Changing objects in 'const member functions' via Mutable

We saw last time an example of 'Const Member Functions' (also known as 'Inspectors'). In contrast the member functions without the const suffix are known as 'non-const member functions' or 'mutators'.

Sometimes it may be necessary to modify a data member (variable or object in your class) in a const member function. There are two approaches to do that. One is to use the const_cast and the other to use 'mutable'. I use both the approaches in the example below:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//This example shows how to change objects in const member functions
#include<iostream>

using namespace std;

class ABC
{
public:
  int func1(int a, int b);
  int func2(int a, int b) const;
private:
  int x;
  mutable int y;
};

int ABC::func1(int a, int b)
{
  x = a, y = b;
  cout<<"x = "<<x<<" and y = "<<y<<endl;
  return 0;
}

int ABC::func2(int a, int b) const
{
  //x = a; - COMPILE ERROR because its const
  int *temp = const_cast<int*>(&x); //Removing the const
  *temp = a; //OK now
  y = b; //OK because its defined as mutable
  cout<<"x = "<<x<<" and y = "<<y<<endl;
  return 0;
}

int main()
{
  ABC abc;
  abc.func1(3, 7);
  abc.func2(20, 40);
  return 0;
}

The output is as follows: