Using classes exported from dlls

Download Source Files - 9.2 KB

Introduction

If you try to dynamically use a class exported from a .dll sometimes you get an ugly error because you try to release memory allocated by some other heap manager. For example, you create the object using a factory method and release it calling delete directly. In this case the heap is allocated by the .dll's heap manager and the .exe's heap manager throws an exception when you try to release it.

There are two methods to counter this problem. The easiest is to set Project -> Properties -> Code Generation -> Runtime Library to Multi-threaded Debug DLL both in the .exe and in the .dll.

If you can't do this for whatever reason you have a second option: you must free the memory in the same module (.dll or .exe) in which you have allocated it. The following code shows how to do it:

// d1.dll -> a1.h
#pragma once

#ifdef D1_EXPORTS
#define D1API __declspec(dllexport)
#else
#define D1API __declspec(dllimport)
#endif

class D1API A1
{
public:
    int x;
    static A1* New();
    static void Delete(A1* a1);
};

// d1.dll -> a1.cpp
#include "a1.h"

A1* A1::New()
{
    return new A1;
}

void A1::Delete(A1* a1)
{
    delete a1;
}

// e1.exe -> e1.cpp
#include "../d1/A1.h"

int main()
{
    A1 a1;
    a1.x = 10;

    A1* a2 = new A1; // allocates the memory using e1.exe-s heap manager
    a2->x = 20;
    delete a2; // correct, deletes the memory using e1.exe-s heap manager
    
    A1* a3 = A1::New(); // allocates the memory using d1.dll-s heap manager
    a3->x = 30;
    A1::Delete(a3); // correct, deletes the memory using d1.dll-s heap
                    // manager

    A1* a4 = new A1;
    a4->x = 40;
    A1::Delete(a4); // correct only if Runtime Library is set to 
                    //Multi-threaded Debug DLL

    A1* a5 = A1::New();
    a5->x = 50;
    delete a5; // correct only if Runtime Library is set to Multi-threaded 
               // Debug DLL

    return 0;
}