I have a project where my files have gotten out of hand because of the volume of them. I have started to combine classes of similar types in single header files because you can, and it still is good practice, now can i do the them same for those classes functions, that are declared in the source files.
Is it a good idea to combine several classes functions in a single source file, if the classes are say all sub classes of the same super class?
It is not bad practice to do so, just make sure that if the user only want a part of the system, he can include only one header and get the necessary bits. Having several definitions of class and other things in one header is fine and can even help compilation time if well done. If not well done, it can kill the build time in a bad way. So, just make sure each header gather elements that cannot be separated and that represent one (sub)module.
Look at readable C++ standard library implementation (most are not, depends on the provider) and/or better, look at boost code as good examples of this practice.
I have started to combine classes of similar types in single header files..
It is good approach to implement library, but If it is your implementing client does not require the another extra classes for implementation. for example your header file contain 1 million lines of code after integrating similar type of classed then it took a long time to build.
As per my experience with library design and the standards that i follow in Microsoft technology Library Design Guidelines. In case of .Net platform it does not matter much, but In case of c++ that i got on the apple development Dynamic Library Design Guidelines is that library should maintain these following factor of implementation.
Try to make your header files smaller and concise so that it will be easy to understand and increase the compile time. Sometime i prefer to have single class in single file, some experts suggest to follow this approach. You can combine classes those are much coupled to each other in single file.
Ref:
Multiple classes in a header file vs. a single header file per class
Google C++ Style Guide
