Linker error: "LNK2019: Unresolved external symbol" [duplicate] - c++

Question

This question already has answers here:
What is an undefined reference/unresolved external symbol error and how do I fix it?
(39 answers)
error LNK2019: unresolved external symbol
(2 answers)
Closed 9 years ago.
I am getting linker errors and can not make out the source of the problem. I hope you can help me! The error occurs already when creating an object of the class ClMyClass.
The two errors:
Error 18 error LNK2019: unresolved external symbol "public: virtual __thiscall ClMyClass::~ClMyClass(void)" (??1ClMyClass##UAE#XZ) referenced in function "void __cdecl Test1(void)" (?Test1##YAXXZ)
Error 19 error LNK2019: unresolved external symbol "public: __thiscall ClMyClass::ClMyClass(void)" (??0ClMyClass##QAE#XZ) referenced in function "void __cdecl Test1(void)" (?Test1##YAXXZ)
//Test.cpp
#include "StdAfx.h"
#include "MyClass.hpp"
void Test1(){
ClMyClass oHandle;
}
//ClMyClass.hpp
class ClMyClass: public ClMyClassBase
{
public:
ClMyClass(void);
virtual ~ClMyClass(void);
//methods...
}
//ClMyClass.cpp
ClMyClass::ClMyClass(void):ClMyClassBase(),m_nCallbackError(ERROR_NOT_INITIALIZED), m_eState(eUnknown)
{
//initialize members
}
ClMyClass::~ClMyClass(void)
{
Unassign();
}
EDIT: Added .cpp file

Answer 1

You haven't defined the constructor and destructor that you declared in your class ClMyClass.
ClMyClass::ClMyClass(){
//implementation
}
ClMyClass::~ClMyClass(){
//implementation
}
That's usually done in a separate .cpp file that contains all the class' member functions' implementations.

Answer 2

In your implementation you have not implement the virtual functions. If you use virtual functions, then you must implement all functions which are virtual, even if you don't use them. Otherwise the linker will have the unresolved symbol error you are seeing.
If a function is not used in a class, then will not get an error, because there is no reference to this function. Thus the linker will not see it and doesn't complain. For a virtual function this is different, because they are always generated.