Export a function from a DLL

Question

I have a DLL where I have:

extern "C" __declspec(dllexport) CBase* __cdecl CreateInstance(const char* type)
{
    if ("Derived1" == type)
        return new CDerived1();
    if ("Derived2" == type)
        return new CDerived2();

    return nullptr;
}

and in a console app, I am trying to use this function, loaded manually:

#include "MyDll.h"

    typedef CBase* (*FACTORY)(const char* type);
    HINSTANCE hInstance = LoadLibrary(_T("My.dll"));
    if (nullptr != hInstance)
    {
        FACTORY pFactory = (FACTORY)GetProcAddress(hInstance, "CreateInstance");
        if (pFactory)
        {
            pFactory->SomeMethod();
            pFactory->OtherMethod();
        }
        FreeLibrary(hInstance);
    }

I got:

error C2227: left of '->SomeMethod' must point to class/struct/union/generic type

What I have done wrong ?

Answer 1

Try something like this:

CDerived1 * d = (CDerived1*)pFactory("Derived1");
d->SomeMethod( );

In addition, use strcmp, stricmp, etc. instead of “==”.

Answer 2

I modified the code like this (the code is obvious, but because of late hours I missed):

    FACTORY pFactory1 = (FACTORY)GetProcAddress(hInstance, "CreateInstance");
    if (pFactory1)
    {
        CDerived1* pDerived1 = (CDerived1*)pFactory1("Derived1");
        if (pDerived1)
        {
            pDerived1->SomeMethod();
            pDerived1->Destroy();
        }
    }
    FACTORY pFactory2 = (FACTORY)GetProcAddress(hInstance, "CreateInstance");
    if (pFactory2)
    {
        CDerived2* pDerived2 = (CDerived2*)pFactory2("Derived2");
        if (pDerived2)
        {
            pDerived2->SomeMethod();
            pDerived2->Destroy();    // assert: File: minkernel\crts\ucrt\src\appcrt\heap\debug_heap.cpp - Expression: __acrt_first_block == header
        }
    }

And Destroy() is defined in My.dll:

void Destroy() { delete this; }

Why I got that error ?

Answer 3

This is the code from DLL:

class CBase
{
private:
    int object_id;
    std::string type;

public:
    CBase(const char* type)
    {
        this->type = type;
    }
    ~CBase() {}
    virtual void SomeMethod()
    {
        printf("CBase::SomeMethod");
    }
};

class CDerived1 : public CBase
{
public:
    explicit CDerived1(const char* type)
        :CBase(type)
    {
    }
    virtual void SomeMethod()
    {
        CBase::SomeMethod();
    }
    void Destroy() { delete this; }
};

class CDerived2 : public CBase
{
public:
    explicit CDerived2(const char* type)
        :CBase(type)
    {
    }
    virtual void SomeMethod()
    {
        CBase::SomeMethod();
    }
    void Destroy() { delete this; }
};

extern "C" __declspec(dllexport) CBase* __cdecl CreateInstance(const char* type)
{
     if ("Derived1" == type)
         return new CDerived1();
     if ("Derived2" == type)
         return new CDerived2();

     return nullptr;
}