title: Adding a custom class to a Qt project
body: <p>I've been coding a physics simulation that holds in a custom class and I'm know trying to code a GUI with Qt 4.8 for it.</p>
  
  <p>It seems to me that the right to do that is to add one instance of my class as a member of my MainWindow object in Qt.</p>
  
  <p>My homecooked class works finely on its side but I'm having trouble in adding it to the Qt project. I tried using the usual
     #include "my_custom_class.h" (everything is defined in this header, I usually don't split it in a cpp file)</p>
  
  <p>to my mainwindow.h file and, when compiling I get for every member of my first class an error message : multiple definition of my_custom_class::member1() , etc...</p>
  
  <p>I also tried the:</p>
  
  <p>class my_custom_class;</p>
  
  <p>but it doesn't work either, and I'm not comfortable with that as I don't really understand what it really does.</p>