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Code Snippet - DLL Class Exports
Author:Arkon
Category:Win32API
File Size:~ 1.93 KB
Uploaded at:27-Nov-02 02:54:21 pm
Description:
  This is a way to implement a really small COM like model, check it out! Shows how to "export" classes from DLLs.
  
Well, if you mess with DLLs alot,
I guess you once wanted to export a class as well as functions.

OK, infact there is a nice trick which is to create an instance of the class in the DLL itself
and pass a pointer to the DLL loader.

Now let's get into depth,
every function you want to export in the class, you'll have to make it a virtual function, thus,
the DLL loader will "find" the code of the function in the virtual table of that instance.

The DLL would be something like this for example:

Class1.cpp:
-----------
Class1::Class1()
{
. // You get the idea
.
.
}

void Class1::SetData(int nx)...
void Class1::ShowData()...
Class1::~Class1()...

// Now this is the most important function:
__declspec(dllexport) Class1* MakeClass1Ptr()
{// Return an instance of the Class1, ofcourse don't forget to delete it by yourself!
 return new Class1;
}

Class1.h:
---------
class Class1
{
private:
 int x;
public:
 Class1();
 virtual void SetData(int nx);
 virtual void ShowData();
 virtual ~Class1(); // Yes, dtor can(and should) be a virtual too!
};

Hey, don't forget to export the MakeClass1Ptr function in your DLL's .def file!

Now Loader time:
#include "class1.h" // You'll have to include it, in order to know how the class is built!

HMODULE hDLL = LoadLibrary("Class.dll"); // Loading DLL

typedef Class1* (*MakeClass1ptr)(void);
MakeClass1ptr ptr = (MakeClass1ptr)GetProcAddress(hDLL, "MakeClass1Ptr"); // Getting a pointer to function

Class1* c1;
c1 = ptr(); // Calling the function and getting the pointer to the instance now.

c1->SetData(0xd11c1a77); // Use the virtual functions...
c1->ShowData(); // Ditto

delete c1; // Destroy the class

FreeLibrary(hDLL); // Unload DLL

That was all about it...
It's a nice trick, there are a few more ways
but I chose to show this one, because it's the easiest in my opinion...

VC 6.0 Source Code - http://qsoft.ragestorm.com/tutors/windows/dllclass.zip
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