Unresolved external symbol using static constexpr member

Question

I'm trying to compile this (C++14) code using VC++ 2017.

#include <type_traits>
#include <limits>

struct Templ
{
    template <typename T>
        static constexpr int value = ( std::numeric_limits<T>::min() == 0 );
};
using type = std::conditional_t<Templ::value<unsigned>, bool, double>;

int main()
{
    return 0;
}

I get the following error message:

unresolved external symbol "public: static int const Templ::value" (??$value@I@Templ@@2HB) referenced in function "void __cdecl Templ::`dynamic initializer for 'public: static int const Templ::value''(void)" (??__E??$value@I@Templ@@2HB@Templ@@YAXXZ)|

How can I correctly use conditional_t with a templated static constexpr member as the condition?

Edit. Based on Some programmer dude's answer I came up with this:

struct Templ
{
    template<typename T>
        struct inner
    {
        enum
        {
            value = ( std::numeric_limits<T>::min() == 0 )
        };
    };
};
using type = std::conditional_t<Templ::inner<unsigned>::value, bool, double>;

The error message doesn't fit with the code you show, as the error message mentions Templ::value<double> and you don't have it in your code. Is the code you show truly a minimal reproducible example that generates the error you show? — Some programmer dude
@Someprogrammerdude sorry, updated the code and the error message. — user1768761

Some programmer dude Some programmer dude · Accepted Answer · 2019-12-09T12:58:44

The problem isn't with std::conditional_t, but simply that even if you make the member variable constexpr and initialize it inline in the class, you still need to define it.

A simple solution, considering that your variable is a plain int, is to use an enumeration instead. But then you need to make the structure Templ a template instead:

template <typename T>
struct Templ
{
    enum
    {
        value = (std::numeric_limits<T>::min() == 0)
    };
};
using type = std::conditional_t<Templ<unsigned>::value, bool, double>;

Unresolved external symbol using static constexpr member

2 Answers