• Never #include unnecessary header files.
• Never #include a header when a forward declaration will suffice.
  Prefer to #include when a forward declaration of a stream will suffice.
  A dependency is only mention as a return type: doesn't require a definition, but it is better to add it for the caller convenience (he will be able to use auto val = x.h();)
  ** A dependency is only mention as a parameter type, or bother a parameter type and a return type: doesn't require a definition

Ces deux derniers point ne sont pas vrais en Qt si c'est dans la définition d'un signal ou d'un slot. Qmake génère un moc_.cpp à partir du .h, et il a besoin des définitions des paramètres.