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Const Member Functions in C++

C++ | Const Member Functions: Here, we are going to learn how to declare constant (const) member functions in C++?
Submitted by IncludeHelp, on September 20, 2018 [Last updated : March 01, 2023]

C++ Const Member Functions

A constant (const) member function can be declared by using const keyword, it is used when we want a function that should not be used to change the value of the data members i.e. any type of modification is not allowed with the constant member function.

Example:

Here, we have a class named "Numbers" with two private data members a and b and 4 member functions two are used as setter functions to set the value of a and b and 2 are constant members functions which are using as getter functions to get the value of a and b.

C++ program to implement the const member functions

#include <iostream>
using namespace std;

class Numbers {
private:
    int a;
    int b;

public:
    Numbers()
    {
        a = 0;
        b = 0;
    }

    //setter functions to set a and b
    void set_a(int num1)
    {
        a = num1;
    }
    void set_b(int num2)
    {
        b = num2;
    }

    //getter functions to get value of a and b
    int get_a(void) const
    {
        return a;
    }
    int get_b(void) const
    {
        return b;
    }
};

//Main function
int main()
{
    //declaring object to the class
    Numbers Num;
    //set values
    Num.set_a(100);
    Num.set_b(100);

    //printing values
    cout << "Value of a: " << Num.get_a() << endl;
    cout << "Value of b: " << Num.get_b() << endl;

    return 0;
}

Output

Value of a: 100
Value of b: 200