C++ – C\C++ – Re-using functions across multiple programs

const is pointless when the argument is passed by value since you will not be modifying the caller's object.

Wrong.

It's about self-documenting your code and your assumptions.

If your code has many people working on it and your functions are non-trivial then you should mark const any and everything that you can. When writing industrial-strength code, you should always assume that your coworkers are psychopaths trying to get you any way they can (especially since it's often yourself in the future).

Besides, as somebody mentioned earlier, it might help the compiler optimize things a bit (though it's a long shot).

You seem to be asking about separating definitions from declarations, although there are other uses for header files.

The answer is that C++ doesn't "need" this. If you mark everything inline (which is automatic anyway for member functions defined in a class definition), then there is no need for the separation. You can just define everything in the header files.

The reasons you might want to separate are:

1. To improve build times.
2. To link against code without having the source for the definitions.
3. To avoid marking everything "inline".

If your more general question is, "why isn't C++ identical to Java?", then I have to ask, "why are you writing C++ instead of Java?" ;-p

More seriously, though, the reason is that the C++ compiler can't just reach into another translation unit and figure out how to use its symbols, in the way that javac can and does. The header file is needed to declare to the compiler what it can expect to be available at link time.

So #include is a straight textual substitution. If you define everything in header files, the preprocessor ends up creating an enormous copy and paste of every source file in your project, and feeding that into the compiler. The fact that the C++ standard was ratified in 1998 has nothing to do with this, it's the fact that the compilation environment for C++ is based so closely on that of C.

Converting my comments to answer your follow-up question:

How does the compiler find the .cpp file with the code in it

It doesn't, at least not at the time it compiles the code that used the header file. The functions you're linking against don't even need to have been written yet, never mind the compiler knowing what .cpp file they'll be in. Everything the calling code needs to know at compile time is expressed in the function declaration. At link time you will provide a list of .o files, or static or dynamic libraries, and the header in effect is a promise that the definitions of the functions will be in there somewhere.