small string optimization for vector?

Question

I know several (all?) STL implementations implement a "small string" optimization where instead of storing the usual 3 pointers for begin, end and capacity a string will store the actual character data in the memory used for the pointers if sizeof(characters) <= sizeof(pointers). I am in a situation where I have lots of small vectors with an element size <= sizeof(pointer). I cannot use fixed size arrays, since the vectors need to be able to resize dynamically and may potentially grow quite large. However, the median (not mean) size of the vectors will only be 4-12 bytes. So a "small string" optimization adapted to vectors would be quite useful to me. Does such a thing exist?

I'm thinking about rolling my own by simply brute force converting a vector to a string, i.e. providing a vector interface to a string. Good idea?

Answer 1

Boost 1.58 was just released and it's Container library has a small_vector class based on the LLVM SmallVector.

There is also a static_vector which cannot grow beyond the initially given size. Both containers are header-only.

facebook's folly library also has some awesome containers.

It has a small_vector which can be configured with a template parameter to act like boost's static or small vectors. It can also be configured to use small integer types for it's internal size bookkeeping which given that they are facebook is no surprise :)

There is work in progress to make the library cross platform so Windows/MSVC support should land some day...