Question

If I have a class:

class Odp
{
    int i;
    int b;
    union
    {
         long f;
         struct
         {
               WCHAR* pwszFoo;
               HRESULT hr;
         };
    };

}

Union means that, of all values listed, it can only take on one of those values at a time? How does that work in terms of accessing these variables? How would I access hr directly? If I set hr, what happens if I try to access f?

Answer 1

This is a very fraught area in the C++ standard – basically a union instance, per the standard can only be treated at any one time as if it contained one “active” member – the last one written to it. So:

union U {
   int a;
   char c;
};

then:

U u;
u.a = 1;
int n = u.a;
u.c = 2;
char c = u.c;

is OK, but:

U u;
u.a = 1;
char c = u.c;

is not. However, there are vast volumes of existing code that say that both are OK. and in neither, or any, case will an exception be thrown for an “invalid” access. The C++ language uses exceptions exceptionally (!) sparingly.

Basically, if you find yourself using unions in your C++ code to deal with anything but C libraries, something is wrong.