C++ Union, Struct, Member type

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

It just means you can access the same memory as either the long, or the struct.

To access hr:

Odp o1;
o1.hr;

Interesting link: http://www.cplusplus.com/forum/general/18816/

Answer 2

Right, with a union the same memory locations will be used to represent a single one of the members at any given time. So if you have an instance of the union and set the value of hr, you will get garbage if you then try to read the value of f.

Try using the following to access hr:

union a;
a.hr = NULL;

Answer 3

Every time you set (write to) a member of a union, you essentially make it "active". You are only allowed to read the currently active member of the union. It means that it is your responsibility to remember somehow which member is active at each moment in time.

Attempting to access the inactive member of a union leads to undefined behavior.

Keep in mind also that your code is not valid C++. There's no such thing as "anonymous struct" in C++. Your struct member has to have a name. If your compiler accepts it, it is just a non-standard extension supported by your specific compiler.

Answer 4

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.