c++ Declaration Rect structure with union

Question

I want to declare Rect structure:

struct{
   float x,y;
   float width,height;
}Rect;

And union variables x,y to 'pos' and width,height to 'size' Vector2f structure:

struct{
   float x,y;
}Vector2f;

How can i do it with union?

Rect rect; 
//rec.x; rec.y; rect.pos; rect.pos.x; rect.pos.y; 
//rect.width; rect.height; rect.size; rect.size.x; rect.size.y;

Don't think Union does what *you* think it does : http://en.cppreference.com/w/cpp/language/union — Caribou, Nov 18 '12 at 11:24
Related: http://stackoverflow.com/q/4527955/399317 (also shows an alternative solution with accessor functions rather than a union) — Kos, Nov 18 '12 at 11:41

score 2 · Answer 1 · answered Nov 18 '12 at 11:25

You’ve got the syntax wrong: the name of the struct comes before the body, not after it:

struct Rect {
   float x, y;
   float width, height;
};

There, now you’re good to go.

But note that “union” means something completely different in C++. A union is a data structure which, like a struct, groups objects. But while every instance of a struct can hold multiple values simultaneously, an instance of a union can only hold a single value at a time. They have their uses, but those are pretty rare and there are usually better (and type safe) ways of accomplishing the same.

score 2 · Accepted Answer · answered Nov 18 '12 at 11:32

You're looking for anonymous unions. The syntax is:

struct Rect {

   union {
       Vector2f pos;
       struct {
           float x,y;
       };
   };
   union {
       Vector2f size;
       struct {
           float width, height;
       };
   };

};

Demo: http://ideone.com/JgqABu

(I don't recommend doing that though; I'd just KISS and use the vectors.)

c++ Declaration Rect structure with union

2 Answers2