What's the REAL difference between .h and .cpp files?

Question

This question was posted several times on StackOverflow, but most of the answers stated something similar to ".h files are supposed to contain declarations whereas .cpp files are supposed to contain their definitions/implementation". I've noticed that simply defining functions in .h files works just fine. What's the purpose of declaring functions in .h files but defining and implementing them in .cpp files? Does it really reduce compile time? What else?

Answer 1

Practically: the conventions around .h files are in place so that you can safely include that file in multiple other files in your project. Header files are designed to be shared, while code files are not.

Let's take your example of defining functions or variables. Suppose your header file contains the following line:

header.h:

int x = 10;

code.cpp:

#include "header.h"

Now, if you only have one code file and one header file this probably works just fine:

g++ code.cpp -o outputFile

However, if you have two code files this breaks:

header.h:

int x = 10;

code1.cpp:

#include "header.h"

code2.cpp:

#include "header.h"

And then:

g++ code1.cpp -c  (produces code1.o)
g++ code2.cpp -c  (produces code2.o)
g++ code1.o code2.o -o outputFile

This breaks, specifically at the linker step, because now you have two symbols in the same executable that have the same symbol, and the linker doesn't know what's it's supposed to do with that. When you include your header in code1 you get a symbol "x" and when you include your header in code2 you get another symbol "x". The linker doesn't know your intention here, so it throws an error:

code2.o:(.data+0x0): multiple definition of `x'
code1.o:(.data+0x0): first defined here
collect2: error: ld returned 1 exit status

Which again is just the linker saying that it can't resolve the fact that you now have two symbols with the same name in the same executable.