c++ - Qt signals and slots in different classes -

i have class x slot, , class y signal. i'm setting connection class x, , created public method in class y emit signal class x (i'm not sure step necessary).

then, if call method class x, signal emitted, , slot executed. if emit signal class y, slot never executed , don't understand why.

may stablish connection @ class y?

this pseudo-code tries explain want:

class x  : public qwidget {     q_object  x(){     connect(y::getinstance(), signal(updatesignal(int)), this, slot(updatestatus(int)));     y::getinstance().emitsignal(somevalue); // works }  public slots:     void updatestatus(int value);  }  class y : public qobject {  q_object  y(){  }  public:     y getinstance();     void emitsignal(int value) {          emit updatesignal(value);     }   signal:     void updatesignal(int value); }  class z : public y { z(){  }  init(){  emitsignal(somevalue); // doesn't work } } 

remember connections not between classes, between instances. if emit signal , expect connected slots called, must emitted on instance on connection made. that's problem.

assuming y singleton:

if connect( y::getinstance(), ... )

and y::getinstance() new y() @ point, constructor of y called before connection set up. that, signal emitted slot not listen yet.

apart that, best 1 of following things, though not emit signal in constructor of y these approaches:

• use third class z knows both x , y, , connection there
• dependency injection. means x gets instance of y in constructor:

example dependency injection constructor:

x::x( y* const otherclass ) {     connect( otherclass, signal( ... ), this, slot( ... ) }