Dynamicly load DLL classes

[DJMaze]

In all my years of coding i never dynamicly loaded classes from a DLL.
However the last weeks i've discovered this is a "must have" in certain circumstances.

For example i wrote a MP3 player with a flat database system, but somehow my users pushed it over the limit by adding 100.000+ music files in the database which made it very slow. So i needed to figure out a way how to control a database in a server/client behavior in many ways like SQLite, MySQL, Postgre and flatfile.
Now there's an issue because each database engine has his own function calls but your program only wants to load one of them but don't want the overhead of everything in your system.

The solution is DLL's and SO's with functions. But functions are limited and don't allow multiple instances easily unless you identify each so i've decided to use classes in libraries.

But how do you export them ?

Actualy i've found a good examples on the internet which made my task complete. The example was called 'expclass' but i totally forgot where i found it so credits to author will be attached when i find it again

But i have an problem, i can't add attachments in here so could i have permission or something ?

[Locutus]

What is your actual question?

Dynamically loading C++ classes isn't really something you want to do directly, due to "name-mangling", among other things. However, there is another way.

What you generally do is to create functions that create and delete an instance of your class.

Suppose you've got a class "MyClass"

extern "C" MyClass *create(int some_parm)
{
    return new MyClass(some_parm);
}

extern "C" void destroy(MyClass *p)
{
    delete p;
}

The "destroy" function is needed because you can't guarantee that the implementation of new/delete in the application and the plugin are compatible.

Of course when writing plugins, you usually derive from a common base class for all your plugins, so you'd get something more like this:

extern "C" MyBaseClass *create(int some_parm)
{
    return new MyClass(some_parm);
}

extern "C" void destroy(MyBaseClass *p)
{
    delete p;
}

Most of the time people wrap all that into a macro, so all you'd have to do in the implementation of your plugin would be something like:

EXPORT_PLUGIN(MyClass)

http://www.tldp.org/HOWTO/C++-dlopen/ has some more information on the subject.