从.h和.cpp文件定义纯虚函数会产生链接器错误？

Question

I was trying to put together a 'framework' for all my lab works, but then I ran into a frustrating linker error dealing with the implementation of pure virtual function..

When I define the pure virtual function from a .cpp file ( like returntype classname::function() {.....} ) I get a linker error, telling me that the definition of the pure virtual function is not provided...

However, when i simply put the definition to the header file, it works well.. I know i sound confusing... but the code below will surely help you see what is happening..

Can anyone please help me understand why is this happening ?

The project contains 4 files, ( 2 headers, and 2 cpp files)

1 > FrameWork.h :

 #ifndef _FRAMEWORK
 #define _FRAMEWORK

 #include<iostream>

 class labTest

 {
   public :
      virtual void execute() = 0;
 };
#endif

======================================

2 >Stack_Array.h :

#include "FrameWork.h"
#include<iostream>
using namespace std;

template <class T>
class Stack_Array : public labTest
  {

       public: 
         virtual void execute();
  };

======================================

3 > Stack_Array.cpp :

#include "Stack_Array.h"
template<class T>

 virtual void Stack_Array<T>::execute(void) // I beleive i am defining the pure virtual function here, but my compiler ll not agree.
  {
      std::cout << "Test";
  }

======================================

4 > Main_Run.cpp :

#include<istream>
#include"FrameWork.h"
#include"Stack_Array.h"
#include<vector>
using namespace std;

void main()
  {
   vector<labTest*> list(5);
   vector<labTest*>::iterator it;
   it = list.begin();
   Stack_Array<int>* sa = new Stack_Array<int>();

   list.insert(it,sa);
   list[0]->execute();
   getchar();

  }

=========================================

Build Output :

1>------ Rebuild All started: Project: Lab FrameWork, Configuration: Debug Win32 ------
1>Build started 11/20/2012 6:16:48 PM.
1>InitializeBuildStatus:
1>  Touching "Debug\Lab FrameWork.unsuccessfulbuild".
1>ClCompile:
1>  Stack_Array.cpp
1>  Main_Run.cpp
1>  Generating Code...
1>Main_Run.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Stack_Array<int>::execute(void)" (?execute@?$Stack_Array@H@@UAEXXZ)
1>C:\Users\BSP-4\Documents\Visual Studio 2010\Projects\SFML\Lab FrameWork\Debug\Lab FrameWork.exe : fatal error LNK1120: 1 unresolved externals
1>
1>Build FAILED.
1>
1>Time Elapsed 00:00:01.64

========== Rebuild All: 0 succeeded, 1 failed, 0 skipped =======================

It works if I make my Stack_Array.h :

#include "FrameWork.h"
#include<iostream>
using namespace std;

template <class T>
class Stack_Array : public labTest
{

    public: 
    void execute() // give the definition here instead of Stack_Array.cpp and it will work !
       {
           cout << "Test Success !!";
       }
};

I am sure its some silly thing.. Did I overlooked something ?.. but am still in need of help....

Thanks in advance...

Answer 1

A definition of a template must be available in all translation units in which it is used (unless explicit specialisation/instantiation is involved, which is not the case here). In other words, member functions of class templates must be defined in header files.

Answer 2

void Stack_Array<T>::execute(void) is only defined within the compilation unit where it is defined. Outside of Stack_Array.cpp, your compiler has no idea how to implement execute on Stack_Array<T> . In general, template instantiation requests are not passed from one execution unit to another. Now, you can fix this by either putting the implementation of Stack_Array<T> into header files, or by explicitly instantiating the <T> that you want to export in Stack_Array.cpp .

C++ has tried to add support for cross-compilation-unit exporting and requesting of template instantiations, but it is tricky.

The easiest solution is to move your implementation into a header file, and make sure the methods are inline (or in the body of the class).