Why won't this code run? I am confused : <pre><code>// main.cpp
#include &lt;iostream&gt;
#include "add.h"

int main(){
  std::cout &lt;&lt; "Hello World\n";
  std::cout &lt;&lt; add(3,4) &lt;&lt; std::endl;
  return 0;
}

// add.h
#ifndef add_h
#define add_h

int add(int x, int y);

#endif /* add_h */

// add.cpp
#include "add.h"

int add(int x, int y){
  return x + y;
}
</code></pre>

<p>I am compiling with 'g++ -std=c++11 main.cpp -o main'. I keep getting linker errors. I copied it exactly from a tutorial too.</p>
