kaifsm / svector

Small Vector optimization

ankerl::svector 🚚

ankerl::svector is an std::vector-like container that can hold some elements on the stack without the need for any allocation. There are lots of small vector implementations (absl, boost, folly, llvm, ...) but this here is special by how compact it is.

The smallest ankerl::svector is just 8 bytes large (on 64bit machines). E.g. ankerl::svector<std::byte, 7> can store 7 bytes inline on the stack without allocating, yet it's sizeof() is only 8. As far as I know, no other implementation is that compact. Most implementation need at least 24 or 32 bytes.

Here is a table comparing the amount of bytes you can store on the stack for a given total stack size of the vector:

	8	16	24	32	40	48	56	64
boost::container::small_vector	-	-	-	8	16	24	32	40
absl::InlinedVector	-	-	16	24	32	40	48	56
ankerl::svector	7	15	23	31	39	47	55	63

In short:

• 24 bytes overhead for boost::container::small_vector
• 8 bytes overhead for absl::InlinedVector
• 1 (one!) byte overhead for ankerl::svector.

Note that boost's small_vector cheats a bit. E.g. boost::container::small_vector<std::byte, 15> won't give you 15 elements on the stack, but only 8 bytes. It seems to round down to the closest multiple of 8 bytes.

Disclaimer

ankerl::svector is new and relatively untested! I have implemented and tested all of std::vector's API though, and have 100% test coverage. Still, using it might set your computer on fire.