What means scope resolution of structure in a variable declaration?

Question

I found this code in MySQL source in file set_var.h and I'm not sure what does ulong SV::*offset mean.

In short it looks like:

struct SV {...}

class A {

    ulong SV::*offset;

    A(ulong SV::*offset_arg): offset(offset_arg) {...}
};

class B {

    DATE_TIME_FORMAT *SV::*offset;

    B(DATE_TIME_FORMAT *SV::*offset_arg) : offset(offset_arg) {...}
}

and so on.

Answer 1

ulong SV::*offset; is a member of the class A named offset which points to a member of the class SV of type ulong. It's used like this :

#include <iostream>

using ulong = unsigned long;
struct SV {
    ulong x, y, z;
};

int main()
{
    // A pointer to a ulong member of SV
    ulong SV::*foo;

    // Assign that pointer to y
    foo = &SV::y;

    // Make an instance of SV to test
    SV bar;
    bar.x = 10;
    bar.y = 20;
    bar.z = 30;

    // Dereference with an instance of SV
    // Returns "20" in this case
    std::cout << bar.*foo;

    return 0;
}