function2 VS sparsepp

Fixed size function wrappers are not only limited and also a waste of space. Use a configurable function wrapper like fu2 with a stack allocator and set small object optimizer capacity to 0.

std::function wasn't cutting it for me, but I ended up going with fu2::function. It comes with a lot of features (almost too many), but the header size and compile times are comparable to , and it fit my needs pretty well.

Thank you, function2 is also used heavily in Idle as a type-erasure backend for continuable that is the primary asynchronous task abstraction.

You can write your own std::function, too, nor is it the only STL type that can take a capturing lambda (std::packaged_task for example).

A capturing lambda is just a class with an operator(). It's complicated to do what std::function does, but fully possible.

In fact, custom std::function replacements have better lambda support than std::function itself. Such as unique_function in https://github.com/Naios/function2 which can handle non-copyable lambdas.

Also, I'm testing https://github.com/greg7mdp/sparsepp which is based on google's sparsehash

Hey thanks for the mention /u/jtoomim. I see that you used sparsepp in CoinsViewCache. A phmap::flat_hash_map would probably be similar for memory usage, but likely faster (because of parallel lookup with SSE instructions and contiguous memory storage in one array).