C++ Visual Studio Windows: how to declare but not define extern function in dll - c++

Question

I have a project that is compiled into a library and declares a certain function to be implemented by the user of the library:
//To be defined by user
Application* CreateApplication();
When compiling the code into a shared library on Linux this works perfectly. Any user of the library can define an implementation for the declared function and it can be used inside the library. If the user of the library forgets to define an implementation, they will get an error pointing this out.
I'm now in the process of porting the library to Windows, where it is supposed to be compiled into a dll. However, I'm running into problems as the linker used by Visual Studio is complaining:
unresolved external symbol Application* __cdecl CreateApplication(void)
I tried adding the extern keyword to indicate that the definition of the function is somewhere else, but this didn't work.
Why can't I declare (but not define) a function in a dll like this? How should I fix my code so it works both on Linux and on Windows?

Answer 1

What you are attempting to do only works in a static library, it cannot work in a dynamic library like a DLL. For that, you will have to change the code to use a function pointer instead. The application that is using the DLL can pass in the address of the desired function from its own code, and the DLL can then assign that address to a variable that it uses as needed, eg:
HEADER:
#ifndef MYLIB_H
#ifndef MYLIB_H
#ifdef COMPILING_MY_LIB
#define MY_EXPORT __declspec(dllexport)
#else
#define MY_EXPORT __declspec(dllimport)
#endif
// declare Application as needed...
typedef Application (*lpCreateApplicationFunc)();
#ifdef __cplusplus
extern "C" {
#endif
MY_EXPORT void SetCreateApplicationFunc(lpCreateApplicationFunc func);
#ifdef __cplusplus
}
#endif
#endif
DLL:
#define COMPILING_MY_LIB
#include "MyLib.h"
//To be defined by user
lpCreateApplicationFunc CreateApplication = NULL;
void SetCreateApplicationFunc(lpCreateApplicationFunc func)
{
CreateApplication = func;
}
void doSomething()
{
Application *app = NULL;
if (CreateApplication)
app = (*CreateApplication)();
if (app)
{
...
}
}
EXE:
#include "MyLib.h"
Application MyCreateApplicationFunc()
{
...
}
// during startup, call this...
SetCreateApplicationFunc(&MyCreateApplicationFunc);