How Does the Signals Library Improve Your Programs?

• Flexible multicast callbacks for functions and function objects

• A robust mechanism for triggering and handling events

• Compatibility with function object factories, such as Boost.Bind and Boost.Lambda

The Boost.Signals library reifies signals and slots, where a signal is something that can be "emitted," and slots are connections that receive such signals. This is a well-known design pattern that goes under a few different namesObserver, signals/slots, publisher/subscriber, events (and event targets)but these names all refer to the same thing, which is a one-to-many relation between some source of information and instances that are interested in knowing when that information changes. There are many cases where this design pattern is used; one of the most obvious is in GUI code, where certain actions (for example, the user clicks a button) are loosely connected to some kind of action (the button changes its appearance, and some business logic is performed). There are many more cases where signals and slots are useful to decouple the trigger of an action (signal) from the code that handles it (one or more slots). This can be used to dynamically alter the behavior of the handling code, to allow multiple handlers of the same signal, or to reduce type dependencies through an abstract connection between types via signals and slots. With Boost.Signals, it is possible to create signals that accept slots with any given function signaturethat is, slots that accept arguments of arbitrary types. This approach makes the library very flexible; it accommodates the signaling needs of virtually any domain. By decoupling the source of the signal and the handlers thereof, systems become more robust in terms of both physical and logical dependencies. It's possible to let the signaling types be totally ignorant of the slot types, and vice versa. This is imperative to achieve a higher level of reusability, and it can help break cyclic dependencies. So, a signals and slots library isn't only about object-oriented callbacks, it's also about the robustness of the whole system to which it is applied.