Linker error when trying to link to a dll - c++

Question

I have a Dll Which has 4 files Header files-1.StreamReader.h 2.StreamWriter.h Source Files-StreamReader.cpp 2.StreamWriter.cpp in windows VS2008.
StreamReader.h
Class StreamReader{
public:
static __declspec(dllexport) StreamReader* GetInstance();
//other functions
private:
StreamReader(){}
~StreamReader(){}
static StreamReader *m_pInstance;
};
StreamReader.cpp
StreamReader *StreamReader::m_pInstance=NULL;
StreamReader *StreamReader::GetInstance()
{
return((m_pInstance==NULL)?m_pInstance=new StreamReader:m_PInstance);
}
//other functions
I have a very similar structure for StreamWriter and a GetInstance() there as well.
When I link to this dll statically in a exe it complains during compilation
error LNK2001: unresolved external symbol "private: static class
StreamReader * StreamReader::m_pInstance"
(?m_pInstance#StreamReader##0PAV1#A)
error LNK2019: unresolved
external symbol "public: static class StreamReader * __cdecl
StreamReader::GetInstance(void)"
(?GetInstance#StreamReader##SAPAV1#XZ) referenced in function _main
The exe is also written in c++. But it can find StreamWriter symbol. The StreamWriter and StreamReader files are essentially the same except for the fact that one reads and one writes to a file. What am i missing? Thank you

Answer 1

Make sure the .cpp files are actually in the project and not just in the folder. That way, the compiler knows to compile them and produce the object files that the linker is looking for.

Answer 2

When you import a dll you need to have dllimport declared.
You have
static __declspec(dllexport) StreamReader* GetInstance();
you need
static __declspec(dllimport) StreamReader* GetInstance();
you can use the same header file for this by using a preprocessor definition
#ifdef _EXPORTING
#define CLASS_DECLSPEC __declspec(dllexport)
#else
#define CLASS_DECLSPEC __declspec(dllimport)
#endif
static CLASS_DECLSPEC StreamReader* GetInstance();
make sure to add _EXPORTING preprocessor definition to the project that exports the DLL