What does "in-situ without memory allocation" mean for folly implementation of String?

Question

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 ?

Answer 1

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.