signalsslots

Signalsslots is a software design pattern used to enable communication between objects in an event-driven system. The core idea is that an object can emit a signal to announce that something has happened, and other objects provide slots—callable handlers—that respond to that signal. This arrangement promotes loose coupling, since the emitter does not need to know which objects will handle the event.

In a typical signalsslots implementation, signals are defined by the emitting object and can carry data as parameters. Slots are functions or methods that are invoked in response to a signal. A connection mechanism pairs a specific signal with one or more slots. When the signal is emitted, all connected slots are invoked, with the signal’s parameters passed along. Many implementations support flexible parameter matching (slots may have fewer parameters than the signal) and offer different connection types to handle synchronous versus asynchronous invocation, including multithreaded scenarios.

The best-known example of signalsslots is the system used by the Qt framework. In Qt, classes derived from QObject can declare signals and slots, and a central mechanism called the meta-object system manages the connections via a function like connect. Modern Qt supports type-safe connections using function pointers or functors. Signals are emitted without knowledge of who receives them, and slots can be added or removed at runtime, enabling dynamic and extensible software architectures.

Advantages of signalsslots include reduced coupling, easier reusability, and improved testability, as behavior can be extended by adding new connections without modifying existing code. Limitations include potential runtime errors if connections are misconfigured and the need for careful lifecycle management in object lifetimes, particularly in multithreaded contexts. Alternatives include observer patterns or event bus systems.