Including .cpp files

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.