Comparing SDL Custom Events to Signal/Slot Mechanisms (e.g., Qt)

Comparing SDL's custom event system to signal/slot mechanisms (like those found in frameworks such as Qt, Boost.Signals2, or sigc++) highlights different approaches to object communication and decoupling.

Both are implementations of the Observer pattern but differ significantly in their mechanics and philosophy.

Signal/Slot Mechanisms (e.g., Qt)

• Concept: Objects ("emitters" or "senders") define "signals," which they emit when their state changes or an action occurs. Other objects ("receivers" or "listeners") define "slots," which are essentially normal member functions. You explicitly connect a sender's signal to a receiver's slot. When the signal is emitted, all connected slots are invoked.
• Connection: Connections are typically established explicitly at runtime (e.g., QObject::connect(sender, &Sender::mySignal, receiver, &Receiver::mySlot);). The framework manages these connections.
• Invocation: Slots can often be invoked either directly (synchronously, like a function call, within the sender's thread) or queued (asynchronously, processed later in the receiver's event loop), depending on the connection type and thread affinities.
• Type Safety: Usually provide strong compile-time type safety. The signature of the signal (argument types) must be compatible with the signature of the connected slot. Mismatches are often caught by the compiler.
• Data Passing: Data is passed as arguments to the signal, which are then forwarded to the connected slots.
• Coupling: Creates a managed coupling. The sender doesn't need to know which specific objects are listening, but the connection itself establishes a direct link managed by the framework. Receivers often know about the sender type to connect.
• Infrastructure: Often requires framework support, like Qt's Meta-Object Compiler (MOC) and QObject base class, to manage signal/slot discovery and invocation.

SDL Custom Event System

• Concept: Components ("pushers") create SDL_Event structures (with a custom type and optional data in code, data1, data2) and push them onto a central, global event queue using SDL_PushEvent. Other components ("handlers" or "pollers") retrieve events from this queue using SDL_PollEvent and check the type to see if they are interested.
• Connection: There is no direct connection. Pushers add events to the queue without knowing who, if anyone, will process them. Handlers poll the queue without knowing who pushed the events. Communication is mediated entirely through the central queue.
• Invocation: Always asynchronous. Events are added to the queue and processed later when the handler calls SDL_PollEvent (typically in the main loop).
• Type Safety: Weak type safety for custom data. data1 and data2 are void*, requiring runtime casting (static_cast) and careful programmer discipline to ensure correctness. Type errors are not caught at compile time.
• Data Passing: Limited to the SDL_UserEvent struct format. Complex data requires pointers and careful lifetime management.
• Coupling: Highly decoupled. Pushers and handlers have no direct knowledge of each other; their only interaction point is the shared SDL event queue.
• Infrastructure: Relies solely on SDL's core event queue mechanism. No extra compilation steps or mandatory base classes are needed beyond including SDL headers and linking the library.

Key Differences Summarized

Conclusion

SDL's custom event system provides a simple, highly decoupled, queue-based communication mechanism that integrates naturally with the main SDL event loop and offers built-in thread-safe pushing. Its main weakness is the lack of type safety for custom data.

Signal/slot mechanisms offer superior type safety and more explicit control over connections, potentially supporting both synchronous and asynchronous invocation. However, they typically involve more framework overhead and establish a different (though still managed) form of coupling between components.

The choice depends on the project's needs regarding type safety, desired coupling model, required invocation types (sync/async), and integration with the existing framework (SDL loop vs. a dedicated meta-object system).