How can I initialize a static data member of a class template with type traits?

Question

I tried to use something like this but the initialization seems not to work. When I remove the type trait, then it works as expected.

template<typename _T, typename = std::enable_if_t<std::is_integral<_T>::value>>
struct Foo
{
    static int bar;
};

template<typename _T>
int Foo<_T>::bar = 0;

How to properly initialize such a static variable?

Answer 1

That's because you used std::enable_if not quite properly.

template<typename T, typename = std::enable_if_t<std::is_integral<T>::value>>
struct Foo;

template <typename T>
struct Foo<T, false> //Partial specialization
{
  // if T is not integral, Foo becomes empty class
};

template <typename T>
struct Foo<T, true> //Partial specialization
{
    static int bar;
};

And then:

template<typename T>
int Foo<T, true>::bar = 0;

I changed _T to T , because name defined as _X , __X or __x are reserved for internal implementation.