title: c++ accessing functions from another file
body: <p>I'm learning c++ and having a bit of trouble with how you share methods with different files and or classes.</p>
  
  <p>if I make a function called increment(int a){a++;  return a;}</p>
  
  <p>and have it defined in a.cpp how would I call it in main.cpp?
  Any help is appreciated, thanks.</p>