对从静态实例引用的类模板的静态成员的未定义引用

时间:2014-09-13 16:19:51

标签: c++ templates linker-errors undefined-reference static-order-fiasco

请查看以下内容:

#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()
{
}

因此Blob Blob::prototype;似乎崩溃了,因为Factory<0>::map尚未实例化,所以我尝试用该行实例化它:

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

但它不会编译(使用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

1 个答案:

答案 0 :(得分:2)

而不是Factory<N>::map专门化 <0>,而只是显式地实例化整个类:

template class Factory<0>;

取代//template <> ...

DEMO


<强>更新

对于Visual Studio,即使模板在第一次使用之前显式实例化,似乎仍然无法初始化静态字段,您可以选择专门化整个类:

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;

或定义Factory<0>的字段(虽然我不知道为什么VS会接受并且不会触发错误,因为语法无效):

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

DEMO 2