/cpp_inline_functions.html[Inline Functions]With an inline function, the compiler tries to expand the code in the body of the function in place of a call to the function.

|7|link:../cplusplus/cpp_this_pointer.html[this Pointer]Every object has a special pointer *this* which points to the object itself.

|8|link:../cplusplus/cpp_pointer_to_class.html[Pointer to C++ Classes]A pointer to a class is done exactly the same way a pointer to a structure is. In fact a class is really just a structure with functions in it.

|9|link:../cplusplus/cpp_static_members.html[Static Members of a Class]Both data members and function members of a class can be declared as static.

|===


== C++ Inheritance

One of the most important concepts in object-oriented programming is that of inheritance. Inheritance allows us to define a class in terms of another class, which makes it easier to create and maintain an application. This also provides an opportunity to reuse the code functionality and fast implementation time.

When creating a class, instead of writing completely new data members and member functions, the programmer can designate that the new class should inherit the members of an existing class. This existing class is called the *base* class, and the new class is referred to as the *derived* class.

The idea of inheritance implements the *is a* relationship. For example, mammal IS-A animal, dog IS-A mammal hence dog IS-A animal as well and so on.


=== Base and Derived Classes

A class can be derived from more than one classes, which means it can inherit data and functions from multiple base classes. To define a derived class, we use a class derivation list to specify the base class(es). A class derivation list names one or more base classes and has the form −

[source]

Where access-specifier is one of *public, protected,* or *private*, and base-class is the name of a previously defined class. If the access-specifier is not used, then it is private by default.

Consider a base class *Shape* and its derived class *Rectangle* as follows −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Access Control and Inheritance

A derived class can access all the non-private members of its base class. Thus base-class members that should not be accessible to the member functions of derived classes should be declared private in the base class.

We can summarize the different access types according to - who can access them in the following way −
[%autowidth]
|===

|Access|public|protected|private
|Same class|yes|yes|yes
|Derived classes|yes|yes|no
|Outside classes|yes|no|no
|===

A derived class inherits all base class methods with the following exceptions −

. Constructors, destructors and copy constructors of the base class.

. Overloaded operators of the base class.

. The friend functions of the base class.


=== Type of Inheritance

When deriving a class from a base class, the base class may be inherited through *public, protected* or * private* inheritance. The type of inheritance is specified by the access-specifier as explained above.

We hardly use *protected* or * private* inheritance, but *public* inheritance is commonly used. While using different type of inheritance, following rules are applied −

. *Public Inheritance* − When deriving a class from a *public* base class, *public* members of the base class become *public* members of the derived class and *protected* members of the base class become *protected* members of the derived class. A base class's *private* members are never accessible directly from a derived class, but can be accessed through calls to the *public* and *protected* members of the base class.


. *Protected Inheritance* − When deriving from a *protected* base class, *public* and *protected* members of the base class become *protected* members of the derived class.


. *Private Inheritance* − When deriving from a *private* base class, *public* and *protected* members of the base class become *private* members of the derived class.



=== Multiple Inheritance

A C++ class can inherit members from more than one class and here is the extended syntax −

[source]

Where access is one of *public, protected,* or *private* and would be given for every base class and they will be separated by comma as shown above. Let us try the following example −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== C++ Overloading (Operator and Function)

C++ allows you to specify more than one definition for a *function* name or an *operator* in the same scope, which is called *function overloading* and *operator overloading* respectively.

An overloaded declaration is a declaration that is declared with the same name as a previously declared declaration in the same scope, except that both declarations have different arguments and obviously different definition (implementation).

When you call an overloaded *function* or *operator*, the compiler determines the most appropriate definition to use, by comparing the argument types you have used to call the function or operator with the parameter types specified in the definitions. The process of selecting the most appropriate overloaded function or operator is called *overload resolution*.


=== Function Overloading in C++

You can have multiple definitions for the same function name in the same scope. The definition of the function must differ from each other by the types and/or the number of arguments in the argument list. You cannot overload function declarations that differ only by return type.

Following is the example where same function *print()* is being used to print different data types −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Operators Overloading in C++

You can redefine or overload most of the built-in operators available in C++. Thus, a programmer can use operators with user-defined types as well.

Overloaded operators are functions with special names: the keyword "operator" followed by the symbol for the operator being defined. Like any other function, an overloaded operator has a return type and a parameter list.

[source]

declares the addition operator that can be used to *add* two Box objects and returns final Box object. Most overloaded operators may be defined as ordinary non-member functions or as class member functions. In case we define above function as non-member function of a class then we would have to pass two arguments for each operand as follows −

[source]

Following is the example to show the concept of operator over loading using a member function. Here an object is passed as an argument whose properties will be accessed using this object, the object which will call this operator can be accessed using *this* operator as explained below −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Overloadable/Non-overloadableOperators

Following is the list of operators which can be overloaded −
[%autowidth]
|===

|+|-|*|/|%|^
|&|||~|!|,|=
|<|>|<=|>=|++|--
|<<|>>|==|!=|&&|||
|+=|-=|/=|%=|^=|&=
||=|*=|<<=|>>=|[]|()
|->|->*|new|new []|delete|delete []
|===

Following is the list of operators, which can not be overloaded −
[%autowidth]
|===

|::|.*|.|?:
|===


=== Operator Overloading Examples

Here are various operator overloading examples to help you in understanding the concept.
[%autowidth]
|===

|Sr.No|Operators & Example
|1|link:../cplusplus/unary_operators_overloading.html[Unary Operators Overloading]
|2|link:../cplusplus/binary_operators_overloading.html[Binary Operators Overloading]
|3|link:../cplusplus/relational_operators_overloading.html[Relational Operators Overloading]
|4|link:../cplusplus/input_output_operators_overloading.html[Input/Output Operators Overloading]
|5|link:../cplusplus/increment_decrement_operators_overloading.html[ ++ and -- Operators Overloading]
|6|link:../cplusplus/assignment_operators_overloading.html[Assignment Operators Overloading]
|7|link:../cplusplus/function_call_operator_overloading.html[Function call () Operator Overloading]
|8|link:../cplusplus/subscripting_operator_overloading.html[Subscripting [\] Operator Overloading]
|9|link:../cplusplus/class_member_access_operator_overloading.html[Class Member Access Operator -> Overloading]
|===


== Polymorphism in C++

The word *polymorphism* means having many forms. Typically, polymorphism occurs when there is a hierarchy of classes and they are related by inheritance.

C++ polymorphism means that a call to a member function will cause a different function to be executed depending on the type of object that invokes the function.

Consider the following example where a base class has been derived by other two classes −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

The reason for the incorrect output is that the call of the function area() is being set once by the compiler as the version defined in the base class. This is called *static resolution* of the function call, or *static linkage* - the function call is fixed before the program is executed. This is also sometimes called *early binding* because the area() function is set during the compilation of the program.

But now, let's make a slight modification in our program and precede the declaration of area() in the Shape class with the keyword *virtual* so that it looks like this −

[source]

After this slight modification, when the previous example code is compiled and executed, it produces the following result −

[source]

This time, the compiler looks at the contents of the pointer instead of it's type. Hence, since addresses of objects of tri and rec classes are stored in *shape the respective area() function is called.

As you can see, each of the child classes has a separate implementation for the function area(). This is how *polymorphism* is generally used. You have different classes with a function of the same name, and even the same parameters, but with different implementations.


=== Virtual Function

A *virtual* function is a function in a base class that is declared using the keyword *virtual*. Defining in a base class a virtual function, with another version in a derived class, signals to the compiler that we don't want static linkage for this function.

What we do want is the selection of the function to be called at any given point in the program to be based on the kind of object for which it is called. This sort of operation is referred to as *dynamic linkage*, or *late binding*.


=== Pure Virtual Functions

It is possible that you want to include a virtual function in a base class so that it may be redefined in a derived class to suit the objects of that class, but that there is no meaningful definition you could give for the function in the base class.

We can change the virtual function area() in the base class to the following −

[source]

The = 0 tells the compiler that the function has no body and above virtual function will be called *pure virtual function*.


== Data Abstraction in C++

Data abstraction refers to providing only essential information to the outside world and hiding their background details, i.e., to represent the needed information in program without presenting the details.

Data abstraction is a programming (and design) technique that relies on the separation of interface and implementation.

Let's take one real life example of a TV, which you can turn on and off, change the channel, adjust the volume, and add external components such as speakers, VCRs, and DVD players, BUT you do not know its internal details, that is, you do not know how it receives signals over the air or through a cable, how it translates them, and finally displays them on the screen.

Thus, we can say a television clearly separates its internal implementation from its external interface and you can play with its interfaces like the power button, channel changer, and volume control without having any knowledge of its internals.

In C++, classes provides great level of *data abstraction*. They provide sufficient public methods to the outside world to play with the functionality of the object and to manipulate object data, i.e., state without actually knowing how class has been implemented internally.

For example, your program can make a call to the *sort()* function without knowing what algorithm the function actually uses to sort the given values. In fact, the underlying implementation of the sorting functionality could change between releases of the library, and as long as the interface stays the same, your function call will still work.

In C++, we use *classes* to define our own abstract data types (ADT). You can use the *cout* object of class *ostream* to stream data to standard output like this −

[source]

Here, you don't need to understand how *cout* displays the text on the user's screen. You need to only know the public interface and the underlying implementation of ‘cout’ is free to change.


=== Access Labels Enforce Abstraction

In C++, we use access labels to define the abstract interface to the class. A class may contain zero or more access labels −

. Members defined with a public label are accessible to all parts of the program. The data-abstraction view of a type is defined by its public members.


. Members defined with a private label are not accessible to code that uses the class. The private sections hide the implementation from code that uses the type.


There are no restrictions on how often an access label may appear. Each access label specifies the access level of the succeeding member definitions. The specified access level remains in effect until the next access label is encountered or the closing right brace of the class body is seen.


=== Benefits of Data Abstraction

Data abstraction provides two important advantages −

. Class internals are protected from inadvertent user-level errors, which might corrupt the state of the object.


. The class implementation may evolve over time in response to changing requirements or bug reports without requiring change in user-level code.


By defining data members only in the private section of the class, the class author is free to make changes in the data. If the implementation changes, only the class code needs to be examined to see what affect the change may have. If data is public, then any function that directly access the data members of the old representation might be broken.


=== Data Abstraction Example

Any C++ program where you implement a class with public and private members is an example of data abstraction. Consider the following example −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

Above class adds numbers together, and returns the sum. The public members - *addNum* and *getTotal* are the interfaces to the outside world and a user needs to know them to use the class. The private member *total* is something that the user doesn't need to know about, but is needed for the class to operate properly.


=== Designing Strategy

Abstraction separates code into interface and implementation. So while designing your component, you must keep interface independent of the implementation so that if you change underlying implementation then interface would remain intact.

In this case whatever programs are using these interfaces, they would not be impacted and would just need a recompilation with the latest implementation.


== Data Encapsulation in C++

All C++ programs are composed of the following two fundamental elements −

. *Program statements (code)* − This is the part of a program that performs actions and they are called functions.


. *Program data* − The data is the information of the program which gets affected by the program functions.


Encapsulation is an Object Oriented Programming concept that binds together the data and functions that manipulate the data, and that keeps both safe from outside interference and misuse. Data encapsulation led to the important OOP concept of *data hiding*.

*Data encapsulation* is a mechanism of bundling the data, and the functions that use them and *data abstraction* is a mechanism of exposing only the interfaces and hiding the implementation details from the user.

C++ supports the properties of encapsulation and data hiding through the creation of user-defined types, called *classes*. We already have studied that a class can contain *private, protected * and *public* members. By default, all items defined in a class are private. For example −

[source]

The variables length, breadth, and height are *private*. This means that they can be accessed only by other members of the Box class, and not by any other part of your program. This is one way encapsulation is achieved.

To make parts of a class *public* (i.e., accessible to other parts of your program), you must declare them after the *public* keyword. All variables or functions defined after the public specifier are accessible by all other functions in your program.

Making one class a friend of another exposes the implementation details and reduces encapsulation. The ideal is to keep as many of the details of each class hidden from all other classes as possible.


=== Data Encapsulation Example

Any C++ program where you implement a class with public and private members is an example of data encapsulation and data abstraction. Consider the following example −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

Above class adds numbers together, and returns the sum. The public members *addNum* and *getTotal * are the interfaces to the outside world and a user needs to know them to use the class. The private member *total* is something that is hidden from the outside world, but is needed for the class to operate properly.


=== Designing Strategy

Most of us have learnt to make class members private by default unless we really need to expose them. That's just good *encapsulation*.

This is applied most frequently to data members, but it applies equally to all members, including virtual functions.


== Interfaces in C++ (Abstract Classes)

An interface describes the behavior or capabilities of a C++ class without committing to a particular implementation of that class.

The C++ interfaces are implemented using *abstract classes* and these abstract classes should not be confused with data abstraction which is a concept of keeping implementation details separate from associated data.

A class is made abstract by declaring at least one of its functions as *pure virtual* function. A pure virtual function is specified by placing "= 0" in its declaration as follows −

[source]

The purpose of an *abstract class* (often referred to as an ABC) is to provide an appropriate base class from which other classes can inherit. Abstract classes cannot be used to instantiate objects and serves only as an *interface*. Attempting to instantiate an object of an abstract class causes a compilation error.

Thus, if a subclass of an ABC needs to be instantiated, it has to implement each of the virtual functions, which means that it supports the interface declared by the ABC. Failure to override a pure virtual function in a derived class, then attempting to instantiate objects of that class, is a compilation error.

Classes that can be used to instantiate objects are called *concrete classes*.


=== Abstract Class Example

Consider the following example where parent class provides an interface to the base class to implement a function called *getArea()* −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

You can see how an abstract class defined an interface in terms of getArea() and two other classes implemented same function but with different algorithm to calculate the area specific to the shape.


=== Designing Strategy

An object-oriented system might use an abstract base class to provide a common and standardized interface appropriate for all the external applications. Then, through inheritance from that abstract base class, derived classes are formed that operate similarly.

The capabilities (i.e., the public functions) offered by the external applications are provided as pure virtual functions in the abstract base class. The implementations of these pure virtual functions are provided in the derived classes that correspond to the specific types of the application.

This architecture also allows new applications to be added to a system easily, even after the system has been defined.


== C++ Files and Streams

So far, we have been using the *iostream* standard library, which provides *cin* and *cout* methods for reading from standard input and writing to standard output respectively.

This tutorial will teach you how to read and write from a file. This requires another standard C++ library called *fstream*, which defines three new data types −
[%autowidth]
|===

|Sr.No|Data Type & Description
|1|*ofstream*
This data type represents the output file stream and is used to create files and to write information to files.

|2|*ifstream*
This data type represents the input file stream and is used to read information from files.

|3|*fstream*
This data type represents the file stream generally, and has the capabilities of both ofstream and ifstream which means it can create files, write information to files, and read information from files.

|===

To perform file processing in C++, header files <iostream> and <fstream> must be included in your C++ source file.


=== Opening a File

A file must be opened before you can read from it or write to it. Either *ofstream* or *fstream* object may be used to open a file for writing. And ifstream object is used to open a file for reading purpose only.

Following is the standard syntax for open() function, which is a member of fstream, ifstream, and ofstream objects.

[source]

Here, the first argument specifies the name and location of the file to be opened and the second argument of the *open()* member function defines the mode in which the file should be opened.
[%autowidth]
|===

|Sr.No|Mode Flag & Description
|1|*ios::app*
Append mode. All output to that file to be appended to the end.

|2|*ios::ate*
Open a file for output and move the read/write control to the end of the file.

|3|*ios::in*
Open a file for reading.

|4|*ios::out*
Open a file for writing.

|5|*ios::trunc*
If the file already exists, its contents will be truncated before opening the file.

|===

You can combine two or more of these values by *OR*ing them together. For example if you want to open a file in write mode and want to truncate it in case that already exists, following will be the syntax −

[source]

Similar way, you can open a file for reading and writing purpose as follows −

[source]

=== Closing a File

When a C++ program terminates it automatically flushes all the streams, release all the allocated memory and close all the opened files. But it is always a good practice that a programmer should close all the opened files before program termination.

Following is the standard syntax for close() function, which is a member of fstream, ifstream, and ofstream objects.

[source]

=== Writing to a File

While doing C++ programming, you write information to a file from your program using the stream insertion operator (<<) just as you use that operator to output information to the screen. The only difference is that you use an *ofstream* or *fstream* object instead of the *cout* object.


=== Reading from a File

You read information from a file into your program using the stream extraction operator (>>) just as you use that operator to input information from the keyboard. The only difference is that you use an *ifstream* or *fstream* object instead of the *cin* object.


=== Read and Write Example

Following is the C++ program which opens a file in reading and writing mode. After writing information entered by the user to a file named afile.dat, the program reads information from the file and outputs it onto the screen −

[source]

When the above code is compiled and executed, it produces the following sample input and output −

[source]

Above examples make use of additional functions from cin object, like getline() function to read the line from outside and ignore() function to ignore the extra characters left by previous read statement.


=== File Position Pointers

Both *istream* and *ostream* provide member functions for repositioning the file-position pointer. These member functions are *seekg* ("seek get") for istream and *seekp* ("seek put") for ostream.

The argument to seekg and seekp normally is a long integer. A second argument can be specified to indicate the seek direction. The seek direction can be *ios::beg* (the default) for positioning relative to the beginning of a stream, *ios::cur* for positioning relative to the current position in a stream or *ios::end* for positioning relative to the end of a stream.

The file-position pointer is an integer value that specifies the location in the file as a number of bytes from the file's starting location. Some examples of positioning the "get" file-position pointer are −

[source]

== C++ Exception Handling

An exception is a problem that arises during the execution of a program. A C++ exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero.

Exceptions provide a way to transfer control from one part of a program to another. C++ exception handling is built upon three keywords: *try, catch,* and *throw*.

. *throw* − A program throws an exception when a problem shows up. This is done using a *throw* keyword.


. *catch* − A program catches an exception with an exception handler at the place in a program where you want to handle the problem. The *catch* keyword indicates the catching of an exception.


. *try* − A *try* block identifies a block of code for which particular exceptions will be activated. It's followed by one or more catch blocks.


Assuming a block will raise an exception, a method catches an exception using a combination of the *try* and *catch* keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch as follows −

[source]

You can list down multiple *catch* statements to catch different type of exceptions in case your *try* block raises more than one exception in different situations.


=== Throwing Exceptions

Exceptions can be thrown anywhere within a code block using *throw* statement. The operand of the throw statement determines a type for the exception and can be any expression and the type of the result of the expression determines the type of exception thrown.

Following is an example of throwing an exception when dividing by zero condition occurs −

[source]

=== Catching Exceptions

The *catch* block following the *try* block catches any exception. You can specify what type of exception you want to catch and this is determined by the exception declaration that appears in parentheses following the keyword catch.

[source]

Above code will catch an exception of *ExceptionName* type. If you want to specify that a catch block should handle any type of exception that is thrown in a try block, you must put an ellipsis, ..., between the parentheses enclosing the exception declaration as follows −

[source]

The following is an example, which throws a division by zero exception and we catch it in catch block.

[source]

Because we are raising an exception of type *const char**, so while catching this exception, we have to use const char* in catch block. If we compile and run above code, this would produce the following result −

[source]

=== C++ Standard Exceptions

C++ provides a list of standard exceptions defined in *<exception>* which we can use in our programs. These are arranged in a parent-child class hierarchy shown below −

image::https://www.iokays.com/tutorialspoint/cplusplus/_images/cpp_exceptions.jpg[C++ Exceptions Hierarchy]

Here is the small description of each exception mentioned in the above hierarchy −
[%autowidth]
|===

|Sr.No|Exception & Description
|1|*std::exception*
An exception and parent class of all the standard C++ exceptions.

|2|*std::bad_alloc*
This can be thrown by *new*.

|3|*std::bad_cast*
This can be thrown by *dynamic_cast*.

|4|*std::bad_exception*
This is useful device to handle unexpected exceptions in a C++ program.

|5|*std::bad_typeid*
This can be thrown by *typeid*.

|6|*std::logic_error*
An exception that theoretically can be detected by reading the code.

|7|*std::domain_error*
This is an exception thrown when a mathematically invalid domain is used.

|8|*std::invalid_argument*
This is thrown due to invalid arguments.

|9|*std::length_error*
This is thrown when a too big std::string is created.

|10|*std::out_of_range*
This can be thrown by the 'at' method, for example a std::vector and std::bitset<>::operator[]().

|11|*std::runtime_error*
An exception that theoretically cannot be detected by reading the code.

|12|*std::overflow_error*
This is thrown if a mathematical overflow occurs.

|13|*std::range_error*
This is occurred when you try to store a value which is out of range.

|14|*std::underflow_error*
This is thrown if a mathematical underflow occurs.

|===


=== Define New Exceptions

You can define your own exceptions by inheriting and overriding *exception* class functionality. Following is the example, which shows how you can use std::exception class to implement your own exception in standard way −

[source]

This would produce the following result −

[source]

Here, *what()* is a public method provided by exception class and it has been overridden by all the child exception classes. This returns the cause of an exception.


== C++ Dynamic Memory

A good understanding of how dynamic memory really works in C++ is essential to becoming a good C++ programmer. Memory in your C++ program is divided into two parts −

. *The stack* − All variables declared inside the function will take up memory from the stack.


. *The heap* − This is unused memory of the program and can be used to allocate the memory dynamically when program runs.


Many times, you are not aware in advance how much memory you will need to store particular information in a defined variable and the size of required memory can be determined at run time.

You can allocate memory at run time within the heap for the variable of a given type using a special operator in C++ which returns the address of the space allocated. This operator is called *new* operator.

If you are not in need of dynamically allocated memory anymore, you can use *delete* operator, which de-allocates memory that was previously allocated by new operator.


=== new and delete Operators

There is following generic syntax to use *new* operator to allocate memory dynamically for any data-type.

[source]

Here, *data-type* could be any built-in data type including an array or any user defined data types include class or structure. Let us start with built-in data types. For example we can define a pointer to type double and then request that the memory be allocated at execution time. We can do this using the *new * operator with the following statements −

[source]

The memory may not have been allocated successfully, if the free store had been used up. So it is good practice to check if new operator is returning NULL pointer and take appropriate action as below −

[source]

The *malloc()* function from C, still exists in C++, but it is recommended to avoid using malloc() function. The main advantage of new over malloc() is that new doesn't just allocate memory, it constructs objects which is prime purpose of C++.

At any point, when you feel a variable that has been dynamically allocated is not anymore required, you can free up the memory that it occupies in the free store with the ‘delete’ operator as follows −

[source]

Let us put above concepts and form the following example to show how ‘new’ and ‘delete’ work −

[source]

If we compile and run above code, this would produce the following result −

[source]

=== Dynamic Memory Allocation for Arrays

Consider you want to allocate memory for an array of characters, i.e., string of 20 characters. Using the same syntax what we have used above we can allocate memory dynamically as shown below.

[source]

To remove the array that we have just created the statement would look like this −

[source]

Following the similar generic syntax of new operator, you can allocate for a multi-dimensional array as follows −

[source]

However, the syntax to release the memory for multi-dimensional array will still remain same as above −

[source]

=== Dynamic Memory Allocation for Objects

Objects are no different from simple data types. For example, consider the following code where we are going to use an array of objects to clarify the concept −

[source]

If you were to allocate an array of four Box objects, the Simple constructor would be called four times and similarly while deleting these objects, destructor will also be called same number of times.

If we compile and run above code, this would produce the following result −

[source]

== Namespaces in C++

Consider a situation, when we have two persons with the same name, Zara, in the same class. Whenever we need to differentiate them definitely we would have to use some additional information along with their name, like either the area, if they live in different area or their mother’s or father’s name, etc.

Same situation can arise in your C++ applications. For example, you might be writing some code that has a function called xyz() and there is another library available which is also having same function xyz(). Now the compiler has no way of knowing which version of xyz() function you are referring to within your code.

A *namespace* is designed to overcome this difficulty and is used as additional information to differentiate similar functions, classes, variables etc. with the same name available in different libraries. Using namespace, you can define the context in which names are defined. In essence, a namespace defines a scope.


=== Defining a Namespace

A namespace definition begins with the keyword *namespace* followed by the namespace name as follows −

[source]

To call the namespace-enabled version of either function or variable, prepend (::) the namespace name as follows −

[source]

Let us see how namespace scope the entities including variable and functions −

[source]

If we compile and run above code, this would produce the following result −

[source]

=== The using directive

You can also avoid prepending of namespaces with the *using namespace* directive. This directive tells the compiler that the subsequent code is making use of names in the specified namespace. The namespace is thus implied for the following code −

[source]

If we compile and run above code, this would produce the following result −

[source]

The ‘using’ directive can also be used to refer to a particular item within a namespace. For example, if the only part of the std namespace that you intend to use is cout, you can refer to it as follows −

[source]

Subsequent code can refer to cout without prepending the namespace, but other items in the *std * namespace will still need to be explicit as follows −

[source]

If we compile and run above code, this would produce the following result −

[source]

Names introduced in a *using* directive obey normal scope rules. The name is visible from the point of the *using* directive to the end of the scope in which the directive is found. Entities with the same name defined in an outer scope are hidden.


=== Discontiguous Namespaces

A namespace can be defined in several parts and so a namespace is made up of the sum of its separately defined parts. The separate parts of a namespace can be spread over multiple files.

So, if one part of the namespace requires a name defined in another file, that name must still be declared. Writing a following namespace definition either defines a new namespace or adds new elements to an existing one −

[source]

=== Nested Namespaces

Namespaces can be nested where you can define one namespace inside another name space as follows −

[source]

You can access members of nested namespace by using resolution operators as follows −

[source]

In the above statements if you are using namespace_name1, then it will make elements of namespace_name2 available in the scope as follows −

[source]

If we compile and run above code, this would produce the following result −

[source]

== C++ Templates

Templates are the foundation of generic programming, which involves writing code in a way that is independent of any particular type.

A template is a blueprint or formula for creating a generic class or a function. The library containers like iterators and algorithms are examples of generic programming and have been developed using template concept.

There is a single definition of each container, such as *vector*, but we can define many different kinds of vectors for example, *vector <int>* or *vector <string>*.

You can use templates to define functions as well as classes, let us see how they work −


=== Function Template

The general form of a template function definition is shown here −

[source]

Here, type is a placeholder name for a data type used by the function. This name can be used within the function definition.

The following is the example of a function template that returns the maximum of two values −

[source]

If we compile and run above code, this would produce the following result −

[source]

=== Class Template

Just as we can define function templates, we can also define class templates. The general form of a generic class declaration is shown here −

[source]

Here, *type* is the placeholder type name, which will be specified when a class is instantiated. You can define more than one generic data type by using a comma-separated list.

Following is the example to define class Stack<> and implement generic methods to push and pop the elements from the stack −

[source]

If we compile and run above code, this would produce the following result −

[source]

== C++ Preprocessor

The preprocessors are the directives, which give instructions to the compiler to preprocess the information before actual compilation starts.

All preprocessor directives begin with #, and only white-space characters may appear before a preprocessor directive on a line. Preprocessor directives are not C++ statements, so they do not end in a semicolon (;).

You already have seen a *#include* directive in all the examples. This macro is used to include a header file into the source file.

There are number of preprocessor directives supported by C++ like #include, #define, #if, #else, #line, etc. Let us see important directives −


=== The

The #define preprocessor directive creates symbolic constants. The symbolic constant is called a *macro* and the general form of the directive is −

[source]

When this line appears in a file, all subsequent occurrences of macro in that file will be replaced by replacement-text before the program is compiled. For example −

[source]

Now, let us do the preprocessing of this code to see the result assuming we have the source code file. So let us compile it with -E option and redirect the result to test.p. Now, if you check test.p, it will have lots of information and at the bottom, you will find the value replaced as follows −

[source]

=== Function-Like Macros

You can use #define to define a macro which will take argument as follows −

[source]

If we compile and run above code, this would produce the following result −

[source]

=== Conditional Compilation

There are several directives, which can be used to compile selective portions of your program's source code. This process is called conditional compilation.

The conditional preprocessor construct is much like the ‘if’ selection structure. Consider the following preprocessor code −

[source]

You can compile a program for debugging purpose. You can also turn on or off the debugging using a single macro as follows −

[source]

This causes the *cerr* statement to be compiled in the program if the symbolic constant DEBUG has been defined before directive #ifdef DEBUG. You can use #if 0 statment to comment out a portion of the program as follows −

[source]

Let us try the following example −

[source]

If we compile and run above code, this would produce the following result −

[source]

=== The

The # and ## preprocessor operators are available in C++ and ANSI/ISO C. The # operator causes a replacement-text token to be converted to a string surrounded by quotes.

Consider the following macro definition −

[source]

If we compile and run above code, this would produce the following result −

[source]

Let us see how it worked. It is simple to understand that the C++ preprocessor turns the line −

[source]

Above line will be turned into the following line −

[source]

The ## operator is used to concatenate two tokens. Here is an example −

[source]

When CONCAT appears in the program, its arguments are concatenated and used to replace the macro. For example, CONCAT(HELLO, C++) is replaced by "HELLO C++" in the program as follows.

[source]

If we compile and run above code, this would produce the following result −

[source]

Let us see how it worked. It is simple to understand that the C++ preprocessor transforms −

[source]

Above line will be transformed into the following line −

[source]

=== Predefined C++ Macros

C++ provides a number of predefined macros mentioned below −
[%autowidth]
|===

|Sr.No|Macro & Description
|1|*__LINE__*
This contains the current line number of the program when it is being compiled.

|2|*__FILE__*
This contains the current file name of the program when it is being compiled.

|3|*__DATE__*
This contains a string of the form month/day/year that is the date of the translation of the source file into object code.

|4|*__TIME__*
This contains a string of the form hour:minute:second that is the time at which the program was compiled.

|===

Let us see an example for all the above macros −

[source]

If we compile and run above code, this would produce the following result −

[source]

== C++ Signal Handling

Signals are the interrupts delivered to a process by the operating system which can terminate a program prematurely. You can generate interrupts by pressing Ctrl+C on a UNIX, LINUX, Mac OS X or Windows system.

There are signals which can not be caught by the program but there is a following list of signals which you can catch in your program and can take appropriate actions based on the signal. These signals are defined in C++ header file <csignal>.
[%autowidth]
|===

|Sr.No|Signal & Description
|1|*SIGABRT*
Abnormal termination of the program, such as a call to *abort*.

|2|*SIGFPE*
An erroneous arithmetic operation, such as a divide by zero or an operation resulting in overflow.

|3|*SIGILL*
Detection of an illegal instruction.

|4|*SIGINT*
Receipt of an interactive attention signal.

|5|*SIGSEGV*
An invalid access to storage.

|6|*SIGTERM*
A termination request sent to the program.

|===


=== The signal() Function

C++ signal-handling library provides function *signal* to trap unexpected events. Following is the syntax of the signal() function −

[source]

Keeping it simple, this function receives two arguments: first argument as an integer which represents signal number and second argument as a pointer to the signal-handling function.

Let us write a simple C++ program where we will catch SIGINT signal using signal() function. Whatever signal you want to catch in your program, you must register that signal using *signal* function and associate it with a signal handler. Examine the following example −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

Now, press Ctrl+c to interrupt the program and you will see that your program will catch the signal and would come out by printing something as follows −

[source]

=== The raise() Function

You can generate signals by function *raise()*, which takes an integer signal number as an argument and has the following syntax.

[source]

Here, *sig* is the signal number to send any of the signals: SIGINT, SIGABRT, SIGFPE, SIGILL, SIGSEGV, SIGTERM, SIGHUP. Following is the example where we raise a signal internally using raise() function as follows −

[source]

When the above code is compiled and executed, it produces the following result and would come out automatically −

[source]

== C++ Multithreading

Multithreading is a specialized form of multitasking and a multitasking is the feature that allows your computer to run two or more programs concurrently. In general, there are two types of multitasking: process-based and thread-based.

Process-based multitasking handles the concurrent execution of programs. Thread-based multitasking deals with the concurrent execution of pieces of the same program.

A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution.

Before C++ 11, there is no built-in support for multithreaded applications. Instead, it relies entirely upon the operating system to provide this feature.

This tutorial assumes that you are working on Linux OS and we are going to write multi-threaded C++ program using POSIX. POSIX Threads, or Pthreads provides API which are available on many Unix-like POSIX systems such as FreeBSD, NetBSD, GNU/Linux, Mac OS X and Solaris.


=== Creating Threads

The following routine is used to create a POSIX thread −

[source]

Here, *pthread_create* creates a new thread and makes it executable. This routine can be called any number of times from anywhere within your code. Here is the description of the parameters −
[%autowidth]
|===

|Sr.No|Parameter & Description
|1|*thread*
An opaque, unique identifier for the new thread returned by the subroutine.

|2|*attr*
An opaque attribute object that may be used to set thread attributes. You can specify a thread attributes object, or NULL for the default values.

|3|*start_routine*
The C++ routine that the thread will execute once it is created.

|4|*arg*
A single argument that may be passed to start_routine. It must be passed by reference as a pointer cast of type void. NULL may be used if no argument is to be passed.

|===

The maximum number of threads that may be created by a process is implementation dependent. Once created, threads are peers, and may create other threads. There is no implied hierarchy or dependency between threads.


=== Terminating Threads

There is following routine which we use to terminate a POSIX thread −

[source]

Here *pthread_exit* is used to explicitly exit a thread. Typically, the pthread_exit() routine is called after a thread has completed its work and is no longer required to exist.

If main() finishes before the threads it has created, and exits with pthread_exit(), the other threads will continue to execute. Otherwise, they will be automatically terminated when main() finishes.

*Example*

This simple example code creates 5 threads with the pthread_create() routine. Each thread prints a "Hello World!" message, and then terminates with a call to pthread_exit().

[source]

Compile the following program using -lpthread library as follows −

[source]

Now, execute your program which gives the following output −

[source]

=== Passing Arguments to Threads

This example shows how to pass multiple arguments via a structure. You can pass any data type in a thread callback because it points to void as explained in the following example −

[source]

When the above code is compiled and executed, it produces the following result −

[source]

=== Joining and Detaching Threads

There are following two routines which we can use to join or detach threads −

[source]

The pthread_join() subroutine blocks the calling thread until the specified 'threadid' thread terminates. When a thread is created, one of its attributes defines whether it is joinable or detached. Only threads that are created as joinable can be joined. If a thread is created as detached, it can never be joined.

This example demonstrates how to wait for thread completions by using the Pthread join routine.

[source]

When the above code is compiled and executed, it produces the following result −

[source]

== C++ Web Programming


=== What is CGI?

. The Common Gateway Interface, or CGI, is a set of standards that define how information is exchanged between the web server and a custom script.


. The CGI specs are currently maintained by the NCSA and NCSA defines CGI is as follows −


. The Common Gateway Interface, or CGI, is a standard for external gateway programs to interface with information servers such as HTTP servers.


. The current version is CGI/1.1 and CGI/1.2 is under progress.



=== Web Browsing

To understand the concept of CGI, let's see what happens when we click a hyperlink to browse a particular web page or URL.

. Your browser contacts the HTTP web server and demand for the URL ie. filename.


. Web Server will parse the URL and will look for the filename. If it finds requested file then web server sends that file back to the browser otherwise sends an error message indicating that you have requested a wrong file.


. Web browser takes response from web server and displays either the received file or error message based on the received response.


However, it is possible to set up the HTTP server in such a way that whenever a file in a certain directory is requested, that file is not sent back; instead it is executed as a program, and produced output from the program is sent back to your browser to display.

The Common Gateway Interface (CGI) is a standard protocol for enabling applications (called CGI programs or CGI scripts) to interact with Web servers and with clients. These CGI programs can be a written in Python, PERL, Shell, C or C++ etc.


=== CGI Architecture Diagram

The following simple program shows a simple architecture of CGI −

image::https://www.iokays.com/tutorialspoint/cplusplus/_images/cgiarch.gif[CGI Architecture]


=== Web Server Configuration

Before you proceed with CGI Programming, make sure that your Web Server supports CGI and it is configured to handle CGI Programs. All the CGI Programs to be executed by the HTTP server are kept in a pre-configured directory. This directory is called CGI directory and by convention it is named as /var/www/cgi-bin. By convention CGI files will have extension as *.cgi*, though they are C++ executable.

By default, Apache Web Server is configured to run CGI programs in /var/www/cgi-bin. If you want to specify any other directory to run your CGI scripts, you can modify the following section in the httpd.conf file −

[source]

Here, I assume that you have Web Server up and running successfully and you are able to run any other CGI program like Perl or Shell etc.


=== First CGI Program

Consider the following C++ Program content −

[source]

Compile above code and name the executable as cplusplus.cgi. This file is being kept in /var/www/cgi-bin directory and it has following content. Before running your CGI program make sure you have change mode of file using *chmod 755 cplusplus.cgi* UNIX command to make file executable.


=== My First CGI program

The above C++ program is a simple program which is writing its output on STDOUT file i.e. screen. There is one important and extra feature available which is first line printing *Content-type:text/html\r\n\r\n*. This line is sent back to the browser and specify the content type to be displayed on the browser screen. Now you must have understood the basic concept of CGI and you can write many complicated CGI programs using Python. A C++ CGI program can interact with any other external system, such as RDBMS, to exchange information.


=== HTTP Header

The line *Content-type:text/html\r\n\r\n* is a part of HTTP header, which is sent to the browser to understand the content. All the HTTP header will be in the following form −

[source]

There are few other important HTTP headers, which you will use frequently in your CGI Programming.
[%autowidth]
|===

|Sr.No|Header & Description
|1|*Content-type:*
A MIME string defining the format of the file being returned. Example is Content-type:text/html.

|2|*Expires: Date*
The date the information becomes invalid. This should be used by the browser to decide when a page needs to be refreshed. A valid date string should be in the format 01 Jan 1998 12:00:00 GMT.

|3|*Location: URL*
The URL that should be returned instead of the URL requested. You can use this filed to redirect a request to any file.

|4|*Last-modified: Date*
The date of last modification of the resource.

|5|*Content-length: N*
The length, in bytes, of the data being returned. The browser uses this value to report the estimated download time for a file.

|6|*Set-Cookie: String*
Set the cookie passed through the string.

|===


=== CGI Environment Variables

All the CGI program will have access to the following environment variables. These variables play an important role while writing any CGI program.
[%autowidth]
|===

|Sr.No|Variable Name & Description
|1|*CONTENT_TYPE*
The data type of the content, used when the client is sending attached content to the server. For example file upload etc.

|2|*CONTENT_LENGTH*
The length of the query information that is available only for POST requests.

|3|*HTTP_COOKIE*
Returns the set cookies in the form of key & value pair.

|4|*HTTP_USER_AGENT*
The User-Agent request-header field contains information about the user agent originating the request. It is a name of the web browser.

|5|*PATH_INFO*
The path for the CGI script.

|6|*QUERY_STRING*
The URL-encoded information that is sent with GET method request.

|7|*REMOTE_ADDR*
The IP address of the remote host making the request. This can be useful for logging or for authentication purpose.

|8|*REMOTE_HOST*
The fully qualified name of the host making the request. If this information is not available then REMOTE_ADDR can be used to get IR address.

|9|*REQUEST_METHOD*
The method used to make the request. The most common methods are GET and POST.

|10|*SCRIPT_FILENAME*
The full path to the CGI script.

|11|*SCRIPT_NAME*
The name of the CGI script.

|12|*SERVER_NAME*
The server's hostname or IP Address.

|13|*SERVER_SOFTWARE*
The name and version of the software the server is running.

|===

Here is small CGI program to list out all the CGI variables.

[source]

=== C++ CGI Library

For real examples, you would need to do many operations by your CGI program. There is a CGI library written for C++ program which you can download from link:ftp://ftp.gnu.org/gnu/cgicc/[ftp://ftp.gnu.org/gnu/cgicc/] and follow the steps to install the library −

[source]

You can check related documentation available at link:https://www.gnu.org/software/cgicc/doc/index.html[‘C++ CGI Lib Documentation].


=== GET and POST Methods

You must have come across many situations when you need to pass some information from your browser to web server and ultimately to your CGI Program. Most frequently browser uses two methods to pass this information to web server. These methods are GET Method and POST Method.


=== Passing Information Using GET Method

The GET method sends the encoded user information appended to the page request. The page and the encoded information are separated by the ? character as follows −

[source]

The GET method is the default method to pass information from browser to web server and it produces a long string that appears in your browser's Location:box. Never use the GET method if you have password or other sensitive information to pass to the server. The GET method has size limitation and you can pass upto 1024 characters in a request string.

When using GET method, information is passed using QUERY_STRING http header and will be accessible in your CGI Program through QUERY_STRING environment variable.

You can pass information by simply concatenating key and value pairs alongwith any URL or you can use HTML <FORM> tags to pass information using GET method.


=== Simple URL Example: Get Method

Here is a simple URL which will pass two values to hello_get.py program using GET method.

Below is a program to generate *cpp_get.cgi* CGI program to handle input given by web browser. We are going to use C++ CGI library which makes it very easy to access passed information −

[source]

Now, compile the above program as follows −

[source]

Generate cpp_get.cgi and put it in your CGI directory and try to access using following link −

This would generate following result −

[source]

=== Simple FORM Example: GET Method

Here is a simple example which passes two values using HTML FORM and submit button. We are going to use same CGI script cpp_get.cgi to handle this input.

[source]

Here is the actual output of the above form. You enter First and Last Name and then click submit button to see the result.


=== Passing Information Using POST Method

A generally more reliable method of passing information to a CGI program is the POST method. This packages the information in exactly the same way as GET methods, but instead of sending it as a text string after a ? in the URL it sends it as a separate message. This message comes into the CGI script in the form of the standard input.

The same cpp_get.cgi program will handle POST method as well. Let us take same example as above, which passes two values using HTML FORM and submit button but this time with POST method as follows −

[source]

Here is the actual output of the above form. You enter First and Last Name and then click submit button to see the result.


=== Passing Checkbox Data to CGI Program

Checkboxes are used when more than one option is required to be selected.

Here is example HTML code for a form with two checkboxes −

[source]

The result of this code is the following form −

Below is C++ program, which will generate cpp_checkbox.cgi script to handle input given by web browser through checkbox button.

[source]

=== Passing Radio Button Data to CGI Program

Radio Buttons are used when only one option is required to be selected.

Here is example HTML code for a form with two radio button −

[source]

The result of this code is the following form −

Below is C++ program, which will generate cpp_radiobutton.cgi script to handle input given by web browser through radio buttons.

[source]

=== Passing Text Area Data to CGI Program

TEXTAREA element is used when multiline text has to be passed to the CGI Program.

Here is example HTML code for a form with a TEXTAREA box −

[source]

The result of this code is the following form −

Below is C++ program, which will generate cpp_textarea.cgi script to handle input given by web browser through text area.

[source]

=== Passing Drop down Box Data to CGI Program

Drop down Box is used when we have many options available but only one or two will be selected.

Here is example HTML code for a form with one drop down box −

[source]

The result of this code is the following form −

Below is C++ program, which will generate cpp_dropdown.cgi script to handle input given by web browser through drop down box.

[source]

=== Using Cookies in CGI

HTTP protocol is a stateless protocol. But for a commercial website it is required to maintain session information among different pages. For example one user registration ends after completing many pages. But how to maintain user's session information across all the web pages.

In many situations, using cookies is the most efficient method of remembering and tracking preferences, purchases, commissions, and other information required for better visitor experience or site statistics.


=== How It Works

Your server sends some data to the visitor's browser in the form of a cookie. The browser may accept the cookie. If it does, it is stored as a plain text record on the visitor's hard drive. Now, when the visitor arrives at another page on your site, the cookie is available for retrieval. Once retrieved, your server knows/remembers what was stored.

Cookies are a plain text data record of 5 variable-length fields −

. *Expires* − This shows date the cookie will expire. If this is blank, the cookie will expire when the visitor quits the browser.


. *Domain* − This shows domain name of your site.


. *Path* − This shows path to the directory or web page that set the cookie. This may be blank if you want to retrieve the cookie from any directory or page.


. *Secure* − If this field contains the word "secure" then the cookie may only be retrieved with a secure server. If this field is blank, no such restriction exists.


. *Name = Value* − Cookies are set and retrieved in the form of key and value pairs.



=== Setting up Cookies

It is very easy to send cookies to browser. These cookies will be sent along with HTTP Header before the Content-type filed. Assuming you want to set UserID and Password as cookies. So cookies setting will be done as follows

[source]

From this example, you must have understood how to set cookies. We use *Set-Cookie* HTTP header to set cookies.

Here, it is optional to set cookies attributes like Expires, Domain, and Path. It is notable that cookies are set before sending magic line *"Content-type:text/html\r\n\r\n*.

Compile above program to produce setcookies.cgi, and try to set cookies using following link. It will set four cookies at your computer −

link:../cgi-bin/setcookies.cgi[/cgi-bin/setcookies.cgi]


=== Retrieving Cookies

It is easy to retrieve all the set cookies. Cookies are stored in CGI environment variable HTTP_COOKIE and they will have following form.

[source]

Here is an example of how to retrieve cookies.

[source]

Now, compile above program to produce getcookies.cgi, and try to get a list of all the cookies available at your computer −

link:../cgi-bin/getcookies.cgi[/cgi-bin/getcookies.cgi]

This will produce a list of all the four cookies set in previous section and all other cookies set in your computer −

[source]

=== File Upload Example

To upload a file the HTML form must have the enctype attribute set to *multipart/form-data*. The input tag with the file type will create a "Browse" button.

[source]

The result of this code is the following form −

*Note* − Above example has been disabled intentionally to stop people uploading files on our server. But you can try above code with your server.

Here is the script *cpp_uploadfile.cpp* to handle file upload −

[source]

The above example is for writing content at *cout* stream but you can open your file stream and save the content of uploaded file in a file at desired location.

Hope you have enjoyed this tutorial. If yes, please send us your feedback.