computer science

Signals and slots

Signals and slots is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Signals and slots rather than just read about it. In short: Signals and slots is a language construct introduced in Qt for communication between objects which makes it easy to implement the observer pattern while avoiding boilerplate code. The concept is that GUI widgets, and other objects, can send signals containing event information which can be received by other objects using special member functions known as slots.

Key takeaways

Signals and slots belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Signals and slots to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Signals and slots from memory before moving on to harder problems.

Reference excerpt

Signals and slots is a language construct introduced in Qt for communication between objects which makes it easy to implement the observer pattern while avoiding boilerplate code. The concept is that GUI widgets, and other objects, can send signals containing event information which can be received by other objects using special member functions known as slots. This is similar to C/C++ function pointers, but the signal/slot system ensures the type-correctness of callback arguments. The signal/slot system fits well with the way graphical user interfaces are designed. Similarly, the signal/slot system can be used for other non-GUI usages, for example asynchronous I/O (including sockets, pipes, serial devices, etc.) event notification or to associate timeout events with appropriate object instances and methods or functions. It is easy to use and no registration/deregistration/invocation code need to be written, because Qt's metaobject compiler (MOC) automatically generates the needed infrastructure. A spreadsheet programs update system could be implemented using signals and slots such that when a cell is changed, its dependent cells are notified/updated.

Alternative implementations

There are some implementations of signal/slot systems based on C++ templates, which don't require the extra metaobject compiler, as used by Qt, such as libsigc++, sigslot, vdk-signals, nano-signal-slot, neosigslot, Signals, boost.signals2, Synapse, Cpp::Events, Platinum, JBroadcaster and KDBindings. Common Language Infrastructure (CLI) languages such as C# also supports a similar construct although with a different terminology and syntax: events play the role of signals, and delegates are the slots. Another implementation of signals exists for ActionScript 3.0, inspired by C# events and signals/slots in Qt. Additionally, a delegate can be a local variable, much like a function pointer, while a slot in Qt must be a class member declared as such. The C based GObject system also provides similar functionality via GSignal. In D it is implemented by std.signals.

See also Observer pattern Delegate (CLI)

Libraries Java: sig4j - multi-threaded, type-safe, based on the FunctionalInterface annotation introduced in Java 8. C++: vdk-signals - thread-safe, type-safe, written in C++11 with atomic variables.

References

Worked examples

Example 1 — a first encounter with Signals and slots

Start with the simplest possible case. Write down what Signals and slots claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Signals and slots before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Signals and slots ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Signals and slots

In research: Signals and slots appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Signals and slots in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Signals and slots is common in secondary-school and first-year university syllabi. It links to neighbouring topics Qt (software), so understanding it makes those chapters shorter.
In everyday life: Look for Signals and slots outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Signals and slots in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Signals and slots means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Signals and slots out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Signals and slots in simple terms?

Why does Signals and slots matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Signals and slots?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Signals and slots.