Undefined reference to static member variable when specializing

Question

I have the following code:

    struct All { 
    All() {} 
    ~All() {}

    template <typename T>
    static struct Item {
        T var;
    } item;
    virtual void setVal() noexcept {} 
};

template <typename T>
struct any : public All
{
    public:
        any() : All() {}
        ~any() {}
        T value; 
        void setVal() noexcept override {
            All::item<decltype(value)>.var = value; // Error appears here
        }
};

And the following error:

undefined reference to All:item<int>

I don't understand this error, because item is a static member variable template, and i have to specialize it...

Help me !

Answer 1

Your bug is that you never defined the static member variable. You only declared it. Add a definition:

template <typename T>
All::Item<T> All::item = {};

However, your syntax for defining the type of the member variable template and declaring the variable itself is not accepted by the compilers that I tested. g++ (5.2.0) is fine with the declaration, but complains that All::Item is not a template when specifying the type for the variable definition. clang++ (3.6.0) does not even accept your declaration. Simply separating the definition of the type and the declaration of the variable solved the issue:

template <typename T>
struct Item {
    T var;
};

template <typename T>
static Item<T> item;