Defining pure virtual functions from .h and .cpp files yields linker error?

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...

Angew is no longer proud of SO · Accepted Answer · 2012-11-20 13:50:21Z

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.

Yakk - Adam Nevraumont · Accepted Answer · 2012-11-20 13:49:54Z

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).

Thanks for clearing this. This behavior of c++ was a bit of a surprise to me. And I did not see many discussing this problem out there in forums or such.... thanks ny way. I ll take my code into the header...

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