Signals

A single header-only C++11 implementation of boost::signals2 (okay... 2 header files). The implementation of this library is also heavly inspired by palacaze/sigslot.

Usage

To use this library, just copy the signals.hpp and optional.hpp files to your include folder.

Motivation

I needed a C++11 signals library that matched the functionality of the boost::signals2 library without requiring the entire boost library to be included in the project. I've looked at a few other libraries that offered similar functionality but there were always trade-offs (for example, no support for return values, doesn't work with C++11, etc...).

This library provides a lot of the features of the boost::signals2 library, without the need to include the entire boost library in your project. There are a few (I feel, seldom used) unimplemented features from the boost::signals2 library (such as connection groups).

Introduction

This is an implementation of a signals & slots library. There are three primary interfaces that make up this library:

slot
connection
signal

// TODO: Add UML diagram.

Slot

A slot is a holder for a function object. A function object can be anything that can be invoked. A few examples are:

Free functions
(Pointer to) member functions
Function objects (Functors, or callable function objects)
Lambda expressions
(Pointer to) member data

For simplicity, any of these types of function-like objects will be generally referred to as callable.

A slot is used to hold any of these types of callable objects with any number (and types) of arguments and provide the return value of calling the slot (as an optional value).

The slot also stores its connection state. A connected slot is one that has a valid callable object associated with it. It is possible to invoke a disconnected slot. The return value of a disconnected slot is alwasy a disengaged opt::optional value. See jpvanoosten/optional for more information on the optional variable type used in this library. The return value of a connected slot should be an engaged opt::optional value.

Connection

A connection object is used to manage the connection state of a slot within a signal. If a connection stores a reference to a valid and connected slot, it is in the connected state. The connection object can also be used to temporarily block a slot, unblock the slot, and disconnect the slot from the signal.

Signal

The signal class is probably the most common way of working with the slots & signals. The signal is a container for multiple slots. The signal can be invoked which results in all of the connected slots being invoked.

Hello, World!

The simplest example of using the signals & slots library is to create a signal with a single slot that calls a free function that prints "Hello, World!" to the console.

#include "signals.hpp"
#include <iostream>

void hello_world()
{
    std::cout << "Hello, World!" << std::endl;
}

int main()
{
    // Define a signal that takes no arguments and returns void.
    using signal = sig::signal<void()>;
    signal s;

    // Connect the hello_world function to the signal.
    s.connect(&hello_world);

    // Call the signal.
    s();

    return 0;
}

The hello_world function is a free function that takes no arguments. It's only purpose is to print "Hello, World!" to the default output stream (the console).

In the the main function, a signal is aliased from sig::signal<void()>. sig::signal is a template class that takes the function signature as the template argument for the class. Creating a template alias for the signal is not required but it does make the code easier to read if you need to create multiple signals with the same function signature.

The hello_world function is connected to the signal using the signal::connect method.

The signal is invoked by using the function call operator s().

When you run the program, you should see "Hello, World!" printed in the console.

Hello, World!

Connecting Multiple Slots

Multiple slots can be connected to a signal. In this case, the task of printing "Hello, World!" to the console is split into separate functions.

#include "signals.hpp"
#include <iostream>

void hello()
{
    std::cout << "Hello";
}

void world()
{
    std::cout << ", World!" << std::endl;
}

int main()
{
    // Define a signal that takes no arguments and returns void.
    using signal = sig::signal<void()>;
    signal s;

    // Connect the hello, and world functions to the signal.
    s.connect(&hello);
    s.connect(&world);

    // Call the signal.
    s();

    return 0;
}

In this example, the functions hello and world are connected to the same signal. When the signal is invoked, the text "Hello, World!" is printed to the console.

Hello, World!

Slot Arguments

A slot can hold functions that takes multiple arguments.

#include "signals.hpp"
#include <iostream>

void print_args(float x, float y)
{
    std::cout << "The arguments are " << x << " and " << y << std::endl;
}

void print_sum(float x, float y)
{
    std::cout << "The sum is " << x + y << std::endl;
}

void print_product(float x, float y)
{
    std::cout << "The product is " << x * y << std::endl;
}

void print_difference(float x, float y)
{
    std::cout << "The difference is " << x - y << std::endl;
}

void print_quotient( float x, float y)
{
    std::cout << "The quotient is " << x / y << std::endl;
}

int main()
{
    // Define a signal that takes two floats and returns void.
    using signal = sig::signal<void(float, float)>;
    signal s;

    // Connect the functions to the signal.
    s.connect(&print_args);
    s.connect(&print_sum);
    s.connect(&print_product);
    s.connect(&print_difference);
    s.connect(&print_quotient);

    // Invoke the signal.
    s(5.0f, 3.0f);

    return 0;
}

In this example, five slots are connected to the signal. Each slot takes two floating point values as arguments and returns void. Each slot that is connected to the signal must match the signature of the signal.

The output of running this example should be:

The arguments are 5 and 3
The sum is 8
The product is 15
The difference is 2
The quotient is 1.66667

Signal Return Values

Slots can also return values. If multiple slots are connected to a signal then the result of invoking the signal is determined by the combiner that is associated with the signal. The default combiner is sig::optional_last_value which returns the result of the last slot that is connected to the signal.

#include "signals.hpp"
#include <iostream>

float product(float x, float y) { return x * y; };
float quotient(float x, float y) { return x / y; }
float sum(float x, float y) { return x + y; }
float difference(float x, float y) { return x - y; }

int main()
{
    // Define a signal that takes two floats and returns a float.
    using signal = sig::signal<float(float, float)>;
    signal s;

    // Connect all of the functions to the signal.
    s.connect(&product);
    s.connect(&quotient);
    s.connect(&sum);
    s.connect(&difference);

    // The default combiner returns a opt::optional containing
    // the return value of the last slot in the list, in this case
    // the result of the difference function.
    std::cout << *s(5.0f, 3.0f) << std::endl;

    return 0;
}

In this example, four functions are connected to the signal. When the signal is invoked, all of the functions are invoked but ony the result of the last slot (difference) is returned. If you run this program, the following output should be printed to the console:

It is possible to override the default combiner for the slot by supplying a combiner class as one of the template arguments for the slot.

#include "signals.hpp"
#include <iostream>

template<typename T>
class maximum_value
{
public:
    using result_type = opt::optional<T>;

    template<typename InputIterator>
    result_type operator()(InputIterator first, InputIterator last) const
    {
        result_type max;
        while (first != last)
        {
            // Dereferencing the iterator invokes the connected function.
            result_type tmp = *first;
            if ( tmp > max) max = tmp;

            ++first;
        }
        return max;
    }
};

float product(float x, float y) { return x * y; };
float quotient(float x, float y) { return x / y; }
float sum(float x, float y) { return x + y; }
float difference(float x, float y) { return x - y; }

int main()
{
    // Define a signal that takes two floats and returns a float.
    // The return value of the signal is the maximum value of all connected slots.
    using signal = sig::signal<float(float, float), maximum_value<float>>;
    signal s;

    // Connect all of the functions to the signal.
    s.connect(&product);
    s.connect(&quotient);
    s.connect(&sum);
    s.connect(&difference);

    // The maximum_value combiner returns the maximum
    // value returned by all connected slots.
    // In this case, the result is 15 since 5 * 3 is 15.
    std::cout << *s(5.0f, 3.0f) << std::endl;

    return 0;
}

The combiner class is a function object whose function call operator takes the first and last input iterators which invoke the slot when dereferenced. In this example, the maximum_value combiner is defined which iterates from first to last and invoking the slot by dereferencing the iterator. The result_type type alias indicates to the signal the type of the return value of the combiner. In this case, the combiner returns an opt::optional<T>. If there are no slots connected to the signal, the result is a disengaged optional value. Otherwise, the return value is the maximum value of all the connected slots.

Running the example should result in 15 being printed to the console.

As another example, we may want to return a list (std::vector) of all of the return values of all of the connected slots.

#include "signals.hpp"
#include <iostream>
#include <vector>

template<typename Container>
class aggregate_values
{
public:
    using result_type = Container;

    template<typename InputIterator>
    result_type operator()(InputIterator first, InputIterator last) const
    {
        result_type values;
        while (first != last)
        {
            auto value = *first++;
            if ( value )
                values.push_back(*value);
        }
        return values;
    }
};

float product(float x, float y) { return x * y; };
float quotient(float x, float y) { return x / y; }
float sum(float x, float y) { return x + y; }
float difference(float x, float y) { return x - y; }

int main()
{
    // Define a signal that takes two floats and returns a float.
    // The return value of the signal is a list of the return values of all connected slots.
    using signal = sig::signal<float(float, float), aggregate_values<std::vector<float>>>;
    signal s;

    // Connect all of the functions to the signal.
    s.connect(&product);
    s.connect(&quotient);
    s.connect(&sum);
    s.connect(&difference);

    std::cout << "Aggregate values: ";
    // The aggregate_values combiner returns a list
    // of all of the values from the connected slots.
    for ( auto f : s(5.0f, 3.0f) )
    {
        std::cout << f << " ";
    }
    std::cout << std::endl;

    return 0;
}

In this example, the combiner for the signal returns a list of return values from all connected slots in an std::vector. For the combiner in this example, any container type that provides the push_back method can be used.

The result of running this example should be:

Aggregate values: 15 1.66667 8 2

Member Functions

Connecting a signal to a member function of an instance of a class is simply a matter of passing a pointer to the class instance as the second parameter of the signal::connect method.

#include "signals.hpp"
#include <iostream>

class Calculator
{
public:
    float product(float x, float y)
    {
        return x * y;
    }

    float sum(float x, float y)
    {
        return x + y;
    }

    float difference(float x, float y)
    {
        return x - y;
    }

    float quotient(float x, float y)
    {
        return x / y;
    }
};

int main()
{
    // Define a signal that takes two floats and returns a float.
    using signal = sig::signal<float(float, float)>;
    signal s;

    // Create an instance of calculator.
    Calculator c;

    // Connect all of the functions to the signal.
    s.connect(&Calculator::product, &c);
    s.connect(&Calculator::sum, &c);
    s.connect(&Calculator::quotient, &c);
    s.connect(&Calculator::difference, &c);

    std::cout << *s(5.0f, 3.0f) << std::endl;

    return 0;
}

In this example, the Calculator class defines four methods. Each method is connected to the signal passing an instance of the class as the second parameter to the signal::connect method. Similar to the previous example, since the Calculator::difference method is connected last, the result of invoking the signal is 2.

It is important to note that the signal does not (cannot) track the lifetime of the instance of the object that is passed as the second parameter of the connect method. In this case, the signal must not be invoked on a class instance that goes out of scope.

...
using signal = sig::signal<float(float, float)>;
signal s;

{
    // Create an instance of calculator.
    Calculator c;

    // Connect all of the functions to the signal.
    s.connect(&Calculator::product, &c);
    s.connect(&Calculator::sum, &c);
    s.connect(&Calculator::quotient, &c);
    s.connect(&Calculator::difference, &c);
}   // Oops.. c is out of scope and has been destroyed.

// Invoking the slot now will fail since c is out of scope!
std::cout << *s(5.0f, 3.0f) << std::endl;
...

In the example shown above, an instance of the Calculator class is created and connected to the signal inside a scope block. The signal is invoked outside of that scope block which means that the instance of the Calculator class has been destroyed and very likely a rutime error will occur.

To solve this problem, you can connect a shared pointer with the signal.

Automatic Connection Management

Slots can automatically manage their connection state if the pointer to the instance variable is a shared pointer object (or anything that is convertable to a std::weak_ptr).

#include "signals.hpp"
#include <iostream>
#include <memory>

class Calculator
{
public:
    float product(float x, float y) { return x * y; }
    float sum(float x, float y) { return x + y; }
    float difference(float x, float y) { return x - y; }
    float quotient(float x, float y) { return x / y; }
};

int main()
{
    // Define a signal that takes two floats and returns a float.
    // The return value of the signal is a list of all return values of connected slots.
    using signal = sig::signal<float(float, float)>;
    signal s;

    {    // Create a shared pointer instance of Calculator.
        auto c = std::make_shared<Calculator>();

        // Connect all of the functions to the signal
        // with a shared pointer to the calculator instance.
        s.connect(&Calculator::product, c);
        s.connect(&Calculator::sum, c);
        s.connect(&Calculator::quotient, c);
        s.connect(&Calculator::difference, c);

        // Okay, should return 2.
        std::cout << *s(5.0f, 3.0f) << std::endl;
    } // The shared pointer instance goes out of scope and should be destroyed.

    // Invoking the signal again should result in a disengaged optional value.
    auto result = s(5.0f, 3.0f);
    if ( result )
    {
        std::cout << *result << std::endl;
    }
    else
    {
        std::cout << "Invalid result!" << std::endl;
    }

    return 0;
}

In this example, a shared_ptr instance of the Calculator class is created inside the scope block and connected to the signal. When the signal is invoked inside the scope block, it returns the value 2.

The signal is invoked again outside of the scope block but in this case the shared_ptr instance goes out of scope and is destroyed. This causes the slots to become disconnected and the result of invoking the signal is a disengaged optional value.

The result of executing this example is:

2
Invalid result!

But it would be pretty frustrating if this was the only way to disconnect a slot from a signal. In the following sections, several different methods of connection management are described.

Disconnecting Slots

The return value of the signal::connect method is a connection object. The connection object is used to disconnect the slot from the signal.

#include "signals.hpp"
#include <iostream>

struct HelloWorld
{
    void operator()() const
    {
        std::cout << "Hello, World!" << std::endl;
    }
};

int main()
{
    // Define a signal that takes no arguments and returns void.
    using signal = sig::signal<void()>;
    signal s;

    // Connect the HelloWorld function object to the signal.
    auto c = s.connect(HelloWorld());

    // Call the signal.
    // This should print "Hello, World!" to the console.
    s();

    // Disconnect the slot.
    c.disconnect();

    // Call the signal again.
    // Nothing is printed to the console.
    s();

    return 0;
}

In this example, a callable function object HelloWorld is connected to the signal using the signal::connect method. This method returns the connection object which is stored in c. The signal is invoked causing "Hello, World!" to be printed to the console. The slot is disconnected using the connection::disconnect method and the signal is invoked again but nothing is printed to the console.

Hello, World!

Blocking Slots

Slots can be temporarly blocked by using a connection_blocker object. A connection_bloker can be obtained from the original connection object using the connection::blocker method.

#include "signals.hpp"
#include <iostream>

struct HelloWorld
{
    void operator()() const
    {
        std::cout << "Hello, World!" << std::endl;
    }
};

int main()
{
    // Define a signal that takes no arguments and returns void.
    using signal = sig::signal<void()>;
    signal s;

    // Connect the HelloWorld function object to the signal.
    auto c = s.connect(HelloWorld());

    // Invoke the signal.
    // This should print "Hello, World!" to the console.
    s();

    {
        // Create a connection_blocker which will block the slot until
        // the connection_blocker is destroyed.
        // You can also use c.block() but then you need to call
        // c.unblock() to unblock the slot again.
        auto b = c.blocker();

        // Invoke the signal again.
        // In this case, nothing is printed to the console.
        s();
    }

    // Invoke the slot again.
    // This should print "Hello, World!" to the console.
    s();

    return 0;
}

The HelloWorld callable function object is connected to the signal and the resulting connection object is stored in c.

The signal is invoked which causes "Hello, World!" to be printed to the screen.

Inside the scope block, a connection_blocker is created and stored in the varible b. The signal is invoked again inside the scope block, but nothing is printed to the screen since the slot is blocked. When the connection_blocker goes out of scope, it unblocks the slot.

Outside of the scope block, the signal is invoked again, causing "Hello, World!" to be printed to the console again.

Hello, World!
Hello, World!

Using the connection_blocker is just one method to block a slot from being invoked. The connection::block method and the connection::unblock method can also be used to block and unblock the slot (respectively).

Scoped Connections

The connection object does not automatically disconnect the slot from the signal when it is destroyed. The scoped_connection object can be used to automatically disconnect the slot when the scoped_connection object is destroyed. The signal::connect_scoped method is used to return a scoped_connection object.

#include "signals.hpp"
#include <iostream>

struct HelloWorld
{
    void operator()() const
    {
        std::cout << "Hello, World!" << std::endl;
    }
};

int main()
{
    // Define a signal that takes no arguments and returns void.
    using signal = sig::signal<void()>;
    signal s;

    {
        // Create scoped_connection object.
        // A scoped_connection will disconnect the signal
        // when it goes out of scope.
        auto sc = s.connect_scoped(HelloWorld());

        // Invoke the signal again.
        // This should print "Hello, World!" to the console.
        s();
    }

    // Invoke the slot again.
    // Nothing is printed to the console.
    s();

    return 0;
}

In this example, a slot is connected to the signal using the signal::connect_scoped method which returns a scoped_connection object and is stored in sc.

Inside the scope block, the signal is invoked causing "Hello, World!" to be printed to the console.

When sc goes out of scope, it automatically disconnects the slot from the signal and when the signal is invoked again outside of the scope block, nothing is printed to the screen.

Hello, World!

Disconnecting Equivalent Slots

Similar to signal::connect, slots can be disconnected from the signal using signal::disconnect and passing the callable function object as a parameter. If the callable function object can be matched to an existing slot, it will be removed from the signal.

The function object must be equality comparable (that is, it must define the equality operator ==). If the function objects are not compareable, a sig::not_comparable_exception exception is thrown.

#include "signals.hpp"
#include <iostream>

float product(float x, float y) { return x * y; };
float quotient(float x, float y) { return x / y; }
float sum(float x, float y) { return x + y; }
float difference(float x, float y) { return x - y; }

int main()
{
    // Define a signal that takes two floats and returns a float.
    using signal = sig::signal<float(float,float)>;
    signal s;

    // Connect all of the functions to the signal.
    s.connect(&product);
    s.connect(&quotient);
    s.connect(&sum);
    s.connect(&difference);

    // Should print 2 (the result of difference)
    std::cout << *s(5.0f, 3.0f) << std::endl;

    // Disconnect the last slot.
    s.disconnect(&difference);

    // Should print 8 (the result of sum)
    std::cout << *s(5.0f, 3.0f) << std::endl;

    // Disconnect the first slot.
    // For efficency, sum becomes the first slot.
    s.disconnect(&product);

    // This actually prints 1.6667 since
    // quotient becomes the last slot in the signal.
    std::cout << *s(5.0f, 3.0f) << std::endl;

    s.disconnect(&quotient);

    // Should now print 8 (the result of sum)
    std::cout << *s(5.0f, 3.0f) << std::endl;

    s.disconnect(&sum);

    // All slots disconnected.
    // Invoking the signal now should result
    // in a disengaged opt::optional value.
    auto result = s(5.0f, 3.0f);
    if ( result )
    {
        std::cout << *result << std::endl;
    }
    else
    {
        std::cout << "Result is invalid!" << std::endl;
    }

    return 0;
}

In this example, a few free functions are defined. The functions are connected to the signal and the signal is invoked. The result of invoking the signal is 2 (the result from difference);

Then the last slot is disconnected and the signal is invoked again. This time, the result is 8 (the result from sum).

Then the product slot is disconnected and the signal is invoked again. It is interesting to note that the result that is printed to the console is from quotient despite the fact that sum should be the last slot. This occurs because when product is removed, product is swapped with the last slot and then product is removed from the end of the container. Since the signal uses an std::vector for its internal container, the slots are swapped to avoid having to relocate all of the slots that appear after the slot being removed. This makes the remove constant-time instead of linear in the number of slots being relocated. It is important to be aware of the slot reordering when removing slots from the beginning or middle of the container.

Then the quotient slot is removed and the signal is invoked again, printing 8 to the console (the result of sum).

Finally, sum is removed from the signal and the signal is invoked again. Since there are no slots left, the result of invoking the signal is a disengaged opt::optional value.

2
8
1.66667
8
Result is invalid!

Event Delegates

Using the sig::signal library, it is easy to create an event system that is similar to the C# event system.

Delegate Class

The Delegate class defines a set of callback functions. For simplicity of the example, only functions returning void are allowed but any number of arguments can be used to define a delegate.

#include "signals.hpp"
#include <iostream>

// Delegate that holds function callbacks.
// For simplicity, delegates are limited to void return types.
template<typename... Args>
class Delegate
{
public:
    using signal = sig::signal<void(Args...)>;
    using connection = sig::connection;

    // Adds a function callback to the delegate.
    template<typename Func>
    connection operator+=(Func&& f)
    {
        return m_Callbacks.connect(std::forward<Func>(f));
    }

    // Remove a function callback.
    // Returns the number of functions removed.
    template<typename Func>
    std::size_t operator-=(Func&& f)
    {
        return m_Callbacks.disconnect(std::forward<Func>(f));
    }

    // Invoke the delegate.
    // All connected callbacks are invoked.
    void operator()(Args... args)
    {
        m_Callbacks(std::forward<Args>(args)...);
    }
private:
    signal m_Callbacks;
};

The Delegate class is a wrapper for a sig::signal but provides += and -= operators for connecting and disconnecting slots to the underlying signal (similar to C# delegates). The delegate also defines a function call operator to invoke the registered callbacks.

Event

With the Delegate class defined, a few Events can be defined that take an EventArgs as the only argument to the event callback:

// Base class for all event args.
class EventArgs
{
public:
    EventArgs() = default;
    virtual ~EventArgs() = default;
};

// Define an event that takes a reference to EventArgs
// as it's only argument.
using Event = Delegate<EventArgs&>;

The Event object can be used to register callbacks that takes a reference to an EventArgs as the only argument.

Mouse Motion Event

The Event can be used for generic callback functions, but when you need to pass more information to the callback, you can pass those arguments through the event args. For example, the MouseMotionEventArgs stores the x, and y coordinates of the mouse when the event is triggered.

class MouseMotionEventArgs : public EventArgs
{
public:
    using base = EventArgs;

    MouseMotionEventArgs( int x, int y )
        : base()
        , X(x)
        , Y(y)
    {}

    int X;
    int Y;
};

// Define an event that is fired when the mouse moves over
// the application window.
using MouseMotionEvent = Delegate<MouseMotionEventArgs&>;

The MouseMotionEventArgs stores the x, and y coordinates of the mouse when the event is fired. The MouseMotionEvent is an event delegate that accepts callback functions that take a reference to a MouseMotionEventArgs as the only argument.

Application Class

With a few events defined, an Application class can be created that exposes a few events that the client can register callback functions for.

// The Application class defines a few events that can be handled by
// callback functions elsewhere.
class Application
{
public:
    Application() = default;
    virtual ~Application() = default;

    // Application events.
    Event            Update;
    Event            Render;
    MouseMotionEvent MouseMoved;

protected:
    // Give the WndProc method access to the
    // protected members of this class.
    friend void WndProc();

    virtual void OnUpdate()
    {
        EventArgs e;
        // Invoke event.
        Update(e);
    }

    virtual void OnRender()
    {
        EventArgs e;
        // Invoke event.
        Render(e);
    }

    virtual void OnMouseMoved(int x, int y)
    {
        MouseMotionEventArgs e(x, y);
        // Invoke event.
        MouseMoved(e);
    }
};

The Application class exposes 3 events:

Update
Render
MouseMoved

The client application can register callback functions that are invoked when those events are fired by the application.

As a simple example, let's suppose that the windows process callback function looks like this:

// I know, globals are evil, but it makes the
// example easier.
static Application app;
// Simulate a windows process function.
void WndProc()
{
    app.OnUpdate();
    app.OnRender();
    app.OnMouseMoved(60, 80);
}

Client Code

The client code can register callback functions that are invoked when the events are fired by the application. First, a few callback functions are defined.

// Define a few callback functions that will handle events from the application.
void OnUpdate(EventArgs& e)
{
    std::cout << "Update game..." << std::endl;
}

void OnRender(EventArgs& e)
{
    std::cout << "Render game..." << std::endl;
}

void OnMouseMoved(MouseMotionEventArgs& e)
{
    std::cout << "Mouse moved: " << e.X << ", " << e.Y << std::endl;
}

Then, the callback functions can be registered with the applications events:

int main()
{
    // Register some callback functions.
    app.Update += &OnUpdate;
    app.Render += &OnRender;
    app.MouseMoved += &OnMouseMoved;

    // Execute the windows event processor
    WndProc();

    // Unregister callback functions
    app.Update -= &OnUpdate;
    app.Render -= &OnRender;
    app.MouseMoved -= &OnMouseMoved;

    // Execute the windows event processor again.
    // This time, nothing should happen.
    WndProc();

    return 0;
}

First, the callback functions are registered with the applications events.

The WndProc function is invoked which simulates the windows processor function. The WndProc function just invokes the applications events which causes the callback functions to be invoked and the following is printed to the console:

Update game...
Render game...
Mouse moved: 60, 80

Next, the callback functions are unregistered from the application's events and the WndProc function is called again. This time, nothing is printed to the console.

Conclusion

The sig::signal library is a C++11 single-header (okay 2 header) library that provides a signal & slot implementation.

Known Issues

sig::signal does not support connection groups (similar to boost::signals2).
sig::signal does not support extended connections (in boost, this is signal::connect_extended).
Currently, the sig::detail::slot_iterator class is used to iterate slots in a Combiner. The iterator should automatically skip blocked or disconnected slots but these are still invoked when the iterator is dereferenced resulting in a disengaged optional value being retured from the slot. Ideally, blocked or disconnected slots should be skipped when the iterator is incremented (using either pre or post-increment operator).
When the sig::detail::slot_iterator is dereferenced in the Combiner, the result of invoking the slot is not cached. This means that dereferencing the iterator in the combiner several times will invoke the slot each time which could potentially be an expensive operation or even change the result that is returned from the slot (if invoking the slot has side-effects). Ideally, the result of invoking the slot should be cached until the iterator is incremented to the next slot.

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signals's Introduction