Mutable Keyword in C++

This C++ program illustrates the concept of mutable keyword in classes. The keyword mutable enables a const member function to change a variable. This explains the concept of logical const-ness of the object where the object changes in a way that it is not apparent through a public interface.

Here is the source code of the C++ program illustrates the concept of mutable keyword in classes. The C++ program is successfully compiled and run on a Linux system. The program output is also shown below.

```
/*
```

 * C++ Program to Implement a Class with Mutable Members

```
 */
```
```
#include <iostream>
```
```
 
```
```
class A {
```
```
    private:
```
```
        int i;
```
```
        mutable int j;
```
```
    public:
```

        A(int ii = 0, int jj = 1) : i(ii), j(jj) { }

```
        void change() const;
```
```
        void print();
```
```
};
```
```
 
```
```
void A::change() const
```
```
{
```
```
    // Mutable variable 
```
```
    j++;
```
```
}
```
```
 
```
```
void A::print()
```
```
{
```

    std::cout << "A::i = " << i << std::endl

              << "A::j = " << j << std::endl;

```
}
```
```
 
```
```
int main()
```
```
{
```
```
    A a;
```
```
 
```

    std::cout << "Before A::change()" << std::endl;

```
    a.print();
```
```
    a.change();
```

    std::cout << "After  A::change()" << std::endl;

```
    a.print();
```
```
}
```

$ a.out
Before A::change()
A::i = 0
A::j = 1
After  A::change()
A::i = 0
A::j = 2

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