Better name for CHAR_BIT?

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).

Answer 2

Unless you're programming for DSPs, a better name for it is 8. POSIX and Windows both require CHAR_BIT==8, and that covers pretty much every interesting target conceivable except DSPs. (I would not consider 1970s mainframes a potentially interesting target anymore, but perhaps someone will beg to differ...)

Answer 3

<climits> is where CHAR_BIT is required to be by the C++ standard. Even if you happened to find it in <bitset>, its not guaranteed to be there, so you're better off going straight to the source. Its not like there's something wrong with including <climits>.

Answer 4

Define "newer". A random Linux system gave me these results:

~> gcc --version
gcc (GCC) 4.1.2 (Gentoo 4.1.2)
[snip]

~> grep CHAR_BIT /usr/include/*.h
/usr/include/limits.h:#  define CHAR_BIT        8

Doesn't that qualify? In C, I think it should always be enough to include limits.h to get CHAR_BIT.

Answer 5

Yes, you should #include <limits.h> to get the definition of CHAR_BIT. It's not a bottleneck at all for your program (see comment below), and later down the line you might find yourself porting to different platforms. Not every implementation has the value of 8

Answer 6

I keep the following function in my .profile to check compiler defined constants. On my system __CHAR_BIT__ is in there, which means no header is required if you can live with the double underscore form, which may only work with gcc.

defines () 
{ 
    touch /tmp/defines.h;
    cpp -dD "$@" /tmp/defines.h | awk '$1!="#"{COUNT+=1;print;}END{printf("count %d\n",COUNT);}' | sort;
    rm /tmp/defines.h
}