模板类可以在 C++ 中有 static 成员吗

Question

Can a template class in C++ have static members? Since it doesn't exist and is imcomplete before it is used, is this possible?

Answer 1

Yes. The static member is declared or defined inside the template< … > class { … } block. If it is declared but not defined, then there must be another declaration which provides the definition of the member.

template< typename T >
class has_static {
    // inline method definition: provides the body of the function.
    static void meh() {}

    // method declaration: definition with the body must appear later
    static void fuh();

    // definition of a data member (i.e., declaration with initializer)
    // only allowed for const integral members
    static int const guh = 3;

    // declaration of data member, definition must appear later,
    // even if there is no initializer.
    static float pud;
};

// provide definitions for items not defined in class{}
// these still go in the header file

// this function is also inline, because it is a template
template< typename T >
void has_static<T>::fuh() {}

/* The only way to templatize a (non-function) object is to make it a static
   data member of a class. This declaration takes the form of a template yet
   defines a global variable, which is a bit special. */
template< typename T >
float has_static<T>::pud = 1.5f; // initializer is optional

A separate static member is created for each parameterization of the template. It is not possible to have a single member shared across all classes generated by the template. For that, you must define another object outside the template. A partially-specialized traits class might help with that.