How Can I Preserve Static Class Members (Including Non-Integral Types) in C Class Templates?

Preserving Static Class Members in Class Templates

In the realm of C , you might encounter a scenario where you wish to define a static data member for a template class, but non-integral data types pose a challenge. This article delves into a solution that allows you to declare static members regardless of their type while ensuring their existence in the compiled code.

Consider the following code snippet:

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

As you've noticed, the compiler throws an error since something_relevant is not an integral data type. The issue stems from the fact that templates are instantiated when used, and since S is a template, you cannot define its members inside a compiled file.

To overcome this obstacle, you can resort to defining the static member in the header file itself, as demonstrated below:

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

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

By defining the static member in the header file, you ensure its out-of-class definition and avoid any compilation errors. Moreover, since it is enclosed within the template, the compiler will handle any potential multiple definitions, ensuring a clean instantiation. This technique allows you to maintain static members within class templates, irrespective of their data type, and guarantees that they will be present in the final compiled code.

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