Including .cpp files

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?

Solution 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.

Solution 2:

There are many reasons to discourage including a .cpp file, but it isn't strictly disallowed. Your example should compile fine.

The problem is probably that you're compiling both main.cpp and foop.cpp, which means two copies of foop.cpp are being linked together. The linker is complaining about the duplication.

Solution 3:

When you say #include "foop.cpp", it is as if you had copied the entire contents of foop.cpp and pasted it into main.cpp.

So when you compile main.cpp, the compiler emits a main.obj that contains the executable code for two functions: main and foo.

When you compile foop.cpp itself, the compiler emits a foop.obj that contains the executable code for function foo.

When you link them together, the compiler sees two definitions for function foo (one from main.obj and the other from foop.obj) and complains that you have multiple definitions.