Static Template function access static class member

Question

I have a static template function inside a class that needs to access a static map inside the same class but I keep getting a unresolved external error when trying to access the map. Any Ideas?

Here's the code:

 class Singleton
{

private:

    static std::map<size_t, Singleton*> singletons;

public:

    template<typename T>
    static T* Get()
    {
        size_t type = typeid(T).hash_code();

        if (singletons[type] == nullptr)
            singletons[type] = new T();

        return (T*)singletons[type];
    }

};

Error Message:

error LNK2001: unresolved external symbol "private: static class std::map,class std::allocator > > Singleton::singletons" (?singletons@Singleton@@0V?$map@IPAVSingleton@@U?$less@I@std@@V?$allocator@U?$pair@$$CBIPAVSingleton@@@std@@@3@@std@@A)

Add `std::map Singleton::singletons;` to your implementation file. You must define a static member, which is distinct from the declaration. — François Andrieux, Apr 10 '17 at 14:41
Possible duplicate of [Unresolved external symbol on static class members](http://stackoverflow.com/questions/195207/unresolved-external-symbol-on-static-class-members) — François Andrieux, Apr 10 '17 at 14:42
btw a map of `Singleton`s called `singletons` is kind of an oxymoron — 463035818_is_not_an_ai, Apr 10 '17 at 14:51

score 1 · Accepted Answer · answered Apr 10 '17 at 14:41

1

static class members need to be defined and declared in a compilation unit (in your case singletons member)

You need to add this line in a .cpp file:

std::map<size_t, Singleton*> Singleton::singletons;

answered Apr 10 '17 at 14:41

Raxvan

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Static Template function access static class member

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