Exporting a C++ class from a DLL

Adding a simple working example for exporting a C++ class from a DLL :

The given below example gives you only a short overview of how dll and exe can interact each other (self explanatory ) but it needs more things to add for changing into a production code.

Full sample example is divided in to two part

A. Creating a .dll library (MyDLL.dll)

B. Creating an Application which uses .dll library (Application).

A. .dll project file (MyDLL.dll):

1. dllHeader.h

#ifdef  MYDLL_EXPORTS 
#define DLLCALL __declspec(dllexport)   /* Should be enabled before compiling 
                                           .dll project for creating .dll*/
#else
#define DLLCALL __declspec(dllimport)  /* Should be enabled in Application side
                                          for using already created .dll*/
#endif

// Interface Class
class ImyMath {
public:
    virtual ~ImyMath() {;}
    virtual int Add(int a, int b) = 0;
    virtual int Subtract(int a, int b) = 0;
};

// Concrete Class
class MyMath: public ImyMath {
public:
    MyMath() {}
    int Add(int a, int b);
    int Subtract(int a, int b);
    int a,b;
};

//  Factory function that will return the new object instance. (Only function
//  should be declared with DLLCALL)
extern "C" /*Important for avoiding Name decoration*/
{
    DLLCALL ImyMath* _cdecl CreateMathObject();
};

// Function Pointer Declaration of CreateMathObject() [Entry Point Function]
typedef ImyMath* (*CREATE_MATH) ();

2. dllSrc.cpp

#include "dllHeader.h"

// Create Object
DLLCALL ImyMath* _cdecl CreateMathObject() {
    return new MyMath();
}

int MyMath::Add(int a, int b) {
    return a+b;
}

int MyMath::Subtract(int a, int b) {
    return a-b;
}

B. Application Project which load and link the already created .dll file:

 #include 
#include 
#include "dllHeader.h"

int main()
{
    HINSTANCE hDLL = LoadLibrary(L"MyDLL.dll"); // L".\Debug\MyDLL.dll"

    if (hDLL == NULL) {
        std::cout << "Failed to load library.\n";
    }
    else {
        CREATE_MATH pEntryFunction = (CREATE_MATH)GetProcAddress(hDLL,"CreateMathObject");
        ImyMath* pMath = pEntryFunction();
        if (pMath) {
            std::cout << "10+10=" << pMath->Add(10, 10) << std::endl;
            std::cout << "50-10=" << pMath->Subtract(50, 10) << std::endl;
        }
        FreeLibrary(hDLL);
    }
    std::cin.get();
    return 0;
}