Question

I was seeing some code and definition some where, like

 class A {
       public:
              int a,b;
 };

 main() {
        A a;
        std::cout<<"Test output "<<&A::a<<" "<<&A::b<<std::endl;
 }

 Output
       1 1

What I dont understand is a and b are not static members of A, but when they are accessed like a static member it gives an error, but when accessing the address of it like a static member prints 1. Is there a special meaning behind it, coz I have no idea why it is needed and why it works this way. Thanks.

Answer 1

The expression &A::a when a is a non-static member returns a pointer-to-member-object (or pointer-to-member-function if a were a function). One can use them like this:

A a;
a.a = 1;
a.b = 2;

int (A::*) some_int_member_of_a = &A::a;
std::cout << a.*some_int_member_of_a; // prints 1

a.*some_int_member_of_a = 5;
std::cout << a.*some_int_member_of_a; // now prints 5

The stream output operation is not defined for pointer-to-member values, but there is a conversion from them to bool which is what’s getting printed in your output.