Are .h files treated differently by the compiler or "just" a naming convention?

Question

I'm new to C family programming and compiling. I'm trying to understand what .c/.cpp files vs .h files are on a low technical level. I understand .h files are used to specify an interface and .c or .cpp files for implementation. However I'm wondering if this difference is reflected in how the compiler works or if it is "just" a naming convention to simplify for us humans? Could you in theory have the implemntation in a .h file and still have it compile? Or specify an interface in a .c file? I'm asking to better understand what the compiler actually does.

Answer 1

Technically, there is no difference between any extension as far as the compiler is concerned. It's same as if you had typed manually the contents of any file in place where you put the #include. You can type #include "foo.pdf" and the compiler will include a file foo.pdf successfully, as long as that file contains code (despite the extension)

By convention now, one usually puts declarations in .h/hpp files (or template definitions) and the implementation in .c/cpp files.

Many libraries have a single-file implementation, with the aid of inline variables/functions as well.

Sometimes also the include file does not even exist, for example when including a standard STL file (like string), the compiler might not read the file at all, but cache it/implement it however it wants.

More info on #include in MSDN and CPPReference.

Answer 2

The compiler does what you tell it to do. It will compile files which you specify as its inputs, and as part of this compilation, it will include other files in these input files if you tell it to do so using #include preprocessor directives.

If you tell the compiler to compile a .h file (and pass in command-line options to make it treat it as a source file, such as -x c for gcc), it will compile it just fine. If you #include "a.cpp", it will get included normally.