|  | Home | Libraries | People | FAQ | More | 
The definition of a slot differs amongst signals and slots libraries. Within Boost.Signals, a slot is defined in a very loose manner: it can be any function object that is callable given parameters of the types specified by the signal, and whose return value is convertible to the result type expected by the signal. However, alternative definitions have associated pros and cons that were considered prior to the construction of Boost.Signals.
Slots derive from a specific base
        class: generally a scheme such as this will require
        all user-defined slots to derive from some library-specified
        Slot abstract class that defines a virtual
        function calling the slot. Adaptors can be used to convert a
        definition such as this to a definition similar to that used
        by Boost.Signals, but the use of a large number of small
        adaptor classes containing virtual functions has been found to
        cause an unacceptable increase in the size of executables
        (polymorphic class types require more code than
        non-polymorphic types).
This approach does have the benefit of simplicity of implementation and user interface, from an object-oriented perspective.
Slots constructed from a set of
        primitives: in this scheme the slot can have a
        limited set of types (often derived from a common abstract
        base class) that are constructed from some library-defined set
        of primitives that often include conversions from free
        function pointers and member function pointers, and a limited
        set of binding capabilities. Such an approach is reasonably
        simple and cover most common cases, but it does not allow a
        large degree of flexibility in slot construction. Libraries
        for function object composition have become quite advanced and
        it is out of the scope of a signals and slots library to
        encorporate such enhancements. Thus Boost.Signals does not
        include argument binding or function object composition
        primitives, but instead provides a hook (via the
        visit_each
        mechanism) that allows existing binder/composition libraries
        to provide the necessary information to Signals.
Users not satisfied with the slot definition choice may opt to replace the default slot function type with an alternative that meets their specific needs.
 Users need to have fine control over the connection of
    signals to slots and their eventual disconnection. The approach
    taken by Boost.Signals is to return a
    connection object that enables
    connected/disconnected query, manual disconnection, and an
    automatic disconnection on destruction mode. Some other possible
    interfaces include:
Pass slot to
        disconnect: in this interface model, the
        disconnection of a slot connected with
        sig.connect(slot) is
        performed via
        sig.disconnect(slot). Internally,
        a linear search using slot comparison is performed and the
        slot, if found, is removed from the list. Unfortunately,
        querying connectedness will generally also end up as
        linear-time operations. This model also fails for
        implementation reasons when slots become more complex than
        simple function pointers, member function pointers and a
        limited set of compositions and argument binders: to match the
        slot given in the call to
        disconnect with an
        existing slot we would need to be able to compare arbitrary
        function objects, which is not feasible.
Pass a token to disconnect: this approach identifies slots with a token that is easily comparable (e.g., a string), enabling slots to be arbitrary function objects. While this approach is essentially equivalent to the approach taken by Boost.Signals, it is possibly more error-prone for several reasons:
Connections and disconnections must be paired, so
            the problem becomes similar to the problems incurred when
            pairing new and delete for
            dynamic memory allocation. While errors of this sort would
            not be catastrophic for a signals and slots
            implementation, their detection is generally
            nontrivial.
Tokens must be unique, otherwise two slots will have the same name and will be indistinguishable. In environments where many connections will be made dynamically, name generation becomes an additional task for the user. Uniqueness of tokens also results in an additional failure mode when attempting to connect a slot using a token that has already been used.
More parameterization would be required, because the token type must be user-defined. Additional parameterization steepens the learning curver and overcomplicates a simple interface.
 This type of interface is supported in Boost.Signals
        via the slot grouping mechanism. It augments the
        connection object-based
        connection management scheme.
The Combiner interface was chosen to mimic a call to an algorithm in the C++ standard library. It is felt that by viewing slot call results as merely a sequence of values accessed by input iterators, the combiner interface would be most natural to a proficient C++ programmer. Competing interface design generally required the combiners to be constructed to conform to an interface that would be customized for (and limited to) the Signals library. While these interfaces are generally enable more straighforward implementation of the signals & slots libraries, the combiners are unfortunately not reusable (either in other signals & slots libraries or within other generic algorithms), and the learning curve is steepened slightly to learn the specific combiner interface.
The Signals formulation of combiners is based on the combiner using the "pull" mode of communication, instead of the more complex "push" mechanism. With a "pull" mechanism, the combiner's state can be kept on the stack and in the program counter, because whenever new data is required (i.e., calling the next slot to retrieve its return value), there is a simple interface to retrieve that data immediately and without returning from the combiner's code. Contrast this with the "push" mechanism, where the combiner must keep all state in class members because the combiner's routines will be invoked for each signal called. Compare, for example, a combiner that returns the maximum element from calling the slots. If the maximum element ever exceeds 100, no more slots are to be called.
| Pull | Push | 
|---|---|
| 
struct pull_max {
  typedef int result_type;
  template<typename InputIterator>
  result_type operator()(InputIterator first,
                         InputIterator last)
  {
    if (first == last)
      throw std::runtime_error("Empty!");
    int max_value = *first++;
    while(first != last && *first <= 100) {
      if (*first > max_value)
        max_value = *first;
      ++first;
    }
    return max_value;
  }
};
 | 
struct push_max {
  typedef int result_type;
  push_max() : max_value(), got_first(false) {}
  // returns false when we want to stop
  bool operator()(int result) {
    if (result > 100)
      return false;
    if (!got_first) {
      got_first = true;
      max_value = result;
      return true;
    }
    if (result > max_value)
      max_value = result;
    return true;
  }
  int get_value() const 
  { 
    if (!got_first)
      throw std::runtime_error("Empty!");
    return max_value; 
  }
private:
  int  max_value; 
  bool got_first;
};
 | 
There are several points to note in these examples. The
    "pull" version is a reusable function object that is based on an
    input iterator sequence with an integer value_type,
    and is very straightforward in design. The "push" model, on the
    other hand, relies on an interface specific to the caller and is
    not generally reusable. It also requires extra state values to
    determine, for instance, if any elements have been
    received. Though code quality and ease-of-use is generally
    subjective, the "pull" model is clearly shorter and more reusable
    and will often be construed as easier to write and understand,
    even outside the context of a signals & slots library.
The cost of the "pull" combiner interface is paid in the implementation of the Signals library itself. To correctly handle slot disconnections during calls (e.g., when the dereference operator is invoked), one must construct the iterator to skip over disconnected slots. Additionally, the iterator must carry with it the set of arguments to pass to each slot (although a reference to a structure containing those arguments suffices), and must cache the result of calling the slot so that multiple dereferences don't result in multiple calls. This apparently requires a large degree of overhead, though if one considers the entire process of invoking slots one sees that the overhead is nearly equivalent to that in the "push" model, but we have inverted the control structures to make iteration and dereference complex (instead of making combiner state-finding complex).
 Boost.Signals supports a connection syntax with the form
    sig.connect(slot), but a
    more terse syntax sig += slot has been suggested (and
    has been used by other signals & slots implementations). There
    are several reasons as to why this syntax has been
    rejected:
It's unnecessary: the
        connection syntax supplied by Boost.Signals is no less
        powerful that that supplied by the +=
        operator. The savings in typing (connect()
        vs. +=) is essentially negligible. Furthermore,
        one could argue that calling connect() is more
        readable than an overload of +=.
Ambiguous return type:
        there is an ambiguity concerning the return value of the
        += operation: should it be a reference to the
        signal itself, to enable sig += slot1 += slot2,
        or should it return a
        connection for the
        newly-created signal/slot connection?
Gateway to operators -=,
        +: when one has added a connection operator
        +=, it seems natural to have a disconnection
        operator -=. However, this presents problems when
        the library allows arbitrary function objects to implicitly
        become slots, because slots are no longer comparable.  
 The second obvious addition when one has
        operator+= would be to add a +
        operator that supports addition of multiple slots, followed by
        assignment to a signal. However, this would require
        implementing + such that it can accept any two
        function objects, which is technically infeasible.
 The trackable
      class is the primary user interface to automatic connection
      lifetime management, and its design affects users directly. Two
      issues stick out most: the odd copying behavior of
      trackable, and the limitation requiring users to
      derive from trackable to create types that can
      participate in automatic connection management.
 The copying behavior of
      trackable is essentially
      that trackable subobjects
      are never copied; instead, the copy operation is merely a
      no-op. To understand this, we look at the nature of a
      signal-slot connection and note that the connection is based on
      the entities that are being connected; when one of the entities
      is destroyed, the connection is destroyed. Therefore, when a
      trackable subobject is
      copied, we cannot copy the connections because the connections
      don't refer to the target entity - they refer to the source
      entity. This reason is dual to the reason signals are
      noncopyable: the slots connected to them are connected to that
      particular signal, not the data contained in the signal.
libsigc++ is a C++ signals & slots library that originally started as part of an initiative to wrap the C interfaces to GTK libraries in C++, and has grown to be a separate library maintained by Karl Nelson. There are many similarities between libsigc++ and Boost.Signals, and indeed Boost.Signals was strongly influenced by Karl Nelson and libsigc++. A cursory inspection of each library will find a similar syntax for the construction of signals and in the use of connections and automatic connection lifetime management. There are some major differences in design that separate these libraries:
Slot definitions: slots in libsigc++ are created using a set of primitives defined by the library. These primitives allow binding of objects (as part of the library), explicit adaptation from the argument and return types of the signal to the argument and return types of the slot (libsigc++ is, by default, more strict about types than Boost.Signals). A discussion of this approach with a comparison against the approach taken by Boost.Signals is given in Choice of Slot Definitions.
Combiner/Marshaller interface: the equivalent to Boost.Signals combiners in libsigc++ are the marshallers. Marshallers are similar to the "push" interface described in Combiner Interface, and a proper treatment of the topic is given there.
Microsoft has introduced the .NET Framework and an associated set of languages and language extensions, one of which is the delegate. Delegates are similar to signals and slots, but they are more limited than most C++ signals and slots implementations in that they:
Require exact type matches between a delegate and what it is calling.
Only return the result of the last target called, with no option for customization.
Must call a method with this already
          bound.