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ankerl::svector 🚚
ankerl::svector is an std::vector-like container that can hold some elements on the heap 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. E.g. ankerl::svector<std::byte, 7> can store 7 bytes on the heap 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 heap for a given total heap 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 heap, 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.