Question

I was reading the documentation of RegisterClassEx function and found that the return type of the function is ATOM.

ATOM RegisterClassEx(CONST WNDCLASSEX *lpwcx);

While creating a window using CreateWindowEx we can either pass the classname or ATOM Class in lpClassName.

So is there any significance of passing ATOM over classname ? or is it just an alternate way?

Thanks.

Answer 1

ATOM is an alias to a string, like an index to a system array of strings. In the context of window classes, it’s an alias to the class name.

So, it’s just a different way of doing the same thing, but in theory this is more efficient.

Where I think ATOMs make more sense to use is in GetProp / SetProp, where you may be looking up that string for every windows message processed. Maybe you can gain some performance by using an ATOM; anyway it’s so easy to use that you might as well.

Interestingly, ATOMs tables are system-wide as well, so if two processes register a window class of the same name, they will receive the same ATOM value, even though the class info will be different.