Notes

Signals are declared, never defined. Qt’s moc auto-generates the
implementation at compile time - writing a function body for one yourself
causes a duplicate-definition link error.

signals:
    void sig_somethingHappened(int value);    // no .cpp definition, ever

public slots:
    void slot_onSomethingHappened(int value); // this one you do implement

emit sig_somethingHappened(5); broadcasts to every slot connected via
connect(...) - the emitter doesn’t know or care who (if anyone) is
listening. This is what decouples, e.g., a rendering widget from the main
window that displays its data.

Gotcha: string-based connect() fails silently

connect(ui->button, SIGNAL(clicked()), ui->widget, SLOT(slot_addNode()));

If slot_addNode doesn’t exactly match a real slot’s name (typo, or
renamed on one side and not the other), this is not a compile error - it
prints a runtime console warning (“No such slot…”) and the connection
is simply never made. Verify by actually clicking the control and
watching for an effect, not just by a successful build.

Related

Virtual functions and polymorphism
Callback functions