Question

I have trying to define a class DS with the declaration in DS.h and implementation at DS.cpp
The code is very small so here is the listing:

 /*
  * DS.h
  */

 #ifndef DS_H_
 #define DS_H_

 #include "Node.h"

 template<class T>
 class DS
 {
     public:
         static const int BST;
         static const int SLL;
         static const int DLL;

         DS(const int);
         ~DS();

     private:
         int m_type;
         Node<T> *head;
 };

 #endif /* DS_H_ */

And,

 /*
  * DS.cpp
  */

 #include "DS.h"

 template<class T> const int DS<T>::BST = 0;
 template<class T> const int DS<T>::SLL = 1;
 template<class T> const int DS<T>::DLL = 2;

 template<class T>
 DS<T>::DS(const int type) :
     m_type(type), head(0)
 {
 }

 template<class T>
 DS<T>::~DS()
 {
 }

The main program is:

 #include "DS.h"

 int main()
 {
     DS<int> *sll1 = new DS<int> (DS<int>::SLL);
     delete sll1;
     return 0;
 }

When I try to compile this program, I get the following error:

 g++ -O2 -g -Wall -Wextra -Weffc++ -fmessage-length=0   -c -o Node.o Node.cpp
 g++ -O2 -g -Wall -Wextra -Weffc++ -fmessage-length=0   -c -o DS.o DS.cpp
 g++ -O2 -g -Wall -Wextra -Weffc++ -fmessage-length=0   -c -o main.o main.cpp
 DS.h: In instantiation of ?DS<int>?:
 main.cpp:13:   instantiated from here
 DS.h:15: warning: ?class DS<int>? has pointer data members
 DS.h:15: warning:   but does not override ?DS<int>(const DS<int>&)?
 DS.h:15: warning:   or ?operator=(const DS<int>&)?
 g++ -o ds.exe Node.o DS.o main.o 
 main.o: In function `main':
 /cygdrive/c/Documents and Settings/ansrivastava/My Documents/src/ds/main.cpp:13: undefined reference to `DS<int>::SLL'
 /cygdrive/c/Documents and Settings/ansrivastava/My Documents/src/ds/main.cpp:13: undefined reference to `DS<int>::DS(int)'
 /cygdrive/c/Documents and Settings/ansrivastava/My Documents/src/ds/main.cpp:14: undefined reference to `DS<int>::~DS()'
 collect2: ld returned 1 exit status
 make: *** [ds.exe] Error 1

Now, if I remove all the code from DS.cpp and paste it into DS.h, everything compiles fine. Any idea what am I doing wrong?

Answer 1

Now, if I remove all the code from DS.cpp and paste it into DS.h, everything compiles fine. Any idea what am I doing wrong?

See this entry in the C++ FAQ about separate compilation.