Does FENV_ACCESS pragma exist in C++11 and higher?

Question

Reading a bug report for clang not supporting FENV_ACCESS pragma I've come across a comment:

Setting the rounding mode without using #pragma STDC FENV_ACCESS ON invokes undefined behavior. See C11 7.6.1/2. (This pragma does not exist in C++, so <cfenv> is unusable, but that's not our fault...)

Does this pragma really not exist in C++, rendering <cfenv> unusable? I've tried to search for it in the C++11 standard, but it really isn't mentioned at all. Are pragmas inherited from C along with function prototypes? Or are they actually not needed to avoid UB, since the C++ standard doesn't say anything about behavior being undefined when the pragma is not used (due to not mentioning the pragma at all)?

Answer 1

I searched the 2015 standard draft text and found no occurrences of FENV_ACCESS. http://cppreference.com also has nothing about it.

However, http://cplusplus.com does mention it (since it is not in the standard I think we must assume that this is advisory information at best):

http://www.cplusplus.com/reference/cfenv/FENV_ACCESS/

Quoting from cplusplus.com: (emphasis mine)

If set to on, the program informs the compiler that it might access the floating-point environment to test its status flags (exceptions) or run under control modes other than the one by default.

If set to off, the compiler may perform certain optimizations that can subvert these tests and mode changes, and thus accessing the floating-point environment in the cases described above, causes undefined behavior.

Whether the state of this pragma by default is on or off depends on the compiler settings and library implementation.

Given the unsettling lack of clarity, I would want to avoid its use if at all possible.

As ever, if the use was unavoidable I'd want to encapsulate it into one class that I can specialise and test for each architecture.

And then document the existence of this class and the trouble it may cause if the compiler, environment or library implementation is upgraded.

Update:

There is a very brief mention of the header in the c++ standard:

§ 26.3 The floating-point environment [cfenv]

...

2 The header defines all functions, types, and macros the same as Clause 7.6 of the C standard.

Update:

Further information here: http://en.cppreference.com/w/cpp/preprocessor/impl

My reading of this is that the pragma is defined by the C11 standard, not the C++11 standard. Therefore use in a c++ program is strictly implementation/un-defined.