Const keyword in C++

This section will discuss the const keyword in the C++ programming language. It is the const keywords used to define the constant value that cannot change during program execution. It means once we declare a variable as the constant in a program, the variable’s value will be fixed and never be changed. If we try to change the value of the const type variable, it shows an error message in the program.

Use const keywords with different parameters:

Use const variable
Use const with pointers
Use const pointer with variables
Use const with function arguments
Use const with class member functions
Use const with class data members
Use const with class objects

1. Const variable

It is a const variable used to define the variable values that never be changed during the execution of a program. And if we try to modify the value, it throws an error.

Syntax

Example: Program to use the const keyword in C++

Let’s create a program to demonstrate the use of the const keyword in the C++ programming language.

  #include   #include   using namespace std;  int main ()  {  // declare the value of the const  const int num = 25;  num = num + 10;  return 0;  }  

Output

It shows the compile-time error because we update the assigned value of the num 25 by 10.

Example: Let’s create a simple program to demonstrate the use of the const keyword in the C++ programming language.

  /* create a program to use the const keyword with different data types in the C++. */  #include   #include   using namespace std;  int main ()  {  // declare variables  const int x = 20;  const int y = 25;  int z;  // add the value of x and y  z = x + y;  cout << ” The sum of the x and y is: ” << z << endl;  return 0;  }  

Output

 The sum of the x and y is: 42

In the above program, we declared and assigned the const variables x and y at initial. And then, store the result of two const variables to the ‘z’ variable to print the sum.

Note: While the declaration of the const variable in the C++ programming, we need to assign the value of the defined variables at the same time; else, it shows the compile-time error.

2. Constant pointer

To create a const pointer, we need to use the const keyword before the pointer’s name. We cannot change the address of the const pointer after its initialization, which means the pointer will always point to the same address once the pointer is initialized as the const pointer.

Example: Program to demonstrate the constant pointer using the const keyword

Let’s consider an example to use the const keyword with the constant pointer in the C++ programming language.

  #include   using namespace std;  int main ()  {  // declaration of the integer variables  int x = 10, y = 20;    // use const keyword to make constant pointer  int* const ptr = &x //  const integer ptr variable point address to the variable x    // ptr = &y // now ptr cannot changed their address  *ptr = 15; // ptr can only change the value  cout << ” The value of x: ” << x << endl;  cout << ” The value of ptr: ” << *ptr << endl;  return 0;  }   

Output

The value of x: 15  The value of ptr: 15

In the above program, pointer ptr pointing to the address of int variable ‘x’ and ptr variable cannot be changed their address once it initialized, but the pointer ptr can change the value of x.

3. Pointer to constant variable

It means the pointer points to the value of a const variable that cannot change.

Declaration of the pointer to the constant variable:

Here, x is a pointer that point to a const integer type variable, and we can also declare a pointer to the constant variable as,

In this case, y is a pointer to point a char type const variable.

Example: Program to use the const keyword with a pointer to constant variable

  #include   using namespace std;  int main ()  {  // declare integer variable  int x = 7, y = 10;    const int *ptr = &x // here x become constant variable  cout << ” n The initial value of ptr:” << *ptr;  cout << ” n The value of x: ” <;>

Output

The initial value of ptr: 7  The value of x: 7  The value of y: 10  The value of ptr: 10

In the above program, pointer ptr points to the const int (x) variable, and the value of the int (x) variable can never change.

4. Constant function Arguments

We can declare the function arguments as the constant argument using the const keyword. And if the value of function arguments is declared const, it does not allow changing its value.

Syntax

In the above syntax, return_type is represented whether the function will return a value or not. The fun_name() function contains a const argument whose value can never be changed once it defines in the program.

Example: Let’s consider an example to use the const keyword with function arguments in the C++ programming language.

  #include   using namespace std;  // create an integer Test() function contains an argument num  int Test (const int num)  {  // if we change the value of the const argument, it thwrows an error.  // num = num + 10;  cout << ” The value of num: ” << num << endl;  return 0;  }  int main ()  {  // call function  Test(5);  }  

Output

The value of num: 5

In the above program, the num is a constant argument, and we cannot update the num value. If we update the value of the num variable, it returns the compile-time error.

5. Const Member function of class

A const is a constant member function of a class that never changes any class data members, and it also does not call any non-const function.
It is also known as the read-only function.
We can create a constant member function of a class by adding the const keyword after the name of the member function.

Syntax

In the above syntax, mem_fun() is a member function of a class, and the const keyword is used after the name of the member function to make it constant.

Example: Program to use the const keyword with the member function of class

Let’s consider an example to define the const keyword with the member function of the class.

  class ABC  {  // define the access specifier  public:    // declare int variables  int A;  // declare member function as constant using const keyword  void fun () const  {   A = 0; // it shows compile time error  }  };    int main ()  {  ABC obj;  obj.fun();  return 0;  }  

Output

The above code throws a compilation error because the fun() function is a const member function of class ABC, and we are trying to assign a value to its data member ‘x’ that returns an error.

6. Constant Data Members of class

Data members are like the variable that is declared inside a class, but once the data member is initialized, it never changes, not even in the constructor or destructor. The constant data member is initialized using the const keyword before the data type inside the class. The const data members cannot be assigned the values during its declaration; however, they can assign the constructor values.

Example: Program to use the const keyword with the Data members of the class

  /* create a program to demonstrate the data member using the const keyword in C++. */  #include   using namespace std;  // create a class ABC  class ABC  {    public:   // use const keyword to declare const data member  const int A;  // create class constructor  ABC ( int y) : A(y)  {  cout << ” The value of y: ” << y << endl;  }  };  int main ()  {  ABC obj( 10); // here ‘obj’ is the object of class ABC  cout << ” The value of constant data member ‘A’ is: ” << obj.A << endl;  // obj.A = 5; // it shows an error.  // cout << obj.A << endl;  return 0;  }  

Output

The value of y: 10  The value of constant data member 'A' is: 10

In the above program, the object obj of the ABC class invokes the ABC constructor to print the value of y, and then it prints the value of the const data member ‘A’ is 10. But when the ‘obj.A’ assigns a new value of ‘A’ data member, it shows a compile-time error because A’s value is constant and cannot be changed.

7. Constant Objects

When we create an object using the const keyword, the value of data members can never change till the life of the object in a program. The const objects are also known as the read-only objects.

Syntax

The const is a keyword used before the class’ object’s name in the above syntax to make it a constant object.

Example: Let’s create a program to use the constant objects in the C++ programming language.

  #include   using namespace std;  class ABC  {  public:  // define data member  int A;  // create constructor of the class ABC  ABC ()  {  A = 10; // define a value to A  }  };  int main ()  {  // declare a constant object  const ABC obj;  cout << ” The value of A: ” << obj.A << endl;  // obj.A = 20; // It returns a compile time error  return 0;  }  

Output

The value of A: 10

In the above program, we assign the value of A is 10, the compiler prints “The value of A: 10”, and when we assign the value of A to 20, the object of the class returns the compile time error in the program.

Program to find the area of a circle using the const keyword

Let’s consider an example to print the area of a circle using the const keyword in the C++ programming language.

  #include   using namespace std;  int main ()  {  // declare variables  int r;  const double PI = 3.14;  double result;  cout << ” Input the radius of a circle: ” << endl;  cin >> r;  result = PI * r * r;  cout << ” The area of a circle is: ” << result;  }  

Output

Input the radius of a circle:   5  The area of a circle is: 78.5

Next TopicMemset in C++

Const keyword in C++

Const keyword in C++

1. Const variable

Note: While the declaration of the const variable in the C++ programming, we need to assign the value of the defined variables at the same time; else, it shows the compile-time error.

2. Constant pointer

3. Pointer to constant variable

4. Constant function Arguments

5. Const Member function of class

6. Constant Data Members of class

7. Constant Objects

Program to find the area of a circle using the const keyword

State variable analysis: Examples with explanation

C++ Deque

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