I’ve been asked multiple times how I would implement a signal / slot mechanism in modern C++. Here is the answer!

1. Qt Signal Slot Template Classes

Qt's signals and slots implementation is not as fast as a template-based solution. While emitting a signal is approximately the cost of four ordinary function calls with common template implementations, Qt requires effort comparable to about ten function calls. So if you need to pass some complex information to a slot, create a class derived from QObject and use that. 1 Well, C does have some high-level constructs like boost::bind and the new C0x standard support for lambdas, but I don't believe either approach is formally supported by Qt's connect methods yet. When the number of pending resultReadyAt or resultsReadyAt signals exceeds the limit, the computation represented by the future will be throttled automatically. The computation will resume once the number of pending signals drops below the limit. Example: Starting a computation and getting a slot callback when it's finished. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type.

What’s the Signal / Slot Pattern?

So basically it allows for event based inter-object communication. In my opinion it’s intuitive to use and produces easily readable code when used in a moderate amount. And the big plus: It can be added to your program with one simple template class!

There are many libraries around (refer to the linked Wikipedia article) implementing this pattern, but it’s so easy to implement on you own that I would recommend to do this without an additional dependency. All you need is the header I posted below. And it’s a good exercise.

The signal template class

Below you can find the entire class. Because this class is using variadic templates you can define signals which pass any kind of data to their slots. Basically you can create signals which allow for arbitrary slot signatures. The emit method will accept the same argument types you declared as template parameters for the Signal class. The class is documented with comments and should be quite understandable. Further below you will find two usage examples.

Simple usage

The example below creates a simple signal. To this signal functions may be connected which accept a string and an integer. A lambda is connected and gets called when the emit method of the signal is called.

When you saved the Signal class as Signal.hpp and the above example as main.cpp you can compile the example with:

And if you execute the resulting application you will get the following output:

Advanced usage

This example shows the usage with classes. A message gets displayed when the button is clicked. Note that neither the button knows anything of a message nor does the message know anything about a button. That’s awesome! You can compile this example in the same way as the first.

You may also connect member functions which take arguments (Thank you FlashingChris for pointing out how to do this without std::placeholders!). In the following example Alice and Bob may say something and the other will hear it:

Issues & next steps

There are two drawbacks in this simple implementation: It’s not threadsafe and you cannot disconnect a slot from a signal from within the slot callback. Both problems are easy to solve but would make this example more complex.

Using this Signal class other patterns can be implemented easily. In a follow-up post I’ll present another simple class: the Property. This will allow for a clean implementation of the observer pattern.

Have some fun coding events in C++!

Edit on 2020-10-19:

• Add move-copy-constructor and move-assignment-operator
• Add emit_for_all_but_one and emit_for methods.
• Remove previously added std::forward.

Qt Signal Slot Template Classes