Calling a single slot from a signal isn't very interesting, so we can make the Hello, World program more interesting by splitting the work of printing "Hello, World!" into two completely separate slots. The first slot will print "Hello" and may look like this:

struct Hello 
{
  void operator()() const
  {
    std::cout << "Hello";
  }
};

The second slot will print ", World!" and a newline, to complete the program. The second slot may look like this:

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

Like in our previous example, we can create a signal sig that takes no arguments and has a void return value. This time, we connect both a hello and a world slot to the same signal, and when we call the signal both slots will be called.

boost::signal<void ()> sig;

sig.connect(Hello());
sig.connect(World());

sig();

boost::signal0<void> sig;

sig.connect(Hello());
sig.connect(World());

sig();

By default, slots are called in first-in first-out (FIFO) order, so the output of this program will be as expected:

Hello, World!

Slots are free to have side effects, and that can mean that some slots will have to be called before others even if they are not connected in that order. The Boost.Signals library allows slots to be placed into groups that are ordered in some way. For our Hello, World program, we want "Hello" to be printed before ", World!", so we put "Hello" into a group that must be executed before the group that ", World!" is in. To do this, we can supply an extra parameter at the beginning of the connect call that specifies the group. Group values are, by default, ints, and are ordered by the integer < relation. Here's how we construct Hello, World:

boost::signal<void ()> sig;
sig.connect(1, World());
sig.connect(0, Hello());
sig();

boost::signal0<void> sig;
sig.connect(1, World());
sig.connect(0, Hello());
sig();

This program will correctly print "Hello, World!", because the Hello object is in group 0, which precedes group 1 where the World object resides. The group parameter is, in fact, optional. We omitted it in the first Hello, World example because it was unnecessary when all of the slots are independent. So what happens if we mix calls to connect that use the group parameter and those that don't? The "unnamed" slots (i.e., those that have been connected without specifying a group name) can be placed at the front or back of the slot list (by passing boost::signals::at_front or boost::signals::at_back as the last parameter to connect, respectively), and defaults to the end of the list. When a group is specified, the final parameter describes where the slot will be placed within the group ordering. If we add a new slot to our example like this:

struct GoodMorning
{
  void operator()() const
  {
    std::cout << "... and good morning!" << std::endl;
  }
};

sig.connect(GoodMorning());

... we will get the result we wanted:

Hello, World!
... and good morning!

Signals can propagate arguments to each of the slots they call. For instance, a signal that propagates mouse motion events might want to pass along the new mouse coordinates and whether the mouse buttons are pressed.

As an example, we'll create a signal that passes two float arguments to its slots. Then we'll create a few slots that print the results of various arithmetic operations on these values.

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;
}

boost::signal<void (float, float)> sig;

sig.connect(&print_sum);
sig.connect(&print_product);
sig.connect(&print_difference);
sig.connect(&print_quotient);

sig(5, 3);

boost::signal2<void, float, float> sig;

sig.connect(&print_sum);
sig.connect(&print_product);
sig.connect(&print_difference);
sig.connect(&print_quotient);

sig(5, 3);

This program will print out the following:

The sum is 8
The product is 15
The difference is 2
The quotient is 1.66667

So any values that are given to sig when it is called like a function are passed to each of the slots. We have to declare the types of these values up front when we create the signal. The type boost::signal<void (float, float)> means that the signal has a void return value and takes two float values. Any slot connected to sig must therefore be able to take two float values.

Just as slots can receive arguments, they can also return values. These values can then be returned back to the caller of the signal through a combiner. The combiner is a mechanism that can take the results of calling slots (there many be no results or a hundred; we don't know until the program runs) and coalesces them into a single result to be returned to the caller. The single result is often a simple function of the results of the slot calls: the result of the last slot call, the maximum value returned by any slot, or a container of all of the results are some possibilities.

We can modify our previous arithmetic operations example slightly so that the slots all return the results of computing the product, quotient, sum, or difference. Then the signal itself can return a value based on these results to be printed:

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; }

boost::signal<float (float x, float y)> sig;

sig.connect(&product);
sig.connect(&quotient);
sig.connect(&sum);
sig.connect(&difference);

std::cout << sig(5, 3) << std::endl;

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; }

boost::signal2<float, float, float> sig;

sig.connect(&product);
sig.connect(&quotient);
sig.connect(&sum);
sig.connect(&difference);

std::cout << sig(5, 3) << std::endl;

This example program will output 2. This is because the default behavior of a signal that has a return type (float, the first template argument given to the boost::signal class template) is to call all slots and then return the result returned by the last slot called. This behavior is admittedly silly for this example, because slots have no side effects and the result is the last slot connect.

A more interesting signal result would be the maximum of the values returned by any slot. To do this, we create a custom combiner that looks like this:

template<typename T>
struct maximum
{
  typedef T result_type;

  template<typename InputIterator>
  T operator()(InputIterator first, InputIterator last) const
  {
    // If there are no slots to call, just return the
    // default-constructed value
    if (first == last)
      return T();

    T max_value = *first++;
    while (first != last) {
      if (max_value < *first)
        max_value = *first;
      ++first;
    }
  
    return max_value;
  }
};

The maximum class template acts as a function object. Its result type is given by its template parameter, and this is the type it expects to be computing the maximum based on (e.g., maximum<float> would find the maximum float in a sequence of floats). When a maximum object is invoked, it is given an input iterator sequence [first, last) that includes the results of calling all of the slots. maximum uses this input iterator sequence to calculate the maximum element, and returns that maximum value.

We actually use this new function object type by installing it as a combiner for our signal. The combiner template argument follows the signal's calling signature:

boost::signal<float (float x, float y), 
              maximum<float> > sig;

boost::signal2<float, float, float, 
               maximum<float> > sig;

Now we can connect slots that perform arithmetic functions and use the signal:

sig.connect(&quotient);
sig.connect(&product);
sig.connect(&sum);
sig.connect(&difference);

std::cout << sig(5, 3) << std::endl;

The output of this program will be 15, because regardless of the order in which the slots are connected, the product of 5 and 3 will be larger than the quotient, sum, or difference.

In other cases we might want to return all of the values computed by the slots together, in one large data structure. This is easily done with a different combiner:

template<typename Container>
struct aggregate_values
{
  typedef Container result_type;

  template<typename InputIterator>
  Container operator()(InputIterator first, InputIterator last) const
  {
    return Container(first, last);
  }
};

Again, we can create a signal with this new combiner:

boost::signal<float (float, float), 
    aggregate_values<std::vector<float> > > sig;

sig.connect(&quotient);
sig.connect(&product);
sig.connect(&sum);
sig.connect(&difference);

std::vector<float> results = sig(5, 3);
std::copy(results.begin(), results.end(), 
    std::ostream_iterator<float>(cout, " "));

boost::signal2<float, float, float,
    aggregate_values<std::vector<float> > > sig;

sig.connect(&quotient);
sig.connect(&product);
sig.connect(&sum);
sig.connect(&difference);

std::vector<float> results = sig(5, 3);
std::copy(results.begin(), results.end(), 
    std::ostream_iterator<float>(cout, " "));

The output of this program will contain 15, 8, 1.6667, and 2. It is interesting here that the first template argument for the signal class, float, is not actually the return type of the signal. Instead, it is the return type used by the connected slots and will also be the value_type of the input iterators passed to the combiner. The combiner itself is a function object and its result_type member type becomes the return type of the signal.

The input iterators passed to the combiner transform dereference operations into slot calls. Combiners therefore have the option to invoke only some slots until some particular criterion is met. For instance, in a distributed computing system, the combiner may ask each remote system whether it will handle the request. Only one remote system needs to handle a particular request, so after a remote system accepts the work we do not want to ask any other remote systems to perform the same task. Such a combiner need only check the value returned when dereferencing the iterator, and return when the value is acceptable. The following combiner returns the first non-NULL pointer to a FulfilledRequest data structure, without asking any later slots to fulfill the request:

struct DistributeRequest {
  typedef FulfilledRequest* result_type;

  template<typename InputIterator>
  result_type operator()(InputIterator first, InputIterator last) const
  {
    while (first != last) {
      if (result_type fulfilled = *first)
        return fulfilled;
      ++first;
    }
    return 0;
  }
};

Slots aren't expected to exist indefinately after they are connected. Often slots are only used to receive a few events and are then disconnected, and the programmer needs control to decide when a slot should no longer be connected.

The entry point for managing connections explicitly is the boost::signals::connection class. The connection class uniquely represents the connection between a particular signal and a particular slot. The connected() method checks if the signal and slot are still connected, and the disconnect() method disconnects the signal and slot if they are connected before it is called. Each call to the signal's connect() method returns a connection object, which can be used to determine if the connection still exists or to disconnect the signal and slot.

boost::signals::connection c = sig.connect(HelloWorld());
if (c.connected()) {
// c is still connected to the signal
  sig(); // Prints "Hello, World!"
}

c.disconnect(); // Disconnect the HelloWorld object
assert(!c.connected()); c isn't connected any more

sig(); // Does nothing: there are no connected slots

Slots can be temporarily "blocked", meaning that they will be ignored when the signal is invoked but have not been disconnected. The block member function temporarily blocks a slot, which can be unblocked via unblock. Here is an example of blocking/unblocking slots:

boost::signals::connection c = sig.connect(HelloWorld());
sig(); // Prints "Hello, World!"

c.block(); // block the slot
assert(c.blocked());
sig(); // No output: the slot is blocked

c.unblock(); // unblock the slot
sig(); // Prints "Hello, World!"

The boost::signals::scoped_connection class references a signal/slot connection that will be disconnected when the scoped_connection class goes out of scope. This ability is useful when a connection need only be temporary, e.g.,

{
  boost::signals::scoped_connection c = sig.connect(ShortLived());
  sig(); // will call ShortLived function object
}
sig(); // ShortLived function object no longer connected to sig

One can disconnect slots that are equivalent to a given function object using a form of the disconnect method, so long as the type of the function object has an accessible == operator. For instance:

void foo();
void bar();

signal<void()> sig;

sig.connect(&foo);
sig.connect(&bar);

// disconnects foo, but not bar
sig.disconnect(&foo);

void foo();
void bar();

signal0<void> sig;

sig.connect(&foo);
sig.connect(&bar);

// disconnects foo, but not bar
sig.disconnect(&foo);

Boost.Signals can automatically track the lifetime of objects involved in signal/slot connections, including automatic disconnection of slots when objects involved in the slot call are destroyed. For instance, consider a simple news delivery service, where clients connect to a news provider that then sends news to all connected clients as information arrives. The news delivery service may be constructed like this:

class NewsItem { /* ... */ };

boost::signal<void (const NewsItem&)> deliverNews;

class NewsItem { /* ... */ };

boost::signal1<void, const NewsItem&> deliverNews;

Clients that wish to receive news updates need only connect a function object that can receive news items to the deliverNews signal. For instance, we may have a special message area in our application specifically for news, e.g.,:

struct NewsMessageArea : public MessageArea
{
public:
  // ...

  void displayNews(const NewsItem& news) const
  {
    messageText = news.text();
    update();
  }
};

// ...
NewsMessageArea newsMessageArea = new NewsMessageArea(/* ... */);
// ...
deliverNews.connect(boost::bind(&NewsMessageArea::displayNews, 
                                newsMessageArea, _1));

However, what if the user closes the news message area, destroying the newsMessageArea object that deliverNews knows about? Most likely, a segmentation fault will occur. However, with Boost.Signals one need only make NewsMessageArea trackable, and the slot involving newsMessageArea will be disconnected when newsMessageArea is destroyed. The NewsMessageArea class is made trackable by deriving publicly from the boost::signals::trackable class, e.g.:

struct NewsMessageArea : public MessageArea, public boost::signals::trackable
{
  // ...
};

At this time there is a significant limitation to the use of trackable objects in making slot connections: function objects built using Boost.Bind are understood, such that pointers or references to trackable objects passed to boost::bind will be found and tracked.

Warning: User-defined function objects and function objects from other libraries (e.g., Boost.Function or Boost.Lambda) do not implement the required interfaces for trackable object detection, and will silently ignore any bound trackable objects. Future versions of the Boost libraries will address this limitation.

Signal/slot disconnections occur when any of these conditions occur:

These events can occur at any time without disrupting a signal's calling sequence. If a signal/slot connection is disconnected at any time during a signal's calling sequence, the calling sequence will still continue but will not invoke the disconnected slot. Additionally, a signal may be destroyed while it is in a calling sequence, and which case it will complete its slot call sequence but may not be accessed directly.

Signals may be invoked recursively (e.g., a signal A calls a slot B that invokes signal A...). The disconnection behavior does not change in the recursive case, except that the slot calling sequence includes slot calls for all nested invocations of the signal.

Slots in the Boost.Signals library are created from arbitrary function objects, and therefore have no fixed type. However, it is commonplace to require that slots be passed through interfaces that cannot be templates. Slots can be passed via the slot_type for each particular signal type and any function object compatible with the signature of the signal can be passed to a slot_type parameter. For instance:

class Button 
{
  typedef boost::signal<void (int x, int y)> OnClick;

public:
  void doOnClick(const OnClick::slot_type& slot);

private:
  OnClick onClick;
};

void Button::doOnClick(
      const OnClick::slot_type& slot
    )
{
  onClick.connect(slot);
}

void printCoordinates(long x, long y)
{
  std::cout << "(" << x << ", " << y << ")\n";
}

void f(Button& button)
{
  button.doOnClick(&printCoordinates);
}

class Button 
{
  typedef boost::signal2<void,int,int> OnClick;

public:
  void doOnClick(const OnClick::slot_type& slot);

private:
  OnClick onClick;
};

void Button::doOnClick(
      const OnClick::slot_type& slot
    )
{
  onClick.connect(slot);
}

void printCoordinates(long x, long y)
{
  std::cout << "(" << x << ", " << y << ")\n";
}

void f(Button& button)
{
  button.doOnClick(&printCoordinates);
}

The doOnClick method is now functionally equivalent to the connect method of the onClick signal, but the details of the doOnClick method can be hidden in an implementation detail file.