[colonwqbang]

I think it can be both things.

Haven't you ever seen someone do

    struct Thing;
    struct OtherThing;

in lieu of just including "thing.h"? I see it frequently in real life code bases and I can't see a reason for it other than compilation time optimisation.

[gumby]

Sure, I do that all the time too. But you can't call a method (or look inside Thing, or pass it as an argument, only a pointer to it) without including the definition.

Hmm, there might be some interesting linker hacks to patch things up post compilation. But then you'd want some way to do the forward declaration for cases where Thing could have been passed in registers...

[DerekL]

This is sometimes required to break dependency cycles. Also, you can use this to rearrange declarations in the same file.