Exporting C++ Classes from an MFC 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 #defines 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 #defines 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.)