On: Efficient string copying and concatenation in C

[nneonneo]

C++ implementations sometimes optimize for storing short strings inside the string structure itself. One approach looks something like this on 64-bit machines:

    struct string {
        union {
            struct {
                char *ptr;
                size_t capacity;
            };
            char str[16];
        } data;
        size_t length;
    };

It looks complex, but it’s actually a really nice design because it requires no separate allocation for strings less than 16 bytes long, which is a common case, and the strings are relatively compact. And, it stores a capacity parameter which allows you to know when it’s safe to grow the string without allocating, making it possible to implement efficient repeated concatenation.