To the best of my knowledge, the C++ standard knows no difference between header files and source files. As far as the language is concerned, any text file with legal code is the same as any other. However, although not illegal, including source files into your program will pretty much eliminate any advantages you would’ve got from separating your source files in the first place.
Essentially, what #include does is tell the preprocessor to take the entire file you’ve specified, and copy it into your active file before the compiler gets its hands on it. So when you include all the source files in your project together, there is fundamentally no difference between what you’ve done, and just making one huge source file without any separation at all.
“Oh, that’s no big deal. If it runs, it’s fine,” I hear you cry. And in a sense, you’d be correct. But right now you’re dealing with a tiny tiny little program, and a nice and relatively unencumbered CPU to compile it for you. You won’t always be so lucky.
If you ever delve into the realms of serious computer programming, you’ll be seeing projects with line counts that can reach millions, rather than dozens. That’s a lot of lines. And if you try to compile one of these on a modern desktop computer, it can take a matter of hours instead of seconds.
“Oh no! That sounds horrible! However can I prevent this dire fate?!” Unfortunately, there’s not much you can do about that. If it takes hours to compile, it takes hours to compile. But that only really matters the first time — once you’ve compiled it once, there’s no reason to compile it again.
Unless you change something.
Now, if you had two million lines of code merged together into one giant behemoth, and need to do a simple bug fix such as, say, x = y + 1, that means you have to compile all two million lines again in order to test this. And if you find out that you meant to do a x = y - 1 instead, then again, two million lines of compile are waiting for you. That’s many hours of time wasted that could be better spent doing anything else.
“But I hate being unproductive! If only there was some way to compile distinct parts of my codebase individually, and somehow link them together afterwards!” An excellent idea, in theory. But what if your program needs to know what’s going on in a different file? It’s impossible to completely separate your codebase unless you want to run a bunch of tiny tiny .exe files instead.
“But surely it must be possible! Programming sounds like pure torture otherwise! What if I found some way to separate interface from implementation? Say by taking just enough information from these distinct code segments to identify them to the rest of the program, and putting them in some sort of header file instead? And that way, I can use the #include preprocessor directive to bring in only the information necessary to compile!”
Hmm. You might be on to something there. Let me know how that works out for you.