Question

I am working on a plugin system in C++ whereby a C++ executable loads a dll and runs plugin_start(someclass&) via GetProcAddress.

I fully understand how to pass function pointers to the dll, and visa versa, and how the dll may use anything defined in a header file, but I would like the dll to be able to use someclass where someclass is declared in someclass.h BUT DEFINED in someclass.cpp.

The catch is, someclass is compiled into the calling executable which means when the dll tries to call a function it gets a linker error. I even understand why this is, what I don’t understand is how to achieve what I want.

I imagine I can pass a pointer to the object, and a pointer to the function ie someclass* somefunction* and then call it as someclass->*somefunction() but this means I would have to pass a pointer to every function in every class.

Is there an easier way to do this, or should I stick to C-style functions and function pointers alone and forget trying to pass entire classes between the two?

Thanks,
Ben

#ifndef EVENTREGISTRAR_H
#define EVENTREGISTRAR_H
#include <vector>

typedef void (__stdcall *error_callback_t)(const char *error);

class EventRegistrar
{

public:
    void OnError(error_callback_t fn);
    void FireError(const char *error);


private:
    std::vector<error_callback_t> errors;

};
#endif

— Cpp

#include "PluginLoader.h"
void EventRegistrar::OnError(error_callback_t fn)
{
    this->errors.push_back(fn);
}

void EventRegistrar::FireError(const char *error)
{
    for (std::vector<error_callback_t>::iterator it = this->errors.begin();
        it != this->errors.end(); ++it)
    {
        (*it)(error);
    }
}

— DLL

#include "../plugin.h"
#include <stdio.h>
void __stdcall error(const char *error) { printf("Error: %s\n",error); }
extern "C" int __stdcall plugin_start(plugin_start_data& data)
{
    error_callback_t fn = error;
    data.events.OnError(fn);
    return LOAD_SUCCESS;
}

–Error

Error   1   error LNK2001: unresolved external symbol "public: void __thiscall EventRegistrar::OnError(void (__stdcall*)(char const *))" (?OnError@EventRegistrar@@QAEXP6GXPBD@Z@Z) D:\Files\C++ Workspace\BLib\BLib\Example Plugin\main.obj    Example Plugin

Answer 1

I did something like this a long time ago. I simply used a straight C interface to keep things simple.

There may be a better way but I think passing a pointer to the object is your best and most straight-forward approach.