How to avoid dynamically allocating small objects?

Question

I found that many times I only need a small std::map (say less than 10 keys), or a small std::vector containing only a few elements, and I think it's really a waste of performance to always dynamically allocate them, especially in structures like std::map<std::string, std::string>, std::vector<std::string>, there're really a lot dynamic allocation involved.

Any good advice? At least reduce the amount of dynamic allocation, better without sacrifying the ease of use. Thanks

Answer 1

You may use stack-allocated memory for small size data (as stack allocations are very fast, basically just a stack-pointer movement; although stack's space is precious and it's a very limited resource), and heap-allocated memory for larger size. In other words, think along the lines of the std::string's small string optimization.

Moreover, to speed up allocations, you could also preallocate big memory chunks on the heap, and then carve smaller allocations inside those chunks, again basically just increasing a pointer inside the chunk. For a sample implementation of this pool allocator technique, consider reading this blog post.

You will find this CppCon 2016 talk interesting as well:

High Performance Code 201: Hybrid Data Structures