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.)