Const Keyword in C++

The const programming language keyword will be covered in this section. The constant value that cannot change during program execution is defined using the const keywords. It implies that once a variable is designated as a constant in a program, its value cannot be altered.

• A function can be invoked on any kind of object, including const and non-const objects, when it is defined as const.
• An object must be initialized at the moment of declaration whenever it is specified as const. However, only constructors make it feasible to initialize an object when declaring.

Const keywords with various parameters:

• Using Const variable
• Using pointers, use const
• Using variables, use a const pointer
• When using function arguments use const.
• Const with member functions of classes
• Using class data members, use const
• Using class objects, use const.

Constant Variable

It is a const variable that is used to define variables with constant values during program execution. Additionally, it throws an error if we attempt to change the value.

const data_type variable_name ;   //syntax

Example1: Let's write a program to show how to utilize the C++ programming language's const keyword.

#include < iostream >  
#include < bits/stdc++.h >
#include < stdio >
#include < stdlib >
#include < conio.h >  
using namespace std ;  
int main ( )  
{  
// declare the value of the const  
const int num = 25 ;  
num = num + 10 ;  
return 0 ;  
}  

OUTPUT:

Because we increase the allocated value of the num 25 by 10, the compile-time error is visible.

Example2: Let's write a straightforward program to show how to utilize the C++ programming language's const keyword.

/* create a program to use the const keyword with different data types in the C++. */  
#include < iostream >  
#include < conio.h >
#include <bits/stdc++.h>
#include <stdio>
#Include <stdlib>  
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
.............................
Process executed in 2.11 seconds
Press any key to continue.

Explanation

The const variables x and y were defined and initialized in the program above. Then, display the total by storing the output of two const variables in the 'z' variable.

NOTE: In order to avoid a compile-time error, we must assign the values of the declared variables simultaneously with the definition of the const variable in C++ programming.

Constant Pointer

We must use the const keyword before the pointer's name in order to build a const pointer. The const pointer cannot have its address changed after it is initialised, therefore once it is initialized as a const pointer, it will always point to the same location.

Example3: A program that uses the keyword const to illustrate the constant pointer

Let's look at an example of how to utilize the C++ programming language's const keyword with a constant pointer.

#include < iostream >  
#include < bits/stdc++.h >
#include < stdlib >
#include < stdio >
#include < conio.h >
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
..............................
Process executed in 1.22 seconds
Press any key to continue.

Explanation

The pointer ptr in the program above can modify the value of x but cannot change the address of the int variable 'x' or the ptr variable once they have been created.

Pointer to constant variable

It indicates that the pointer corresponds to a constant variable whose value cannot change.

const int * x ;   //declaration syntax

Here, the variable x is a pointer to a const integer type variable. A pointer to the constant variable can alternatively be declared as

char const * y ; //declaration syntax

In this instance, y is a reference to a variable of type char that is const.

Example4: A program that points to a constant variable and uses the const keyword

#include < iostream >
#include < bits/stdc++.h >
#include < stdio >
#include < stdlib >  
#include < conio.h >
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: " << x ;  
  
// * ptr = 15 ; It is invalid; we cannot directly assign a value to the ptr variable  
ptr = & y ; //  here ptr variable pointing to the non const address 'y'  
  
cout << " \n The value of y: " << y ;  
cout << " \n The value of ptr:" << * ptr ;   
return 0 ;  
}  

OUTPUT:

The initial value of ptr: 7
The value of x: 7
The value of y: 10
The value of ptr: 10
...........................
Process execute in 2.22 seconds
Press any key to continue.

Explanation

In the program above, the const int ( x ) variable is pointed to by pointer ptr, and its value is immutable.

Arguments for constant functions

Using the term const, we may specify the function parameters as constant arguments. Additionally, if a function argument's value is marked const, it is not possible to change it.

return_type fun_name ( const int x )  //syntax
{  
}  

Return type in the aforementioned syntax indicates whether or not the function will return a value. Fun name ( ) has a const parameter, which means that once it is defined in the program, its value cannot be altered.

Example5: Let's look at an example of how to utilize the C++ programming language's const keyword with function parameters.

#include < iostream >
#include < bits/stdc++.h >
#include < stdio >
#include < stdlib >  
#include < conio.h >
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
...................
Process executed in 1.11 seconds
Press any key to continue.

Explanation

The num parameter in the program above is a constant, thus we are unable to change its value. The compile-time error is returned if the value of the num variable is modified.

Another Example:

// C++ program to demonstrate the
// constant function
#include < iostream >
#include < bits/stdc++.h >
#include < stdio >
#include < stdlib >  
#include < conio.h >
using namespace std ;  
// Class Test
class Test {
	int value ; 
public :
	// Constructor
	Test ( int v = 0 )
	{
		value = v ;
	}
	// We get compiler error if we
	// add a line like "value = 100;"
	// in this function.
	int getValue () const
	{
		return value;
	}
	// a nonconst function trying to modify value
	void setValue (int val) {
		value = val;
	}
 }; 
// Driver Code
int main ()
{
	// Object of the class T
	Test t (20); 
	// non-const object invoking const function, no error
	cout << t.getValue() << endl ; 
	// const object
	const Test t_const(10); 
	// const object invoking const function, no error
	cout << t_const.getValue() << endl; 
	// const object invoking non-const function, CTe
	// t_const . setValue(15); 
	// non-const object invoking non-const function, no error
	t . setValue(12); 
	cout << t . getValue ( ) << endl ; 
	return 0 ;
}

OUTPUT:

20

Explanation

In the above program if we try calling the non constant function from a constant object then it will give us an error stating that “passing const test as this argument discard qualifier [ -fpermissive ]”

Const member of class function

• A constant member function of a class is called a const if it never modifies any class data members or calls any non-const functions.
• Additionally called the read-only function.
• By using the const keyword after the member function's name, we may turn a class member function into a constant one.

return_type mem_fun ( ) //syntax
{
}

mem_fun ( ), which is a member function of a class in the syntax above, is made constant by the use of the const keyword after its name.

Example6: A program to utilize the class member function and the const keyword

Let's look at an illustration of how to define the const keyword using a class member function.

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:

Because we are attempting to give a value to the fun ( ) function's data member 'x', which returns an error, which is a const member function of class ABC, the aforementioned code generates a compilation error.

Constant Data Class Member

Data members are similar to variables defined inside of classes, but once initialized, they remain immutable not even in constructors or destructors. The const keyword is used before the class's data type to initialize the constant data member. Const data members cannot have values assigned to them at declaration, however function Object () { [native code] } values may be assigned.

Example7: A program that uses the const keyword with the class's Data members

/* create a program to demonstrate the data member using the const keyword in C++. */   
#include < iostream >
#include < bits/stdc++.h >
#include < stdio >
#include < stdlib >  
#include < conio.h >
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
.........................................................................
Process executed in 1.22 seconds
Press any key to continue.

Explanation

In the program above, the object of the ABC class, named obj, calls the ABC function Object ( ) { [ native code ] } to display the value of y. Next, it publishes the value of the const data member 'A,' which is 10 in the program. However, because A's value is fixed and cannot be modified, a compile-time error is displayed when "obj . A" assigns a new value to the "A" data member.

Constant Objects

The value of data members can never change while an object is being created using the const keyword in a program. The read-only objects are also referred to as const objects.

const class_name obj_name ; //syntax

In the syntax above, the term const is used before the name of the class object to designate that it is a constant object.

Example8: Let's write a program in the C++ programming language that utilises constant objects.

#include < iostream >
#include < bits/stdc++.h >
#include < stdio >
#include < stdlib >  
#include < conio.h >
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
..............................
Process executed in 2.22 seconds
Press any key to continue.

Explanation

In the program above, if we set the value of A to 10, the compiler outputs "The value of A: 10," and if we set the value of A to 20, the class object produces a compile-time error.

A Program that uses the const keyword to calculate the area of a circle

Example9: Let's look at a C++ programming example that uses the const keyword to print a circle's area.

#include < iostream >
#include < bits/stdc++.h >
#include < stdio >
#include < stdlib >  
#include < conio.h >
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
…………………………………………
Process executed in 2.22 seconds
Press any key to continue.

Explanation

In the above example of a program in C++, we have shown how to calculate the area of a circle using constant variable.