C++ operator ->* and .*

Question

Good day,

I've come across this question, but I'm specifically interested in the "object pointed to by member ..." type operators as listed here on Wikipedia.

I have never seen this in the context of actual code, so the concept appears somewhat esoteric to me.

My intuition says they should be used as follows:

struct A
{
    int *p;
};

int main()
{
    {
        A *a = new A();
        a->p = new int(0);
        // if this did compile, how would it be different from *a->p=5; ??
        a->*p = 5;
    }

    {
        A a;
        a.p = new int(0);
        // if this did compile, how would it be different from *a.p=5; ??
        a.*p = 5;
    }

    return 0;
}

But this doesn't compile because p is undeclared. (See example)

Could anyone please provide a real-world example of the use of operator->* and/or .* in C++?

Answer 1

Those operators are used for pointer-to-member objects. You won't come across them very often. They can be used to specify what function or member data to use for a given algorithm operating on A objects, for instance.

Basic syntax:

#include <iostream>

struct A
{
    int i;
    int geti() {return i;}
    A():i{3}{}
};

int main()
{
    {
        A a;
        int A::*ai_ptr = &A::i; //pointer to member data
        std::cout << a.*ai_ptr; //access through p-t-m
    }

    {
        A* a = new A{};
        int (A::*ai_func)() = &A::geti; //pointer to member function
        std::cout << (a->*ai_func)(); //access through p-t-m-f
    }

    return 0;
}