std::make_signed that accepts floating point types

Question

I have a templated class that can only be instantiated for scalar types (integers, floats, etc.) and I want a member typedef to always be the signed variant of the type. That is:

unsigned int -> signed int
signed long long -> signed long long (already signed)
unsigned char -> signed char
float -> float
long double -> long double
etc...

Unfortunately, std::make_signed only works for integral types, not the floating point types. What is the simplest way to do this? I'm looking for something of the form using SignedT = ...;, to be part of my templated class with template parameter T already guaranteed to be scalar.

Answer 1

A simple template alias will do:

#include <type_traits>

template<typename T>
struct identity { using type = T; };

template<typename T>
using try_make_signed =
    typename std::conditional<
        std::is_integral<T>::value,
        std::make_signed<T>,
        identity<T>
        >::type;

And this is how you could test it:

int main()
{
    static_assert(::is_same<
        try_make_signed<unsigned int>::type, int
        >::value, "!");

    static_assert(std::is_same<
        try_make_signed<double>::type, double
        >::value, "!");
}

Here is a live example.