Including .cpp files

c++

Solution

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.

Problem

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?

Original source

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