Unions

[Amon Ra]

I am currently working on the cplusplus.com tutorials. I am at the point where they talk about typedef's and unions...
What are unions? I dont get it...could anyone explain to me?
Thanks in advance.. =)

[HarryW]

Unions are sets of variables that all occupy the same space in memory, so by changing one variable you change the others.

For instance, you could have a union representing a 32-bit colour value in RGBA format like this:

union RGBA
{   long value32;
     struct tagColourBytes
     {   unsigned char red;
          unsigned char green;
          unsigned char blue;
          unsigned char alpha
     } ColourBytes;
}

I'm not certain that would map directly onto an RGBA value (the byte values might be in reverse order) but that's the idea. You could then declare a variable to use the union like this:

RGBA myRGBAvar;
// older C-style declaration would be this:
// union RGBA myRGBAvar;

You can then assign a 32-bit colour value to myRGBAvar.value32, and the values of myRGBAvar.ColourBytes.red, .green, .blue and .alpha would change because you have written to the same memory they occupy.

[kedaman]

Yep, it's the other way round Harry R being least significant byte is last, they're stored ABGR