Short string optimization using custom alocator

[dimztimz]

Some standard containers have good SSO, like std::string, but others have bad like std::wstring, or some have none, like std::vector. We can make any contiguous container have SSO with custom allocator. With this we can replace global or, more precisely, thread-local objects at the places they are used, which we use to avoid lot of memory allocations by reusing the old buffers.

An example for such solution is:

• https://howardhinnant.github.io/stack_alloc.html and
• https://github.com/HowardHinnant/HowardHinnant.github.io/blob/master/short_alloc.h .

A C++ allocator can not give memory from within the same object, thus the following is wrong https://github.com/HowardHinnant/HowardHinnant.github.io/blob/master/stack_alloc.h . Howard Hinnant first wrote the wrong one (stack_alloc.h) and then made the correct one (short_alloc.h).

Another solution is to use new features from C++ https://en.cppreference.com/w/cpp/memory/monotonic_buffer_resource . See a tutorial for that. These features are still not implemented in LLVM's libc++.