VC++ MFC Tutorial: Create MFC Extension Dll, Export MFC Classes

VC++ MFC Tutorial: Create MFC Extension Dll, Export MFC Classes
 
By Steve Driessens.  How to simplify importing and exporting classes from an extension DLL
Exporting C++ classes from extension DLLs and importing those classes into applications can be a little confusing at times.  This article discusses one of many ways to simplify this.  Also discussed is a technique to ensure that your DLL's.LIB file is automatically linked into any application (or other DLL) using your DLL, avoiding the need to alter your project link settings.
When building an extension DLL, you want the compiler/linker to export selected C++ classes, but when building your application you want to import those classes.
Traditionally, this has been done by using the  AFX_CLASS_EXPORT  and  AFX_CLASS_IMPORT  defines (defined in afxv_dll.h). Swapping these  #define s in and out depending on whether you're building the DLL itself or building an application (or another DLL) which uses your exported classes.
If we look at how   AFX_CLASS_EXPORT  and  AFX_CLASS_IMPORT  are defined in afxv_dll.h we see the following.

#define AFX_CLASS_EXPORT __declspec(dllexport)
#define AFX_CLASS_IMPORT __declspec(dllimport)

So, when exporting our classes from our DLL we want the class declarations from the DLL to look like this:-

class __declspec(dllexport) CMyClass : public CObject
{
	...
}

And, when importing our C++ classes into our application we want the class declarations from the DLL to look like this:-

class __declspec(dllimport) CMyClass : public CObject
{
	...
}

OK, so here's how I do things.
In the  stdafx.h  file for the export DLL, include two  #define s at the bottom of the file like this:-

#define _MYLIB_DLLAPI_
#define _MYLIB_NOAUTOLIB_

Now, in the main header file for your DLL, say  mylib.h  (the main 'point of entry' header for your DLL that you will include in you application later), add the following at the top:-

// The following will ensure that we are exporting our C++ classes when 
// building the DLL and importing the classes when build an application 
// using this DLL.

#ifdef _MYLIB_DLLAPI_
    #define MYLIB_DLLAPI  __declspec( dllexport )
#else
    #define MYLIB_DLLAPI  __declspec( dllimport )
#endif

// The following will ensure that when building an application (or another
// DLL) using this DLL, the appropriate .LIB file will automatically be used
// when linking.

#ifndef _MYLIB_NOAUTOLIB_
#ifdef _DEBUG
#pragma comment(lib, "mylibd.lib")
#else
#pragma comment(lib, "mylib.lib")
#endif
#endif

Now, just declare all the C++ classes you want exported from the DLL like this:- (Note: Any C++ classes not declared with  MYLIB_DLLAPI  will not be exported from the DLL)

class MYLIB_DLLAPI CMyClass : public CObject
{
	...
}

So, how does it work? 

When building your DLL,  _MYLIB_DLLAPI_  is defined in the DLL's  stdafx.h  file, so  MYLIB_DLLAPI  is then defined as  __declspec( dllexport )  and your C++ classes will be exported.
When building your application,  _MYLIB_DLLAPI_  isn't defined, so  MYLIB_DLLAPI  will be defined as  __declspec( dllimport )  and your classes will be imported.
The other nifty part is the  _MYLIB_NOAUTOLIB_ .  If  _MYLIB_NOAUTOLIB_  isn't defined, (i.e. when building your application), an entry like  #pragma comment(lib, "mylibd.lib")  appears which tells the linker to automatically link in your DLL's .LIB file.  Hence, there's no need to add the .LIB file to the Object/library modules section in your application project link settings (something I invariable forgot to do!).
The above is basically a 'set and forget' technique.  All you'll ever need to do to use you extension DLL is just include it's header in your application, and all the ugly class export/import stuff is sorted for you.
(I can't remember where I picked up this technique originally, but full credit to it's originator as it's proved invaluable over the years.)
From: http://www.ucancode.net/Visual_C_MFC_Example/MFC-Extension-Dll-Export-VC-Classes.htm