how can I know which object caused a specific event ?

[Radek]

Each handler (the "slot" procedure] can call sender() and get a pointer to the object which emitted the just processed signal. sender() returns a QObject * which you dynamic_cast to the real sender class (for example, QSlider). Comparing the pointer to addresses of the possible senders, you get the object which signal is processed. Note: calling sender() outside a handler (for example, in a procedure called by the handler - this is "outside, too) returns nullptr.