Static member initialization in a class template

This will work

template <typename T>
 struct S
 {

     static double something_relevant;
 };

 template<typename T>
 double S<T>::something_relevant=1.5;

Since C++17, you can now declare the static member to be inline, which will define the variable in the class definition:

template <typename T>
struct S
{
    ...
    static inline double something_relevant = 1.5;
};

live: https://godbolt.org/g/bgSw1u

Just define it in the header:

template <typename T>
struct S
{
    static double something_relevant;
};

template <typename T>
double S<T>::something_relevant = 1.5;

Since it is part of a template, as with all templates the compiler will make sure it's only defined once.

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