Can I use this code in my project?

[DaLLuZZ]

Is there the project license?

[BenBE]

The files are marked as Boost Software License: https://github.com/skarupke/flat_hash_map/blob/2c4687431f978f02a3780e24b8b701d22aa32d9c/unordered_map.hpp#L2

For the license text see https://www.boost.org/users/license.html