fgoujeon/signal

Fast C++17 signal/slot library

fgsig

fgsig is a fast, type-safe, multi-signature, C++17 signal/slot library.

THIS LIBRARY IS AT EARLY DEVELOPMENT STAGE AND SHOULDN'T BE USED IN PRODUCTION!

Signals and Slots

Signals and slots is a mechanism for communication between objects which makes it easy to implement the observer pattern while avoiding boilerplate code.

fgsig lets you send notifications to any number of observers (or slots) with minimum code:

fgsig::signal<void(int)> signal;
auto connection0 = fgsig::connect(signal, [](int value){std::cout << "Hello " << value << '\n';});
auto connection1 = fgsig::connect(signal, [](int value){std::cout << "World " << value << '\n';});
signal.emit(42);

Output:

Hello 42
World 42

Note that keeping the connection instance alive is mandatory to maintain the connection between the signal and the slot. When a connection instance is destroyed, the connection it holds is closed.

Type-Safe

The fgsig::signal class template takes a function signature as template parameter. It won't let you connect a slot whose signature doesn't match.

Multi-Signature

The fgsig::signal class template can actually take more than one function signature as template parameters:

fgsig::signal
<
    void(int),
    void(const std::string&),
    void(const whatever_type_you_want&)
> signal;

std::ostringstream oss;
auto connection = fgsig::connect(signal, [&oss](const auto& value){oss << value << '\n';});

signal.emit(42);
signal.emit("test");
signal.emit(whatever_type_you_want{});

std::cout << oss.str();

Output:

42
test
whatever string

Fast

See benchmark.

Despite its type-safe interface, fgsig internally uses void*-based type erasure, which is the fastest technique of type erasure.

No Dependency

fgsig doesn't depend on any other library than the C++ standard library.

Full Example

Here is how you could use fgsig in a real-life project:

#include <fgl/signals.hpp>
#include <string>
#include <sstream>
#include <iostream>

/*
This class represents a car.
You can fill its fuel tank and drive it.
Also, it sends events using fgsig.
*/
struct car
{
    public:
        //properties
        struct fuel_level_l { unsigned int value = 0; }; //fuel level in L
        struct speed_kmh    { unsigned int value = 0; }; //speed in km/h

        //events
        template<class Property>
        struct property_change_event
        {
            decltype(Property::value) value;
        };
        struct stall_event{};

        using signal = fgsig::signal
        <
            void(const property_change_event<fuel_level_l>&),
            void(const property_change_event<speed_kmh>&),
            void(const stall_event&)
        >;

    public:
        //Connect to signal to receive events.
        template<class Slot>
        auto connect(Slot&& slot)
        {
            return fgsig::connect(signal_, std::forward<Slot>(slot));
        }

        //Add some fuel.
        //Quantity is in L.
        void add_fuel_l(const unsigned int quantity_l)
        {
            set<fuel_level_l>(get<fuel_level_l>() + quantity_l);
        }

        //Drive 20 km at 100 km/h.
        //Return true if the car has enough fuel.
        bool drive_20_km()
        {
            if(get<fuel_level_l>() > 0)
            {
                set<speed_kmh>(100);
                set<fuel_level_l>(get<fuel_level_l>() - 1);
                return true;
            }
            else
            {
                signal_.emit(stall_event{});
                set<speed_kmh>(0);
                return false;
            }
        }

    private:
        //Get value of given property.
        template<class Property>
        const decltype(Property::value)& get() const
        {
            return std::get<Property>(properties_).value;
        }

        //Set value of given property, and send corresponding
        //property_change_event if value changes.
        template<class Property>
        void set(const decltype(Property::value)& new_value)
        {
            auto& current_value = std::get<Property>(properties_).value;
            if(current_value != new_value)
            {
                current_value = new_value;
                signal_.emit(property_change_event<Property>{new_value});
            }
        }

    private:
        std::tuple
        <
            fuel_level_l,
            speed_kmh
        > properties_;

        signal signal_;
};

/*
This class prints out the current state of the given car.
*/
struct car_monitor
{
    public:
        car_monitor(car& c):
            connection_{c.connect([this](const auto& event){handle_event(event);})}
        {
        }

    private:
        void handle_event(const car::property_change_event<car::speed_kmh>& event)
        {
            std::cout << "Speed = " << event.value << " km/h\n";
        }

        void handle_event(const car::property_change_event<car::fuel_level_l>& event)
        {
            std::cout << "Fuel level = " << event.value << " L\n";
        }

        void handle_event(const car::stall_event&)
        {
            std::cout << "Car stalled\n";
        }

    private:
        fgsig::any_connection connection_;
};

int main()
{
    car c;
    car_monitor monitor{c};

    c.add_fuel_l(10);
    while(c.drive_20_km());

    return 0;
}

Output:

Fuel level = 10 L
Speed = 100 km/h
Fuel level = 9 L
Fuel level = 8 L
Fuel level = 7 L
Fuel level = 6 L
Fuel level = 5 L
Fuel level = 4 L
Fuel level = 3 L
Fuel level = 2 L
Fuel level = 1 L
Fuel level = 0 L
Car stalled
Speed = 0 km/h

Limitations

Thread Safety

fgsig doesn't provide any thread safety mechanism.

Users are encouraged to handle thread safety at a higher level. Possible solutions are:

• an implementation of the Active Object design pattern;
• a boost::asio::io_context running on a single thread.