Small object optimization
One of the great things about containers such as std::vector is that they automatically allocate dynamic memory when needed. Sometimes, though, the use of dynamic memory for container objects that only contain a few small elements can hurt performance. It would be more efficient to keep the elements in the container itself and only use stack memory, instead of allocating small regions of memory on the heap. Most modern implementations of std::string will take advantage of the fact that a lot of strings in a normal program are short, and that short strings are more efficient to handle without the use of heap memory.
One alternative is to keep a small separate buffer in the string class itself, which can be used when the string's content is short. This would increase the size of the string class, even when the short buffer is not used.
So, a more memory-efficient solution is to use a union, which can hold a short buffer when the string is in short...