Undefined reference to static member of class temp

问题:

Please take a look at the following:

#include <string>
#include <unordered_map>

template <int N> class Object;
template <int N> class Thing;

template <int N>
class Factory {
    private:
        using FuncPtr = Object<N>*(*)(Thing<N>*);
        static std::unordered_map<std::string, FuncPtr> map;
    public:
        static void insertInMap (const std::string& tag, FuncPtr funcPtr) {
            map.emplace (tag, funcPtr);
        }
};
template <int N> 
std::unordered_map<std::string, typename Factory<N>::FuncPtr> Factory<N>::map;

// won't compile on GCC 4.8.1:
//template <> std::unordered_map<std::string, typename Factory<0>::FuncPtr> Factory<0>::map;  

template <int N> struct Object {};

struct Blob : Object<0> {
    static Blob prototype;
    Blob() {Factory<0>::insertInMap ("Blob", Blob::create);}
    Blob (Thing<0>*) {/* */}
    static Object<0>* create (Thing<0>* x) {return new Blob(x);}
};
Blob Blob::prototype;  // Calls up Factory<0>::insertInMap during compile time, but crashes when run.

int main()
{
}

So it appears that Blob Blob::prototype; crashes because Factory<0>::map has not been instantiated yet, so I try to instantiate it with the line:

template <> std::unordered_map<std::string, typename Factory<0>::FuncPtr> Factory<0>::map;

but it won't compile (with GCC 4.8.1):

C:\Users\Andy\AppData\Local\Temp\ccsGlFeV.o:Practice.cpp:(.text$_ZN7FactoryILi0E
E11insertInMapERKSsPFP6ObjectILi0EEP5ThingILi0EEE[__ZN7FactoryILi0EE11insertInMa
pERKSsPFP6ObjectILi0EEP5ThingILi0EEE]+0x14): undefined reference to `Factory<0>:
:map'
collect2.exe: error: ld returned 1 exit status

回答1:

Instead of specializing Factory<N>::map for <0>, just explicitly instantiate the entire class:

template class Factory<0>;

in place of //template <> ...

DEMO

---

UPDATE

For Visual Studio, which seems to still fail to initialize the static field even the template is explicitly instantiated before first usage, you can alternatively specialize the entire class:

template <>
class Factory<0> {
    private:
        typedef Object<0>*(*FuncPtr)(Thing<0>*);
        static std::unordered_map<std::string, FuncPtr> map;
    public:
        static void insertInMap (const std::string& tag, FuncPtr funcPtr) {
            map.emplace (tag, funcPtr);
    }
};
std::unordered_map<std::string, Factory<0>::FuncPtr> Factory<0>::map;

or define the field for Factory<0> (though I don't know why VS accepts that and does not trigger an error, as the syntax is not valid):

std::unordered_map<std::string, Factory<0>::FuncPtr> Factory<0>::map;

DEMO 2