Question

I occasionally see bits of code similar to this:

class A {
    int b;
}

void foo() {
    int* blah = &A::b;
    // Other stuff.
}

But how could grabbing the address of a class’ member variable without an instance of the class be useful? What does it do?

Answer 1

Your code is ill-formed and will not compile. The type of blah is not int*, it is int (A::*). That is, it is not a “pointer to an int,” it is a “pointer to a data member of class type A whose type is int.”

Note that in order to obtain a pointer to a member, the member must be accessible. To compute &A::b in foo(), b would need to be a public data member, or foo() would need to be a friend of A.

A pointer-to-member does not point to an object. Rather, a pointer-to-member can be bound to an object to get the value of its data member. You can obtain the value of the b data member of an A object by binding the blah pointer-to-member to the instance of A. For example,

A x;
int b_value = x.*blah; // b_value has the value of x.b