unresolved external symbol in C++ DLL project - c++

Question

can I just start by saying I appreciate you taking your time to go through my question and attempting to help. However I have already attempted the solution suggested on here and on here and they haven't worked for me.
This is my problem:
I am attempting to create a serial port class as a VS12 DLL project. I have a header file "SerialDll.h" which is included in my c++ source file "SerialDll.cpp". When I try to build the solution in visual studio 2012, i get the errors below:
Error 11 error LNK1120: 1 unresolved externals C:\Sprint 7\SerialDll\Debug\SerialDll.dll 1 1 SerialDll
Error 10 error LNK2001: unresolved external symbol "__declspec(dllimport) private: static void * MySerial::MySerialPort::serial_port_handle" (__imp_?serial_port_handle#MySerialPort#MySerial##0PAXA) C:\Sprint 7\SerialDll\SerialDll\SerialDll.obj SerialDll
When I try implementing John Zwinck's Solution, this is the error i get:
Error 2 error C2491: 'MySerial::MySerialPort::serial_port_handle' : definition of dllimport static data member not allowed c:\sprint 7\serialdll\serialdll\serialdll.cpp 16 1 SerialDll
This is the code in my header file:
#include <Windows.h>
#ifdef SERIAL_DLL_EXPORTS
#define SERIAL_DLL_API __declspec(dllexport)
#else
#define SERIAL_DLL_API __declspec(dllimport)
#endif
namespace MySerial
{
class MySerialPort
{
private:
static SERIAL_DLL_API HANDLE serial_port_handle;
public:
SERIAL_DLL_API MySerialPort();
SERIAL_DLL_API ~MySerialPort();
};
}
This is the code in my c++ source file, with John Zwinck's solution:
#include "stdafx.h"
#include "SerialDll.h"
#include <stdexcept>
#include <iostream>
using namespace std;
namespace MySerial
{
HANDLE MySerialPort::serial_port_handle;
MySerialPort::MySerialPort()
{
serial_port_handle = INVALID_HANDLE_VALUE;
}
MySerialPort::~MySerialPort()
{
if(serial_port_handle != INVALID_HANDLE_VALUE)
{
CloseHandle(serial_port_handle);
}
serial_port_handle = INVALID_HANDLE_VALUE;
}
}
Hope you guys can help me with a solution or at least refer me to a link with a working solution.
Cheers!

Answer 1

The answer is exactly the same as this answer to the previous question you linked:
https://stackoverflow.com/a/17902142/4323
That is, you have only declared, but not allocated storage for, your static member. You need to add this to your implementation file:
namespace MySerial
{
HANDLE MySerialPort::serial_port_handle;
}

Answer 2

If you are looking to export the class outside the DLL, then you need to use __declspec for the class, and not for each member function/variable. (See http://msdn.microsoft.com/en-us//library/a90k134d.aspx )
Your header file needs to look like :
namespace MySerial
{
class SERIAL_DLL_API MySerialPort
{
private:
static HANDLE serial_port_handle;
public:
MySerialPort();
~MySerialPort();
};
}