I'm working my way through a C++ tutorial, and in one of the articles on the preprocessor/header files the author creates 2 'header files' (not the technical term, of course), example.h and example.cpp. In example.h he puts the forward declarations for the functions and the header guard, and in example.cpp the actual body of the functions. Why is this? I tried putting the body of the function in the example.h file and it worked just fine, so why does he put it separately? Is it customary, or does it pose problems in bigger problems? Or what?
Can someone please enlighten me on this...
C++ has a rule called the one-definition rule that says that every function needs to be defined once and only once (there are a few exceptions to this rule, but we'll ignore them for now.) The function prototype typically included in a header file is a declaration saying that the function exists, and the implementation in the .cpp file is the definition giving the code for the function.
If you put the definition of the function in a header file and then include the header file in multiple places, you'll get linker errors because you're breaking the one definition rule - each .cpp file that's compiled will have its own copy of the definition of the function. On the other hand, if you just put the declaration in the header and then put the definition in a single source file, then there's only one definition and nothing will break.
There are a few exceptions for the one definition rule. First, all inline functions are exempt from ODR, so you could potentially put function definitions in a header file if you mark all the functions
inline, though this is generally not considered a good idea for all but the smallest functions. Second, template functions are exempt from the ODR, which (among other things) is one of the reasons you see template functions defined in headers.