Understanding the Qt Signals and Slots Mechanism Through Source Code Analysis

Qt is a cross‑platform C++ framework for building GUI and non‑GUI applications, using a meta‑object system that relies on the Q_OBJECT macro, the Meta‑Object Compiler (moc), and the signals‑and‑slots mechanism to enable loosely coupled communication between objects.

The basic concepts include signals , which are emitted by objects when their state changes, and slots , which are ordinary member functions that receive those signals. Connections are established via the static method QObject::connect(sender, SIGNAL(signal), receiver, SLOT(slot)) , where the SIGNAL and SLOT macros convert identifiers into string signatures.

When a class contains Q_OBJECT, moc generates a moc_*.cpp file that defines static meta‑data structures (e.g., qt_meta_stringdata_* , qt_meta_data_* ) and implements methods for signal emission and slot invocation. The generated code assigns method indices, stores string data, and registers connections.

Qt defines several connection types: AutoConnection (default, chooses Direct or Queued based on thread context), DirectConnection , QueuedConnection , BlockingQueuedConnection , and UniqueConnection . These affect how and when slot functions are called.

At runtime, emitting a signal triggers QMetaObject::activate , which looks up the meta‑object, iterates over the relevant ConnectionList , and invokes the connected slots according to the chosen connection type. The process involves two linked lists: one per signal (holding all connections for that signal) and one per object (holding all connections where the object is a receiver).

The article demonstrates these steps with a simple Qt demo project: building the environment, writing MainWindow.h and MainWindow.cpp , compiling, and observing console output when a button click emits a signal that ultimately calls the onClean slot. It also warns about potential crashes if a slot deletes objects that are later accessed.

References include the original Qt source code, a blog post on how Qt signals and slots work, and a custom C++ implementation demo.