Compile-time constant id

by

This is sufficient assuming a standards conforming compiler (with respect to the one definition rule):

template<typename T>
class A
{
public:
    static char ID_storage;
    static const void * const ID;
};

template<typename T> char A<T>::ID_storage;
template<typename T> const void * const A<T>::ID= &A<T>::ID_storage;

From the C++ standard 3.2.5 One definition rule [basic.def.odr] (bold emphasis mine):

… If D is a template and is defined in more than one translation
unit, then the last four requirements from the list above shall apply
to names from the template’s enclosing scope used in the template
definition (14.6.3), and also to dependent names at the point of
instantiation (14.6.2). If the definitions of D satisfy all these
requirements, then the program shall behave as if there were a single
definition of D. If the definitions of D do not satisfy these
requirements, then the behavior is undefined.

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