Static member shared across template specializations

Question

Given the following class template, is there any way to have the field a to be the same across all specializations (i.e. A<int>::a is the same lvalue as A<std::string>::a)?

template<class T>
class A final {
private:
    static int a;
};

Yes, just inherit from a non-templated base class which would store the static member. — syntagma, May 21 '16 at 20:39
Possible duplicate of [Sharing static members between template instantiations? (impossible?)](http://stackoverflow.com/questions/3198249/sharing-static-members-between-template-instantiations-impossible) — syntagma, May 21 '16 at 20:39

Barry · Answer 1 · 2016-05-21T20:43:54.390

4

Simply inherit (privately) from a non-template and move the static there:

class ABase {
protected:
    static int a;
}; 

int ABase::a;

template<class T>
class A final : private ABase 
{ };

edited May 21 '16 at 20:43

answered May 21 '16 at 20:39

Barry

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The only issue that I can see with this is that anyone can inherit from `ABase`, is there any way to prevent this? – Nick Mertin May 21 '16 at 20:42
Isn't `final` redundant here? – syntagma May 21 '16 at 20:43
@Nicholas Yeah - but is it worth the effort? Just hide the base somewhere. – Barry May 21 '16 at 20:45
@NicholasMertin Making the base class constructor private and the template class a friend should prevent unintended inheritance. – May 21 '16 at 20:59
@ΔλЛ Not if you don't want anything inheriting from `A`. – Barry May 21 '16 at 21:03
OK, just realized that's what OP wanted. – syntagma May 21 '16 at 21:12

score 1 · Answer 2 · answered May 21 '16 at 20:56

Thank you to those of you who suggested a non-templated base class. A similar solution that I found that removes the issue of an API consumer inheriting from the base class is to have it as a final class with the templated class declared as a friend:

class A_Data final {
private:
    static int a;

    template<class T>
    friend class A;
};

template<class T>
class A final {
};

Static member shared across template specializations

2 Answers2