How to fix LNK2019 unresolved external symbol - c++

Question

I have a very simple threading class I'm using for a project. I've just begin progress on it but I'm stuck because of a LNK2019 error I can't figure out how to fix. I've narrowed down the problem to a single line. Perhaps someone can help guide me on what I need to do to fix it.
The following is the class I'm making:
#ifndef __THREADING_H
#define __THREADING_H
#include <Windows.h>
class Threading {
public:
virtual void run() = 0;
void start();
void stop();
bool isStopped();
void cleanup();
private:
bool stopped;
HANDLE reference;
static DWORD WINAPI start_helperfunction(LPVOID ptr);
};
#endif // __THREADING_H
And the line where I'm getting the error is by 2nd line of the start_helperfunction, with the Threading::start_helperfunction below:
#include "Threading.h"
void Threading::start()
{
stopped = false;
reference = CreateThread(NULL, 0, Threading::start_helperfunction, this, NULL, NULL);
}
Finally the error message I'm getting is:
error LNK2019: unresolved external symbol "private: static unsigned long __stdcall Threading::start_helperfunction(void *)" (?start_helperfunction#Threading##CGKPAX#Z) referenced in function "public: void __thiscall Threading::start(void)" (?start#Threading##QAEXXZ)
I'm not sure what I've done wrong or what to try. I'm sure it's a simple fix. I'm not the most experienced in C++.

Answer 1

You didn't implement start_helperfunction, so the linker can't find it. You need to actually write a static member function with that name. The simplest possible one might look like this:
DWORD WINAPI Threading::start_helperfunction(LPVOID ptr)
{
return 0;
}