From Folly documentation
Small strings (<= 23 chars) are stored in-situ without memory allocation.
Medium strings (24 - 255 chars) are stored in malloc-allocated memory and copied eagerly.
Large strings (> 255 chars) are stored in malloc-allocated memory and copied lazily.
Where are these "Small String" stored ?
This means that the cost of std::string already includes 23 characters. Anything more requires additional allocations.
This means internally the structure looks approximately like:
struct string {
// ...
char internal[23];
char* external;
};
This is presumably to make copying short strings very cheap as no heap operations have to be performed, plus it doesn't require any delete calls to clear.