std::stringstream vs std::string for concatenating many strings

There's no reason to expect std::string's appending functions to be slower than stringstream's insertion functions. std::string will generally be nothing more than a possible memory allocation/copy plus copying of the data into the memory. stringstream has to deal with things like locales, etc, even for basic write calls.

Also, std::string provides ways to minimize or eliminate anything but the first memory allocation. If you reserve sufficient space, every insertion is little more than a memcpy. That's not really possible with stringstream.

Even if it were faster than std::string's appending functions, you still have to copy the string out of the stringstream to do something with it. So that's another allocation + copy, which you won't need with std::string. Though at least C++20 looks set to remove that particular need.

You should use std::stringstream if you need formatting, not just for sticking some strings together.