confused with an operator

Question

I confused when I try to understand the code below. Can anyone explain this hack:

a.*b

Or if a is a pointer to a class:

a->*b

Answer 1

Both of those operators are used to dereference a pointer-to-member. Unlike regular pointers, pointer-to-members cannot be dereferenced by themselves, but must be applied to an actual object of the type. Those binary operators pick the object (or pointer) in the left hand side and apply the pointer to member to it.

struct test {
    int a, b, c;
};
int main() {
   int test::*ptr;
   ptr = &test::a;
   test t;
   t.*ptr = 5;         // set t.a to 5
   ptr = &test::b;
   test *p = &t;
   p->*ptr = 10;       // set t.b to 10 through a pointer
}