Is it permissible to specialize mathematical constants for custom numeric types?-CodePudding

Suppose I create my own floating point type MyFloatingPoint (e.g. to provide increased precision relative to the built-in types). Is it permissible to specialize the constants in the C 20 <numbers> header for this type, or is this undefined behavior?

CodePudding user response：

Yes, but only if the specialization depends on a program-defined type (rather than only built-in / standard library types).

See [math.constants]/2 in the post-C 20 draft of the C standard.

Note that without such a specific permission in the standard, it is generally not allowed to specialize a standard library variable template. See [namespace.std]/3.

CodePudding user response：

According to C 20 draft it seems fine:

26.9.2 (...) 2. Pursuant to 16.5.4.2.1, a program may partially or explicitly specialize a mathematical constant variable template provided that the specialization depends on a program-defined type

CodePudding user response：

No it isn't undefined behavior (be careful when to use that term). I don't see any big issues with that (specialy not after reading the other comments).

#include <numbers>

struct my_floatingpoint_t
{
    double value;
};

namespace std
{
    namespace numbers
    {
        template<>
        constexpr my_floatingpoint_t pi_v<my_floatingpoint_t>{3.14};
    }
}


int main()
{
    auto pi = std::numbers::pi_v<double>;
    auto my_pi = std::numbers::pi_v<my_floatingpoint_t>;
    return 0;
}