c++ - Including .cpp files

Question

I have read in places like here that you have to include .h files and not .cpp files, because otherwise then you get an error. So for example

main.cpp

#include <iostream>
#include "foop.h"

int main(int argc, char *argv[])
{
int x=42;
std::cout << x <<std::endl;
std::cout << foo(x) << std::endl;
return 0;
}

foop.h

#ifndef FOOP_H
#define FOOP_H
int foo(int a);
#endif

foop.cpp

int foo(int a){
    return ++a;
}

works, but if I replace #include "foop.h" with #include "foop.cpp" I get an error (Using Dev C++ 4.9.9.2, Windows):

multiple definition of foo(int)
first defined here

Why is this?

Answer 1

What include does is copying all the contents from the file (which is the argument inside the <> or the "" ), so when the preproccesor finishes its work main.cpp will look like:

// iostream stuff

int foo(int a){
    return ++a;
}

int main(int argc, char *argv[])
{
   int x=42;
   std::cout << x <<std::endl;
   std::cout << foo(x) << std::endl;
   return 0;
}

So foo will be defined in main.cpp, but a definition also exists in foop.cpp, so the compiler "gets confused" because of the function duplication.