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Mutable data member in C++ programming language

Learn: What is Mutable Data member in C++, how to define it? Class example with mutable data member in C++ programming language

What is Mutable data member in C++?

Mutable data member is that member which can always be changed; even if the object is const type. It is just opposite to “const”. Sometime we required to use only one or two data member as a variable and other as a constant. In that situation, mutable is very helpful to manage classes.

To make data member as a mutable, we need to use mutable keyword. It is in-built in C++ language.

Example of Mutable data member in C++

#include <iostream>
using namespace std;

class Sample {
    int x;
    mutable int y;

public:
    Sample(int a = 0, int b = 0)
    {
        x = a;
        y = b;
    }

    //function to set value of x
    void setx(int a = 0)
    {
        x = a;
    }

    //function to set value of y
    //value of y being changed, even if member function is constant.
    void sety(int b = 0) const
    {
        y = b;
    }

    //function to display x and y.
    //this has to be const type, if member function is constant type.
    void display() const
    {
        cout << endl
             << "x: " << x << " y: " << y << endl;
    }
};

int main()
{
    const Sample s(10, 20); // A const object

    cout << endl
         << "Value before change: ";
    s.display();

    s.setx(100); //Error: x can not be changed, because object is constant.
    s.sety(200); //y still can be changed, because y is mutable.

    cout << endl
         << "Value after change: ";
    s.display();

    return 0;
}

Explanation

In this program, statement s.setx(100); will produce an error but statement s.sety(200); will not.

Thus, it is cleared that we can change only the value of y (because of mutable) and we cannot change the value of x.