[Octave-bug-tracker] [bug #61812] Math constants (e.g. M_PI) are not part of C/C++ standard

[Rik]

Follow-up Comment #2, bug #61812 (project octave):

These constants may not be part of the standard, but they are *really* widely
available.

I don't think we need to try for perfection here.  If the Windows OS has a
special exception and doesn't include M_PI then let's just figure out a way to
include a definition for M_PI.

Some ideas:

1) Provide a wrapper for the the cmath header in liboctave/wrappers
2) Do a configure test for Windows and define _USE_MATH_DEFINES on that
platform
3) Define M_PI ourselves if it does not exist.


On a related note, this code for the pi() function in
libinterp/corefcn/data.cc is not the way to do it:


#if defined (M_PI)
  double pi_val = M_PI;
#else
  double pi_val = 4.0 * atan (1.0);
#endif


This requires calculating atan() every single time on platforms, like Windows,
which don't have M_PI built-in.  At the very least the second half should be
tagged with constexpr (and maybe static too).

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