std::expf and std::logf not recognized by gcc 7.2.0
This is a bug in libstdc++, which ships with GCC. It does not fully conform to C++17 (as of v9.1.0 in June 2019). The copyright notice on the version of <cmath>
that Ubuntu uses by default says it was last updated in 2016. Version 9.1.0 does have a #if __cplusplus > 201402L
section, but it doesn’t declare the identifiers required by C++17. There is an open bug report.
It never declares expf
or logf
(nor cosf
, sinf
, etc.) within the std::
namespace, even though C++17 says it shall. The C++11 standard says, “Names that are defined as functions in C shall be defined as functions in the C++ standard library,” and “Each name from the Standard C library declared with external linkage is reserved to the implementation for use as a name with extern "C"
linkage, both in namespace std
and in the global namespace.” However, std::expf
et al. are missing from the table of functions provided by <cmath>
until P0175r1 in June 2016. This was apparently an oversight, but GCC has always made them available only in the global namespace.
The libc++ library does declare them, so compiling with clang++ -std=c++17 -stdlib=libc++
should work. You can also #include <math.h>
to use them in the global namespace, or use the overloaded exp()
, log()
, etc. on float arguments.
If you
#include <cmath>
you will get
float exp ( float arg );
double exp ( double arg );
long double exp ( long double arg );
double exp ( IntegralType arg );
float log ( float arg );
double log ( double arg );
long double log ( long double arg );
double log ( IntegralType arg );
And hence you can call just std::exp
/std::log
and let the compiler figure out the overload for you. If you want to call a mismatching overload (e.g. the float
overload on a double
variable), I find it much more explicit and clear to add a static_cast
in those cases:
double bla = ...;
return std::exp(static_cast<float>(bla));
This is a strange construct any way you write it (e.g. why is bla
not a float
to begin with?), and hiding it in a single-letter function name suffix isn't helping anyone.