`cosf`, `sinf`, etc. are not in `std` [duplicate]

Question

Based on the discussion here, I have reported a bug to the Ubuntu developers.

---

When compiling the following sample c++ program:

#include <cmath>
#include <stdio.h>

int main()
{
    printf("%f\n", std::cosf(0.0f));
}

I get the following error message: error: ‘cosf’ is not a member of ‘std’

Including math.h and using the non-namespaced version works fine. What is going on?

I am using g++ 8.3.0-6ubuntu1 on Ubuntu 19.04.

I am building with g++ --std=c++17 test.cpp

Answer 1

That version of the library (libstdc++8) is not fully conforming to C++17. The copyright notice says it was last updated in 2016. As of June 2019, the latest upstream release is bugged. It does have a #if __cplusplus > 201402L section, but it doesn’t declare the identifiers required by C++17. There is an open bug report.

Looking at /usr/include/c++/8/cmath on Ubuntu, it includes <math.h>, undefines a series of macros for its functions (required by the C standard library) to access their names, imports cos, acos, etc. into the std:: namespace, and then declares the overloaded float and long double overloads as inline.

It never declares cosf 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, it does not explicitly state that std::expf et al. must be supported until P0175r1 in June 2016. This was apparently an oversight.

The libc++ library does declare them, so compiling with clang++ -std=c++17 -stdlib=libc++ should work.