abs vs std::abs, what does the reference say?

Question

Beware, I am talking about ::abs(), not std::abs()

According to the cplusplus.com website, abs is supposed to behave different

Answer 1

The official references say... it's a mess. Pre-C++11 and C11:

• Officially, including <cmath> introduced nothing in ::; all of the functions were in std::. Practically, only export was less respected, and different compilers did very different things. If you included <cmath>, you used std:: everywhere, or what you got varied from compiler to compiler.

• C didn't provide any overloads: abs took an int, and was declared in <stdlib.h>, fabs took double, and was declared in <math.h>.

• If you included <math.h> in C++, it's not clear what you got, but since none of the implementers seemed to care about the standard anyway (see the first point above)...

Roughly speaking, either you included <cmath>, and prefixed all of the uses with std::, or you included <math.h>, and used fabs if you wanted support for floating point (and the various suffixes for types other than int or double).

C++11 and C11 added a few new twists:

• <cmath> is now allowed (but not required) to introduce the symbols in :: as well. One more thing which can vary depending on the implementation. (The goal here was to make existing implementations conformant.)

• C has a new header, <tgmath.h>, which uses compiler magic to make the functions in <math.h> behave as if they were overloaded as in C++. (So it doesn't apply to abs, but only to fabs.) This header had not been added to C++, for the obvious reason that C++ doesn't need any compiler magic for this.

All in all, the situation has become slightly worse, and my recommendations above still hold. Include either <math.h> and <stdlib.h>, and use abs/fabs and their derivated (e.g. labs, fabsf, etc.) exclusively, or include <cmath>, and use std::abs exclusively. Anything else, and you'll run into portabiity problems.