c++ mutable

#include <iostream>
using std::cout;

class Test {
public:
	int x;

	// defining mutable variable y
	// now this can be modified
	mutable int y;

	Test()
	{
		x = 4;
		y = 10;
	}
};

int main()
{
	// t1 is set to constant
	const Test t1;

	// trying to change the value
	t1.y = 20;
	cout << t1.y;

	// Uncommenting below lines
	// will throw error
	// t1.x = 8;
	// cout << t1.x;
	system("PAUSE");
	return 0;
}

　20