How to Properly Initialize Static Class Members in C Class Templates?

Defining Static Class Members in Class Templates

In C , static data members within class templates cannot be initialized directly when defined within the class declaration. When dealing with non-integral types, this limitation poses a challenge.

Consider the following code:

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

This code cannot compile because something_relevant is not an integral type. However, the solution lies in defining the member outside the class declaration.

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

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

In this approach, the static member is first declared within the class definition. Then, the definition is provided outside the class, after the template declaration. This approach ensures that the static member is defined once across all instantiations of the S template.

The compiler will handle the definition process. When encountering the member declaration within the class, it will recognize that it is part of a template. Upon encountering the definition outside the class, the compiler will replace the template parameter T with the actual type being used in the instantiation. This ensures that each instantiation of the S template has its own copy of the static member something_relevant.

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