Looking at the C 2011 standard, it looks like "_Bool" is implementation defined ( https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf , pg. 57 of the PDF; the page labeled 39 section 6.2.5 paragraph 2): " An object declared as type _Bool is large enough to store the values 0 and 1."

I believe everybody uses a single byte for that -- a single byte can store the values 0 and 1 -- but it looks like they aren't required to.

I believe C++ specifies "bool" is one byte; it's definitely never larger than a "char".

As far as std::vector<bool>, the fact that each value is defined at taking up a single bit inside the std::vector<bool> doesn't really say anything about how large each bool would be outside of the std::vector<bool>. std::vector<bool> was arguably a case of the Committee being too clever ( https://isocpp.org/blog/2012/11/on-vectorbool ).