C++: link error with static const variables in template class

Question

I'm developing a class that receive as parameter a template class object. Since it has a template object, I'm defining all methods in the .h file (no .cpp).

I have static const variables that I declare in .h file and I initialize them in the .cpp file (this is the only purpose of the .cpp file in my case). I access these static const variables in some methods.

I can compile this class without problems. The problem appears when I try to include the header file in the file that will execute it, then I have the following error:

UnitTest.obj : Error LNK2001: unresolved external symbol "public: static char const * const A::FNUMBER

I have tried to define methods in the .cpp file, but then when I try to compile I have linker errors in all methods of the mentioned template class.

Here an example of code:

---

Header file (.h)

#ifndef A_H
#define A_H

#include "TemplateClass.h"

typedef TemplateClass<int, char> A_TemplateClass;

class A{

    public:
        static const float FNUMBER;

        //Due to I refer a template class, I have to implement all my methods in .h file,
        //else I have a lot of linker errors (1 for each template class method)
        A(A_TemplateClass &atc) : aTemplateClass(atc) {};

        void method1(){
            //Use the static const variable
            //If I do not access this variable, there is no problems.
            float f = FNUMBER + 3.2;
            [...]
        }

        void method2(){
            //Use the static const variable
            //If I do not access this variable, there is no problems.
            float f = FNUMBER + 3.3;
            [...]
        }


    private:
        A_TemplateClass aTemplateClass;

};


#endif

---

.cpp file

#include "A.h"

const float A::FNUMBER = 1.3f;

---

UnitTest.cpp file

#include "TemplateClass.h"
#include "A.h"

int main(){
    TemplateClass<int, char> x;
    A a(x);
    float f = A::FNUMBER; //Linker ERROR
    //If I delete the above line, there are no errors.
}

As I say, If I do not use the static const variables inside the class, there are no errors. What I'm doing wrong? Thanks.

Answer 1

Are you sure that you are compiling/linking with A.cpp/Ao? Your code should compile with g++ UnitTest.cpp A.cpp assuming TemplateClass.h doesn't have an external .cpp file.

Answer 2

包括包含static const变量初始化的.cpp文件，即可解决此问题。