How can const make a function overloads?

Question

I wrote this code in C++:

class Foo
{
public:
    int& fun(){return var;}       // 1st fun
    int fun() const {return var;}  // 2rd fun
 private:
    int var;
};
int main()
{
    Foo foo;
    int i = foo.fun();
    return 0;
}

I know that C++ cannot discriminate overloading function by return value,but why when I added a const to 2rd function ,overloading can work ? What the 'const' have done ?

Answer 1

Compiler cannot discriminate by return type because return values can undergo conversion before the assignment is performed. The object on which the function is invoked, on the other hand, is a parameter (albeit an implicit one) to the function, so the compiler can discriminate on it.