small string optimization for vector?

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(poi开发者_JAVA技巧nters). 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?

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...

You can borrow the SmallVector implementation from LLVM. (header only, located in LLVM\include\llvm\ADT)

If T is a POD type why not basic_string instead of vector??

It was discussed years ago (and a few of the names in that thread may look a bit familiar :-) ), but I don't know of an existing implementation. I don't think I'd try to adapt std::string to the task. The exact requirements on the type over which std::basic_string aren't well stated, but the standard is pretty clear that it's only intended for something that acts a lot like char. For types that are substantially different, it might still work, but it's hard to say what would happen -- it was never intended for, and probably hasn't been tested with many types other than small integers.

A fully conforming implementation of std::vector is a lot of work. But implementing a usable subset of std::vector from scratch (even including a small vector optimization) won't usually be terribly difficult. If you include a small vector optimization, I'm reasonably certain you can't meet all the requirements on std::vector though.

In particular, swapping or moving a vector where you've stored actual data in the vector object means you'll need to swap/move actual data items, where the requirements on std::vector are predicated on its storing only a pointer to the data, so it can normally¹ swap or move the contents just by manipulating the pointers, without actually touching the data items themselves at all. As such, it's required to be able to do these things without throwing, even if manipulating the data items themselves would/will throw. As such, a small vector optimization will preclude meeting those requirements.

On the other hand, as noted above, one of the requirements on std::string is that it can only store items that can be manipulated without throwing. As such, if std::string is a viable option at all, implementing your own vector-like container probably won't need to worry about those details a lot either.

1. There is one case where you end up having to swap/move actual data items, even in an actual std::vector: if the two vectors use different allocators, then you have to allocate space for the objects in the destination via that vector's allocator.