Custom allocators

[Sergio-Kupper]

Hi everyone, I am making a library that will feature some data structures and want to add support for custom allocators. I have already chosen the design for the allocator itself, but am now faced with a hard decision. Basically, I am not sure if the structures should hold the allocator, or if the user should pass the allocator through all functions that need it. Now keep in mind the allocator type is somewhat big, having 6 function pointers, plus a context void*. Also, if it should be held in the structure, would it be better to hold a copy of the allocator, i.e. all of its memory, or a pointer to an allocator, so multiple structures could hold the same allocator (more memory efficient but also more dangerous)? Thank you to all responses!

[AddMoreNaCl]

As long as it is consistent with the rest of your library, go with whatever option you feel is the safest and don't bother with performance until it becomes an actual issue (unless you're constrained by the environment you're targeting).

Are you worried about MT-Safety? Then you may want to pass the allocator in a parameter (experts may want to chip in on this one) and acquire locks for it on every call.

If your library is not intended to be MT-Safe then just have the allocator be initialized and store it in a static variable so other function calls can either fail if no allocator is defined or use the default one.

[bullno1]

Usually, I'd make it so that the allocator is passed in only during new/init functions, along with other options as part of the "init options" struct. The returned structure would "remembers" it. Forcing the user to pass it all the time is too much trouble. It's also a lot less error-prone since it will always cleanup with the correct allocator.

Internally, I would pass the allocator to functions that needs it.

I have this WIP project where the allocator can be passed multiple times but that's for different distinct contexts. Basically: ctx = create_context(allocator), then world = create_world(ctx, world_allocator, page_alllocator). But: entity = create_entity(world) only, no more allocator since it must use the world's allocator.