#include "stdio.h"
class a
{
public:
void f(){printf("non const version\n");};
void f()const{printf("const version\n");};
};

void main()
{
a instanceofa;
const a constantinstanceofa;
instanceofa.f();
constantinstanceofa.f();
}

in fact, besides meaning it should change nothing of the instance, it also stands for a selector.