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

[Markus Mützel]

URL:
  <https://savannah.gnu.org/bugs/?61812>

                 Summary: Math constants (e.g. M_PI) are not part of C/C++
standard
                 Project: GNU Octave
            Submitted by: mmuetzel
            Submitted on: Thu 13 Jan 2022 03:40:23 PM CET
                Category: Interpreter
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Build Failure
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: 7.0.90
         Discussion Lock: Any
        Operating System: Any

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Details:

Math constants are not defined in standard C or C++. They are a GNU/POSIX
extension. See: https://en.cppreference.com/w/cpp/header/cmath

At least on Windows, compilation of headers that use any of those math
constants fails if a "strict" dialect is selected (e.g. `-std=c++11`).

One header that uses e.g. M_PI is `liboctave/util/oct-cmplx.h`.

C++20 defines `std::numbers::pi` in `<numbers>`:
https://en.cppreference.com/w/cpp/numeric/constants

On Windows, the older C math constants can be forced to be defined by
`<cmath>` with `#define _USE_MATH_DEFINES` prior to including this header.
That also works if a "strict" C++ dialect is selected.

Should we do that? Or avoid using these non-standard constants in headers
entirely? Which alternatives are there?




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