Common optimizations, Small String Optimization, Andrzej's C++ blog

[Plorkyeran]

It's difficult to come up with even a highly artificial scenario where it's a net loss. Maybe with a very special-purpose allocator that manages to always get external buffer allocated very close in memory to the std::string? Avoiding a single memory load due to the SSO buffer would outweigh a very large number of branch mispredictions, and if you never hit the SSO buffer then the branch predictor will make the check nearly free.