Ye, everyone know it is:

    #define PI 3.1415927410125732421875

that is the float closest to pi or that there is the GNU extension USE_FUNKY_DEFINES-something with M_PI for cmath. (I know you meant constexpr:ions).

Long time ago when computers were 32-bit I used to write something like this in C++:

    inline double pi()
    {
        __asm fldpi;
    }

On old enough compilers, this will get you 80-bit version of pi, more precision than even double. Completely useless now, modern compilers no longer emit x87 code.

math.h has M_PI in posix.

The point is that using macros for constants is frowned upon in modern codebases, just like all other use cases for macros that break type safety and the one definition rule.

Yes, I understand the point. But to your comment in a literal sense, if I were doing code review I would probably suggest using M_PI. Which happens to be a macro.

Which actually highlights that many things you want to use for practical purposes are C rather than C++, predate innovations such as const or namespaces, etc. It's possible and common to understand the downsides and still reach for the macro.