Weird linker problem with static const class members

Please tell me, why gcc linker gives me the following error: "test_class::test_struct::constVar", referenced from: __ZN12lu_test_class27test_struct6constVar$non_lazy_ptr in test_class.o ?

My code (test_class.h):

class test_class
{
    struct test_struct
    {
         static const int constV开发者_JAVA技巧ar = 0;
    };
};

All references to constVar are in test_class scope in a usual static member access form: test_struct::constVar.

Provide the definition of the static member outside the class

const int test_class::test_struct::constVar;

This works for me.