https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78042
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Dominik Muth from comment #0) > Obviously int abs(int) is called here all the time. However I expect the > corresponding overloads to be selected according to > http://en.cppreference.com/w/cpp/numeric/math/abs. Your expectation is wrong. If you include <cstdlib> then you get all the required overloads in namespace std (and maybe, unportably, some or all in the global namespace as well). If you include <stdlib.h> then you get all the required overloads in the global namespace (and maybe, unportably, some or all in namespace std as well). So if you include <cstdlib> and call ::abs then your code is not portable or reliable and doesn't behave as specified by the C++ standard. If you call std::abs (either explicitly, or after "using std::abs" or "using namespace std") then it would work, or if you included <stdlib.h> then it would work.
