Pros and cons of using dlls

Question

I wonder about create every game engine module (render, input, sound, etc...) into dlls (renderer.dll, input.dll, etc...). What are pros and cons in your opinion of divide game into set of dlls ? I see only need to write game module in separate dll, to have posibility to create mods.

Answer 1

Putting code into seperate Windows DLLs sounds promosing in theory, but in practice you should be aware of certain limitations.

1. You have to declare all your classes and global functions you want to use from outside as "exportable" using

   __declspec( dllexport ) class Foo {};
   

   when building the library. Clients of your DLL must "see" the include like this:

   __declspec( dllimport ) class Foo {};
   

   That is why you see a makro infront of class declarations in DLL headers.

2. Resources like FILE* pointers can not be shared under all circumstances (different CRTs) between DLLs. That means if you open the file in one DLL, you cannot write to it from the client app or another DLL, nor close it.

   http://msdn.microsoft.com/en-us/library/ms235460%28VS.80%29.aspx

3. Memory management: You cannot allocate memory in the DLL and free it in another DLL or in client application under all circumstances (different CRTs).

4. Exporting template classes can be a real pain, especially if they use static member variables.

If you just want to reuse your code without recompiling, there is another option, static libraries (.lib). It is precompiled code (as in the DLL) but it is linked into your application and has no such limitations as the ones mentioned above, plus Link Time Code Generation still works with the library code.

---

Update: As Sean pointed out in the comments, point 2 and 3 can work when using the same CRT version, as the MSDN link in 2 also shows. However, my advice is to design the DLL without relying on it at all, since there are so many cases where it won't work, like using 2 DLLs with different CRT's or compiled with different Visual Studio versions.

Answer 2

Pros

• You can build DLLs separately.
• It could be faster to re-build one DLL

Cons

• Calling code from DLL is slower
• It would be slower to re-build hole project with all DLLs
• Function names are visible. It is easier to reverse code that uses dynamic DLLs

I do not see sense to have render.dll and input.dll But some RareUsedJoystick.DLL could be made as loadable module to release memory for those who do not use that RareUsedJoystick

Answer 3

Most differences are negligible. DLL's are more flexible but make the building process more complex. I would only use DLL's if:

• Multiple applications need to access the same functionality. For example if you have separate binaries for a editor and the game itself.
• You want yourself or other people be able to replace certain components. For example in Civ IV modders could ship replacement for the game's DLL's to fundamentally change parts of the gameplay.
• You want to load them at runtime to allow to extend the application by third party plugins.

Answer 4

Another advantage is that it can help you structure your code correctly. Any non-logical dependencies between modules will stand out more and it will be more difficult to use such hacks.