C++ difference between struct and union

What’s the difference between a STRUCT and a UNION in C++?

Here’s an example!

A C++ struct will make sure to allocate space for each and every member in the struct.

For instance, if we had

struct VertexStruct
{
    float x,y,z;
};

When you create one, using a line of code like:

VertexStruct vertexstruct;

This is what VertexStruct object will look like in memory.

     vertexstruct
 ___________________
 |     |     |     |
 |     |     |     |
 |_____|_____|_____|
    x     y     z

So if we do

vertexstruct.x = 50;
vertexstruct.y = 10;
vertexstruct.z = 2;

We get this picture in memory:

     vertexstruct
 ___________________
 |     |     |     |
 | 50  | 10  |  2  |
 |_____|_____|_____|
    x     y     z

This is natural and completely what you’d expect.

Contrast STRUCT vs UNION

Now, CONTRAST THAT with a UNION. A UNION uses the SAME EXACT PIECE OF MEMORY for __ALL__ of the members inside of it.

This is kind of counter-intuitive and weird.

An example:

union VertexUnion
{
    float x,y,z;
};

When you create one, using a line of code like:

VertexUnion vertexunion;

You get this picture in memory:

 vertexunion
 _______
 |     |
 |     |
 |_____|
    x    
    y
    z

The above union effectively provides 3 ways by which to access that same single piece of memory (x, y and z).

So if you wrote code like:

vertexunion.x = 10;
vertexunion.y = 2;
vertexunion.z = 5;

You get this picture in memory:

 vertexunion
 _______
 |     |
 |  5  |
 |_____|
    x    
    y
    z

(Last x, y and z all refer to the same piece of memory. Since the last value assigned there is 5, ALL OF x, y and z have the exact same value).

WEIRD!

* Note that the sizeof( any_union ) ends up being the sizeof( largest_member_in_union ).

Download the Visual Studio 2005 project files quick demo.

Demo is hosted by esnips! (they’re awesome!)