Dll compatibility between compilers

Question

Is there some way to make c++ dlls built with diffrent compilers compatible with each other? The classes can have factory methods for creation and destruction, so each compiler can use its own new/delete (since diffrent runtimes have there own heaps).

I tried the following code but it crashed on the first member method:

interface.h

#pragma once

class IRefCounted
{
public:
    virtual ~IRefCounted(){}
    virtual void AddRef()=0;
    virtual void Release()=0;
};
class IClass : public IRefCounted
{
public:
    virtual ~IClass(){}
    virtual void PrintSomething()=0;
};

test.cpp compiled with VC9, test.exe

#include "interface.h"

#include <iostream>
#include <windows.h>

int main()
{
    HMODULE dll;
    IClass* (*method)(void);
    IClass *dllclass;

    std::cout << "Loading a.dll\n";
    dll = LoadLibraryW(L"a.dll");
    method = (IClass* (*)(void))GetProcAddress(dll, "CreateClass");
    dllclass = method();//works
    dllclass->PrintSomething();//crash: Access violation writing location 0x00000004
    dllclass->Release();
    FreeLibrary(dll);

    std::cout << "Done, press enter to exit." << std::endl;
    std::cin.get();
    return 0;
}

a.cpp compiled with g++ g++.exe -shared c.cpp -o c.dll

#include "interface.h"
#include <iostream>

class A : public IClass
{
    unsigned refCnt;
public:
    A():refCnt(1){}
    virtual ~A()
    {
        if(refCnt)throw "Object deleted while refCnt non-zero!";
        std::cout << "Bye from A.\n";
    }
    virtual void AddRef()
    {
        ++refCnt;
    }
    virtual void Release()
    {
        if(!--refCnt)
            delete this;
    }

    virtual void PrintSomething()
    {
        std::cout << "Hello World from A!" << std::endl;
    }
};

extern "C" __declspec(dllexport) IClass* CreateClass()
{
    return new A();
}

EDIT: I added the following line to the GCC CreateClass method, the text was correctly printed to the console, so its defenatly the function call thats killing it.

std::cout << "C.DLL Create Class" << std::endl;

I was wondering, how does COM manage to maintain binary compatibility even across languages, since its basicly all classes with inheritence (although only single) and therefore virtual functions. I'm not massivly bothered if I cant have overloaded operators/functions as long as I can maintain the basic OOP stuff (ie classes and single inheritence).

Answer 1

You can as long as you only use extern "C" functions.

This is because the "C" ABI is well defined, while the C++ ABI is deliberately not defined. Thus each compiler is allowed to define its own.

In some compilers the C++ ABI between different versions of the compiler or even with different flags will generate an incompatable ABI.