BoostBoost.Signals Design Rationale

This document describes the rationale behind some of the major design decisions made for the Boost.Signals library.

Table of Contents


Choice of Slot Definitions

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.

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.

User-level Connection Management

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:

Combiner Interface

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 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).

Connection Interfaces: += operator

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:

trackable rationale

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.

trackable copying behavior

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.

Why derivation from trackable?

For trackable to work properly, there are two constraints:

Clearly, deriving from trackable meets these two guidelines. We have not yet found a superior solution.

libsigc++

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:

.NET delegates

Microsoft has introduced the .NET Framework and an associated set of languages and language extensions, one of which is the delgate. Delegates are similar to signals and slots, but they are more limited than most C++ signals and slots implemetations in that they:


Doug Gregor
Last modified: Fri Oct 11 05:41:04 EDT 2002