Crash Course: events, signals and everything in between

Table of Contents

• Introduction
• Delegate
  • Runtime arguments
  • Lambda support
  • Raw access
• Signals
• Event dispatcher
  • Named queues
• Event emitter

Introduction

Signals are more often than not a core part of games and software architectures in general.
They help to decouple the various parts of a system while allowing them to communicate with each other somehow.

The so called modern C++ comes with a tool that can be useful in this regard, the std::function. As an example, it can be used to create delegates.
However, there is no guarantee that an std::function doesn't perform allocations under the hood and this could be problematic sometimes. Furthermore, it solves a problem but may not adapt well to other requirements that may arise from time to time.

In case that the flexibility and power of an std::function isn't required or if the price to pay for them is too high, EnTT offers a complete set of lightweight classes to solve the same and many other problems.

Delegate

A delegate can be used as a general purpose invoker with no memory overhead for free functions, lambdas and members provided along with an instance on which to invoke them.
It doesn't claim to be a drop-in replacement for an std::function, so don't expect to use it whenever an std::function fits well. That said, it's most likely even a better fit than an std::function in a lot of cases, so expect to use it quite a lot anyway.

The interface is trivial. It offers a default constructor to create empty delegates:

entt::delegate<int(int)> delegate{};

What is needed to create an instance is to specify the type of the function the delegate accepts, that is the signature of the functions it models.
However, attempting to use an empty delegate by invoking its function call operator results in undefined behavior or most likely a crash.

There exist a few overloads of the connect member function to initialize a delegate:

int f(int i) { return i; }
 
struct my_struct {
    int f(const int &i) const { return i; }
};
 
// bind a free function to the delegate
delegate.connect<&f>();
 
// bind a member function to the delegate
my_struct instance;
delegate.connect<&my_struct::f>(instance);

The delegate class also accepts data members, if needed. In this case, the function type of the delegate is such that the parameter list is empty and the value of the data member is at least convertible to the return type.

Free functions having type equivalent to void(T &, args...) are accepted as well. The first argument T & is considered a payload and the function will receive it back every time it's invoked. In other terms, this works just fine with the above definition:

void g(const char &c, int i) { /* ... */ }
const char c = 'c';
 
delegate.connect<&g>(c);
delegate(42);

Function g is invoked with a reference to c and 42. However, the function type of the delegate is still void(int). This is also the signature of its function call operator.
Another interesting aspect of the delegate class is that it accepts functions with a list of parameters that is shorter than that of its function type:

void g() { /* ... */ }
delegate.connect<&g>();
delegate(42);

Where the function type of the delegate is void(int) as above. It goes without saying that the extra arguments are silently discarded internally. This is a nice-to-have feature in a lot of cases, as an example when the delegate class is used as a building block of a signal-slot system.
In fact, this filtering works both ways. The class tries to pass its first count arguments first, then the last count. Watch out for conversion rules if in doubt when connecting a listener!
Arbitrary functions that pull random arguments from the delegate list aren't supported instead. Other feature were preferred, such as support for functions with compatible argument lists although not equal to those of the delegate.

To create and initialize a delegate at once, there are a few specialized constructors. Because of the rules of the language, the listener is provided by means of the entt::connect_arg variable template:

entt::delegate<int(int)> func{entt::connect_arg<&f>};

Aside connect, a disconnect counterpart isn't provided. Instead, there exists a reset member function to use to clear a delegate.
To know if a delegate is empty, it can be used explicitly in every conditional statement:

if(delegate) {
    // ...
}

Finally, to invoke a delegate, the function call operator is the way to go as already shown in the examples above:

auto ret = delegate(42);

In all cases, listeners don't have to strictly follow the signature of the delegate. As long as a listener can be invoked with the given arguments to yield a result that is convertible to the given result type, everything works just fine.

As a side note, members of classes may or may not be associated with instances. If they are not, the first argument of the function type must be that of the class on which the members operate and an instance of this class must obviously be passed when invoking the delegate:

entt::delegate<void(my_struct &, int)> delegate;
delegate.connect<&my_struct::f>();
 
my_struct instance;
delegate(instance, 42);

In this case, it's not possible to deduce the function type since the first argument doesn't necessarily have to be a reference (for example, it can be a pointer, as well as a const reference).
Therefore, the function type must be declared explicitly for unbound members.

Runtime arguments

The delegate class is meant to be used primarily with template arguments. However, as a consequence of its design, it also offers minimal support for runtime arguments.
When used like this, some features aren't supported though. In particular:

• Curried functions aren't accepted.
• Functions with an argument list that differs from that of the delegate aren't supported.
• Return type and types of arguments must coincide with those of the delegate and being at least convertible isn't enough anymore.

Moreover, for a given function type Ret(Args...), the signature of the functions connected at runtime must necessarily be Ret(const void *, Args...).

Runtime arguments can be passed both to the constructor of a delegate and to the connect member function. An optional parameter is also accepted in both cases. This argument is used to pass arbitrary user data back and forth as a const void * upon invocation.
To connect a function to a delegate in the hard way:

int func(const void *ptr, int i) { return *static_cast<const int *>(ptr) * i; }
const int value = 42;
 
// use the constructor ...
entt::delegate delegate{&func, &value};
 
// ... or the connect member function
delegate.connect(&func, &value);

The type of the delegate is deduced from the function if possible. In this case, since the first argument is an implementation detail, the deduced function type is int(int).
Invoking a delegate built in this way follows the same rules as previously explained.

Lambda support

In general, the delegate class doesn't fully support lambda functions in all their nuances. The reason is pretty simple: a delegate isn't a drop-in replacement for an std::function. Instead, it tries to overcome the problems with the latter.
That being said, non-capturing lambda functions are supported, even though some features aren't available in this case.

This is a logical consequence of the support for connecting functions at runtime. Therefore, lambda functions undergo the same rules and limitations.
In fact, since non-capturing lambda functions decay to pointers to functions, they can be used with a delegate as if they were normal functions with optional payload:

my_struct instance;
 
// use the constructor ...
entt::delegate delegate{+[](const void *ptr, int value) {
    return static_cast<const my_struct *>(ptr)->f(value);
}, &instance};
 
// ... or the connect member function
delegate.connect([](const void *ptr, int value) {
    return static_cast<const my_struct *>(ptr)->f(value);
}, &instance);

As above, the first parameter (const void *) isn't part of the function type of the delegate and is used to dispatch arbitrary user data back and forth. In other terms, the function type of the delegate above is int(int).

Raw access

While not recommended, a delegate also allows direct access to the stored callable function target and underlying data, if any.
This makes it possible to bypass the behavior of the delegate itself and force calls on different instances:

my_struct other;
delegate.target(&other, 42);

It goes without saying that this type of approach is very risky, especially since there is no way of knowing whether the contained function was originally a member function of some class, a free function or a lambda.
Another possible (and meaningful) use of this feature is that of identifying a particular delegate through its descriptive traits instead.

Signals

Signal handlers work with references to classes, function pointers and pointers to members. Listeners can be any kind of objects and users are in charge of connecting and disconnecting them from a signal to avoid crashes due to different lifetimes. On the other side, performance shouldn't be affected that much by the presence of such a signal handler.
Signals make use of delegates internally and therefore they undergo the same rules and offer similar functionalities. It may be a good idea to consult the documentation of the delegate class for further information.

A signal handler is can be used as a private data member without exposing any publish functionality to the clients of a class.
The basic idea is to impose a clear separation between the signal itself and the sink class, that is a tool to be used to connect and disconnect listeners on the fly.

The API of a signal handler is straightforward. If a collector is supplied to the signal when something is published, all the values returned by its listeners are literally collected and used later by the caller. Otherwise, the class works just like a plain signal that emits events from time to time.
To create instances of signal handlers it's sufficient to provide the type of function to which they refer:

entt::sigh<void(int, char)> signal;

Signals offer all the basic functionalities required to know how many listeners they contain (size) or if they contain at least a listener (empty), as well as a function to use to swap handlers (swap).

Besides them, there are member functions to use both to connect and disconnect listeners in all their forms by means of a sink:

void foo(int, char) { /* ... */ }
 
struct listener {
    void bar(const int &, char) { /* ... */ }
};
 
// ...
 
entt::sink sink{signal};
listener instance;
 
sink.connect<&foo>();
sink.connect<&listener::bar>(instance);
 
// ...
 
// disconnects a free function
sink.disconnect<&foo>();
 
// disconnect a member function of an instance
sink.disconnect<&listener::bar>(instance);
 
// disconnect all member functions of an instance, if any
sink.disconnect(&instance);
 
// discards all listeners at once
sink.disconnect();

As shown above, listeners don't have to strictly follow the signature of the signal. As long as a listener can be invoked with the given arguments to yield a result that is convertible to the given return type, everything works just fine.
In all cases, the connect member function returns by default a connection object to be used as an alternative to break a connection by means of its release member function.
A scoped_connection can also be created from a connection. In this case, the link is broken automatically as soon as the object goes out of scope.

Once listeners are attached (or even if there are no listeners at all), events and data in general are published through a signal by means of the publish member function:

signal.publish(42, 'c');

To collect data, the collect member function is used instead:

int f() { return 0; }
int g() { return 1; }
 
// ...
 
entt::sigh<int()> signal;
entt::sink sink{signal};
 
sink.connect<&f>();
sink.connect<&g>();
 
std::vector<int> vec{};
signal.collect([&vec](int value) { vec.push_back(value); });
 
assert(vec[0] == 0);
assert(vec[1] == 1);

A collector must expose a function operator that accepts as an argument a type to which the return type of the listeners can be converted. Moreover, it can optionally return a boolean value that is true to stop collecting data, false otherwise. This way one can avoid calling all the listeners in case it isn't necessary.
Functors can also be used in place of a lambda. Since the collector is copied when invoking the collect member function, std::ref is the way to go in this case:

struct my_collector {
    std::vector<int> vec{};
 
    bool operator()(int v) {
        vec.push_back(v);
        return true;
    }
};
 
// ...
 
my_collector collector;
signal.collect(std::ref(collector));

Event dispatcher

The event dispatcher class allows users to trigger immediate events or to queue and publish them all together later.
This class lazily instantiates its queues. Therefore, it's not necessary to announce the event types in advance:

// define a general purpose dispatcher
entt::dispatcher dispatcher{};

A listener registered with a dispatcher is such that its type offers one or more member functions that take arguments of type Event & for any type of event, regardless of the return value.
These functions are linked directly via connect to a sink:

struct an_event { int value; };
struct another_event {};
 
struct listener {
    void receive(const an_event &) { /* ... */ }
    void method(const another_event &) { /* ... */ }
};
 
// ...
 
listener listener;
dispatcher.sink<an_event>().connect<&listener::receive>(listener);
dispatcher.sink<another_event>().connect<&listener::method>(listener);

Note that connecting listeners within event handlers can result in undefined behavior.
The disconnect member function is used to remove one listener at a time or all of them at once:

dispatcher.sink<an_event>().disconnect<&listener::receive>(listener);
dispatcher.sink<another_event>().disconnect(&listener);

The trigger member function serves the purpose of sending an immediate event to all the listeners registered so far:

dispatcher.trigger(an_event{42});
dispatcher.trigger<another_event>();

Listeners are invoked immediately, order of execution isn't guaranteed. This method can be used to push around urgent messages like an is terminating notification on a mobile app.

On the other hand, the enqueue member function queues messages together and helps to maintain control over the moment they are sent to listeners:

dispatcher.enqueue<an_event>(42);
dispatcher.enqueue(another_event{});

Events are stored aside until the update member function is invoked:

// emits all the events of the given type at once
dispatcher.update<an_event>();
 
// emits all the events queued so far at once
dispatcher.update();

This way users can embed the dispatcher in a loop and literally dispatch events once per tick to their systems.

Named queues

All queues within a dispatcher are associated by default with an event type and then retrieved from it.
However, it's possible to create queues with different names (and therefore also multiple queues for a single type). In fact, more or less all functions also take an additional parameter. As an example:

dispatcher.sink<an_event>("custom"_hs).connect<&listener::receive>(listener);

In this case, the term name is misused as these are actual numeric identifiers of type id_type.
An exception to this rule is the enqueue function. There is no additional parameter for it but rather a different function:

dispatcher.enqueue_hint<an_event>("custom"_hs, 42);

This is mainly due to the template argument deduction rules and unfortunately there is no real (elegant) way to avoid it.

Event emitter

A general purpose event emitter thought mainly for those cases where it comes to working with asynchronous stuff.
Originally designed to fit the requirements of uvw (a wrapper for libuv written in modern C++), it was adapted later to be included in this library.

To create an emitter type, derived classes must inherit from the base as:

struct my_emitter: emitter<my_emitter> {
    // ...
}

Handlers for the different events are created internally on the fly. It's not required to specify in advance the full list of accepted events.
Moreover, whenever an event is published, an emitter also passes a reference to itself to its listeners.

To create new instances of an emitter, no arguments are required:

my_emitter emitter{};

Listeners are movable and callable objects (free functions, lambdas, functors, std::functions, whatever) whose function type is compatible with:

void(Type &, my_emitter &)

Where Type is the type of event they want to receive.
To attach a listener to an emitter, there exists the on member function:

emitter.on<my_event>([](const my_event &event, my_emitter &emitter) {
    // ...
});

Similarly, the reset member function is used to disconnect listeners given a type while clear is used to disconnect all listeners at once:

// resets the listener for my_event
emitter.erase<my_event>();
 
// resets all listeners
emitter.clear()

To send an event to the listener registered on a given type, the publish function is the way to go:

struct my_event { int i; };
 
// ...
 
emitter.publish(my_event{42});

Finally, the empty member function tests if there exists at least a listener registered with the event emitter while contains is used to check if a given event type is associated with a valid listener:

if(emitter.contains<my_event>()) {
    // ...
}

This class introduces a nice-to-have model based on events and listeners.
More in general, it's a handy tool when the derived classes wrap asynchronous operations but it's not limited to such uses.