VC++: Using DLLs as "subprograms"

Question

So I just began to try my hand at emulation after years of putting it off and not knowing where to start and I have managed to successfully write my first emulator! Now I am organizing my code in so that I can reuse the code to emulate other systems. I've been toying with the idea of having a shared frontend "platform handler" of sorts that I will compile as my executable whereas I will compile my emulated system code into dlls that the platform handler will use to identify what is available and instantiate from. This would allow me to separate my code into different projects and to leave the option open of using a bulkier front-end with more features or a streamlined "game only" and to share the same dlls between them rather than make two different solutions.

I know how to compile dlls vs executables but I don't know how to link the executable to the custom dll in such a way that I can instantiate a class from it. I'm not even sure what I'm trying to do is technically possible. Do the dll classes need to be static? I've never coded anything like this before or even done much with custom dlls so any help or ideas would be appreciated. I'm using Visual C++ 2010 by the way. Thanks in advance for any advice anyone may have.

This is well covered by the MSDN Library. Start reading here: http://msdn.microsoft.com/en-us/library/1ez7dh12.aspx — Hans Passant, Mar 16 '11 at 18:09

Dave Rager · Answer 1 · 2011-03-17T14:22:51.660

You don't really have to do much different. Just export your classes from the dll like you do for functions. In your app, include the header and link to the generated lib like you usually do. See this page: http://msdn.microsoft.com/en-us/library/81h27t8c%28v=vs.80%29.aspx

Example.h

#ifdef DLL_EXPORT
#define EXPORT_API __declspec(dllexport)
#else
#define EXPORT_API __declspec(dllimport)
#endif

class EXPORT_API Example
{
public:
    Example();
    ~Example();

    int SomeMethod();
};

int EXPORT_API ExampleFuncion();

Example.cpp

#include "Example.h"

Example::Example()
{
    // construct stuff
}

Example::~Example()
{
    // destruct stuff
}

int Example::SomeMethod()
{
    // do stuff
    return 0;
}

int EXPORT_API ExampleFunction()
{
    return 0;
}

In your dll project, define DLL_EXPORT and build. You will get a .lib and .dll output. In your main project where you will be using the dll you do not have to do anything except include the header and link against the .lib. Do not define the DLL_EXPORT symbol in your main project and be sure the .dll is somewhere your application can find it.

If you really want to get clever, this problem is screaming for the factory design pattern. If you design your interface well enough, you can have your dlls register their implementation with your application when they are loaded. You can extend forever without even rebuilding your main executable.

That does sound a lot like what I want to do. Is this OS specific? So far my application is cross-platform friendly and I really want to keep it that way. I get nervous when I am pointed in the direction of the MSDN albeit a good resource regardless. — user663000, Mar 16 '11 at 18:38
No, I've done it on both Windows and Linux. I assume it works on any platform that supports loading shared objects. IIRC on Linux you don't have to do anything special, just create the shared object (dll) and link against it. On Windows you have to specifically declare the class as exported as described in the link. I don't have a code snippet handy and I hesitate to make one up from memory because I'll likely miss something. Have a look at dllimport/dllexport on google. I'll check back when I get home from work and try to provide a better example. — Dave Rager, Mar 16 '11 at 18:50
Wow thank you so much! This looks to be exactly how I want to do things. The factory method sounds like the perfect way to set up the platform handler too. Time to dust off the ol' design patterns book and get coding! — user663000, Mar 17 '11 at 14:43

VC++: Using DLLs as "subprograms"

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