Question

I was just checking an answer and realized that CHAR_BIT isn’t defined by headers as I’d expect, not even by #include <bitset>, on newer GCC.

Do I really have to #include <climits> just to get the “functionality” of CHAR_BIT?

Answer 1

As you may know, whether or not an implementation wants to include other headers is unspecified. It’s allowed, but not mandated. (§17.4.4.1) So you either have to be explicit or know your guarantees.

The only time a C++ header must include another is if it requires a definition in another. For example, <bitset> is required to include <cstddef> for std::size_t, as this is explicitly stated in the standard. (§23.3.5, for this example)

For a counter-example, consider <limits>. It may include <climits> and define the values for numeric_limits in terms of the macros within, and it often does since that’s easiest for an implementation. But all the standard says is things like: “Equivalent to CHAR_MIN, SHRT_MIN, FLT_MIN, DBL_MIN, etc.” but doesn’t say it must to be implemented in terms of those, which means <climits> doesn’t have to be included.

So the only way you can be guaranteed that a CHAR_BIT is defined is by including <climits> or some other header where it’s explicitly stated it must include it. And as far as I can tell, none have to; an implementation is free to just hard-code the value everywhere it’s needed, for example, or include <limits> and use std::numeric_limits<unsigned char>::digits (which is equivalent).