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c++language-lawyerexternlinkage

Extern keyword on definition of static data members and member functions, C++

Does C++ standard allow extern keyword on definition of static data members and member functions (provided that linkage matches) ? For example:

struct A
{
    static int a;    // external linkage
    void f();        // external linkage
};

extern int A::a;
extern void A::f() {}
Solution

  • The extern keyword is not allowed as a storage class specifier on class members. From [dcl.stc]/5:

    [...] The extern specifier cannot be used in the declaration of class members or function parameters. [...]

    Moreover, definitions are declarations, cf. [basic.def]/2:

    A declaration is a definition unless [rules].

    Therefore, the extern keyword is not allowed as a storage class specifier on any form of class member declaration, whether on the first declaration that's part of the class definition or on subsequent declarations that are part of out-of-line member definitions.