tree(3): allow the compare function to return any signed type

[glebius]

Summary

This allows to write very short comparison function when we are
comparing just pointer values:

return ((intptr_t)((uintptr_t)a->ptr - (uintptr_t)b->ptr));

[dougm]

How portable is __typeof? Is it in a standard now, or sometime soon?

[glebius]

In D35722#810520, @dougm wrote:

How portable is __typeof? Is it in a standard now, or sometime soon?

AFAIK, it is not part of standard, yet. The point is that tree.h already requires it for RB_PARENT :)

[dougm]

Point taken on __typeof.

In your example of use

return ((intptr_t)((uintptr_t)a->ptr - (uintptr_t)b->ptr));

can overflow, but (assuming that ptr points to something bigger than char)

return ((intptr_t)((uintptr_t)a->ptr/2 - (uintptr_t)b->ptr/2));

does not. Otherwise, it's okay by me.

[glebius]

Good point. Maybe /2 of the result of the difference.

[alc]

In D35722#810749, @glebius wrote:

Good point. Maybe /2 of the result of the difference.

No, it needs to be done before the subtraction, as Doug wrote it.

[glebius]

Yep, now I also see it. Thanks!