samien
/
boost
mirrorاز https://github.com/boostorg/boost


			
							123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307
							<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN">

<html>
<head>
    <meta name="generator" content="HTML Tidy, see www.w3.org">
    <meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
    <meta name="GENERATOR" content="Microsoft FrontPage 4.0">
    <meta name="ProgId" content="FrontPage.Editor.Document">

    <title>Generic Programming Techniques</title>
</head>

<body bgcolor="#FFFFFF" text="#000000">
    <img src="../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="center"
    width="277" height="86"> 

    <h1>Generic Programming Techniques</h1>

    <p>This is an incomplete survey of some of the generic programming
    techniques used in the <a href="../index.htm">boost</a> libraries.

    <h2>Table of Contents</h2>

    <ul>
      <li><a href="#traits">Traits</a>

      <li><a href="#tag_dispatching">Tag Dispatching</a>

      <li><a href="#type_generator">Type Generators</a>

      <li><a href="#object_generator">Object Generators</a>

      <li><a href="#policies">Policies Classes</a>

      <li><a href="#adaptors">Adaptors</a>
    </ul>

    <h2><a name="traits">Traits</a></h2>

    <p>A traits class provides a way of associating information with a
    compile-time entity (a type, integral constant, or address). For example,
    the class template <tt><a href=
    "http://www.sgi.com/tech/stl/iterator_traits.html">std::iterator_traits&lt;T&gt;</a></tt>
    looks something like this:

    <blockquote>
<pre>
template &lt;class Iterator&gt;
struct iterator_traits {
  typedef ... iterator_category;
  typedef ... value_type;
  typedef ... difference_type;
  typedef ... pointer;
  typedef ... reference;
};
</pre>
    </blockquote>
    The traits' <tt>value_type</tt> gives generic code the type which the
    iterator is "pointing at", while the <tt>iterator_category</tt> can be used
    to select more efficient algorithms depending on the iterator's
    capabilities. 

    <p>A key feature of traits templates is that they're <i>non-intrusive</i>:
    they allow us to associate information with arbitrary types, including
    built-in types and types defined in third-party libraries, Normally, traits
    are specified for a particular type by (partially) specializing the traits
    template.

    <p>For an in-depth description of <tt>std::iterator_traits</tt>, see <a href=
    "http://www.sgi.com/tech/stl/iterator_traits.html">this page</a> provided
    by SGI. Another very different expression of the traits idiom in the
    standard is <tt>std::numeric_limits&lt;T&gt;</tt> which provides constants
    describing the range and capabilities of numeric types.

    <h2><a name="tag_dispatching">Tag Dispatching</a></h2>

    <p>
     A technique that often goes hand in hand with traits classes is
     tag dispatching, which is a way of using function overloading to
     dispatch based on properties of a type. A good example of this is
     the implementation of the <a
     href="http://www.sgi.com/tech/stl/advance.html"><tt>std::advance()</tt></a>
     function in the C++ Standard Library, which increments an
     iterator <tt>n</tt> times. Depending on the kind of iterator,
     there are different optimizations that can be applied in the
     implementation. If the iterator is <a
     href="http://www.sgi.com/tech/stl/RandomAccessIterator.html">random
     access</a> (can jump forward and backward arbitrary distances),
     then the <tt>advance()</tt> function can simply be implemented
     with <tt>i += n</tt>, and is very efficient: constant time. If
     the iterator is <a
     href="http://www.sgi.com/tech/stl/BidirectionalIterator.html">bidirectional</a>,
     then it makes sense for <tt>n</tt> to be negative, we can
     decrement the iterator <tt>n</tt> times.
     </p>
     <p>
     The relation between tag dispatching and traits classes is
     that the property used for dispatching (in this case the
     <tt>iterator_category</tt>) is accessed through a traits class.
     The main <tt>advance()</tt> function uses the <a
     href="http://www.sgi.com/tech/stl/iterator_traits.html"><tt>iterator_traits</tt></a>
     class to get the <tt>iterator_category</tt>. It then makes a call
     the the overloaded <tt>advance_dispatch()</tt> function. 
     The
     appropriate <tt>advance_dispatch()</tt> is selected by the
     compiler based on whatever type the <tt>iterator_category</tt>
     resolves to, either <a
     href="http://www.sgi.com/tech/stl/input_iterator_tag.html">
     <tt>input_iterator_tag</tt></a>, <a
     href="http://www.sgi.com/tech/stl/bidirectional_iterator_tag.html">
     <tt>bidirectional_iterator_tag</tt></a>, or <a
     href="http://www.sgi.com/tech/stl/random_access_iterator_tag.html">
     <tt>random_access_iterator_tag</tt></a>.  A <b><i>tag</i></b> is
     simply a class whose only purpose is to convey some property for
     use in tag dispatching and similar techniques. Refer to <a
     href="http://www.sgi.com/tech/stl/iterator_tags.html">this
     page</a> for a more detailed description of iterator tags.
     </p>
    <blockquote>
<pre>
namespace std {
  struct input_iterator_tag { };
  struct bidirectional_iterator_tag { };
  struct random_access_iterator_tag { };

  namespace detail {
    template &lt;class InputIterator, class Distance&gt;
    void advance_dispatch(InputIterator&amp; i, Distance n, <b>input_iterator_tag</b>) {
      while (n--) ++i;
    }

    template &lt;class BidirectionalIterator, class Distance&gt;
    void advance_dispatch(BidirectionalIterator&amp; i, Distance n, 
		   <b>bidirectional_iterator_tag</b>) {
      if (n &gt;= 0)
	while (n--) ++i;
      else
	while (n++) --i;
    }

    template &lt;class RandomAccessIterator, class Distance&gt;
    void advance_dispatch(RandomAccessIterator&amp; i, Distance n, 
		   <b>random_access_iterator_tag</b>) {
      i += n;
    }
  }

  template &lt;class InputIterator, class Distance&gt;
  void advance(InputIterator&amp; i, Distance n) {
    typename <b>iterator_traits&lt;InputIterator&gt;::iterator_category</b> category;
    detail::advance_dispatch(i, n, <b>category</b>);
  }
}
</pre>
    </blockquote>

    <h2><a name="type_generator">Type Generators</a></h2>

    <p>A <i>type generator</i> is a template whose only purpose is to
    synthesize a single new type based on its template argument(s). The
    generated type is usually expressed as a nested typedef named,
    appropriately <tt>type</tt>. A type generator is usually used to
    consolidate a complicated type expression into a simple one, as in
    <tt>boost::<a href=
    "../libs/utility/filter_iterator.hpp">filter_iterator_generator</a></tt>,
    which looks something like this:

    <blockquote>
<pre>
template &lt;class Predicate, class Iterator, 
    class Value = <i>complicated default</i>,
    class Reference = <i>complicated default</i>,
    class Pointer = <i>complicated default</i>,
    class Category = <i>complicated default</i>,
    class Distance = <i>complicated default</i>
         &gt;
struct filter_iterator_generator {
    typedef iterator_adaptor&lt;
        Iterator,filter_iterator_policies&lt;Predicate,Iterator&gt;,
        Value,Reference,Pointer,Category,Distance&gt; <b>type</b>;
};
</pre>
    </blockquote>

    <p>Now, that's complicated, but producing an adapted filter iterator is
    much easier. You can usually just write:

    <blockquote>
<pre>
boost::filter_iterator_generator&lt;my_predicate,my_base_iterator&gt;::type
</pre>
    </blockquote>

    <h2><a name="object_generator">Object Generators</a></h2>

    <p>An <i>object generator</i> is a function template whose only purpose is
    to construct a new object out of its arguments. Think of it as a kind of
    generic constructor. An object generator may be more useful than a plain
    constructor when the exact type to be generated is difficult or impossible
    to express and the result of the generator can be passed directly to a
    function rather than stored in a variable. Most object generators are named
    with the prefix "<tt>make_</tt>", after <tt>std::<a href=
    "http://www.sgi.com/tech/stl/pair.html">make_pair</a>(const T&amp;, const U&amp;)</tt>.

    <p>Here is an example, using another standard object generator, <tt>std::<a
    href=
    "http://www.sgi.com/tech/stl/back_insert_iterator.html">back_inserter</a>()</tt>:

    <blockquote>
<pre>
// Append the items in [start, finish) to c
template &lt;class Container, class Iterator&gt;
void append_sequence(Container&amp; c, Iterator start, Iterator finish)
{
   std::copy(start, finish, <b>std::back_inserter</b>(c));
}
</pre>
    </blockquote>

    <p>Without using the object generator the example above would look like:
    write:

    <blockquote>
<pre>
// Append the items in [start, finish) to c
template &lt;class Container, class Iterator&gt;
void append_sequence(Container&amp; c, Iterator start, Iterator finish)
{
   std::copy(start, finish, <b>std::back_insert_iterator&lt;Container&gt;</b>(c));
}
</pre>
    </blockquote>

    <p>As expressions get more complicated the need to reduce the verbosity of
    type specification gets more compelling.

    <h2><a name="policies">Policies Classes</a></h2>

    <p>A policies class is a template parameter used to transmit
    behaviors. An example from the standard library is <tt>std::<a
    href="http://www.dinkumware.com/htm_cpl/memory.html#allocator"></a></tt>,
    which supplies memory management behaviors to standard <a
    href="http://www.sgi.com/tech/stl/Container.html">containers</a>.

    <p>Policies classes have been explored in detail by <a href=
    "mailto:andrewalex@hotmail.com">Andrei Alexandrescu</a> in <a href=
    "http://www.cs.ualberta.ca/~hoover/cmput401/XP-Notes/xp-conf/Papers/7_3_Alexandrescu.pdf">
    this paper</a>. He writes:

    <blockquote>
      <p>Policy classes are implementations of punctual design choices. They
      are inherited from, or contained within, other classes. They provide
      different strategies under the same syntactic interface. A class using
      policies is templated having one template parameter for each policy it
      uses. This allows the user to select the policies needed.

      <p>The power of policy classes comes from their ability to combine
      freely. By combining several policy classes in a template class with
      multiple parameters, one achieves combinatorial behaviors with a linear
      amount of code.
    </blockquote>

    <p>Andrei's description of policies describe their power as being derived
    from their granularity and orthogonality. Boost has probably diluted the
    distinction in the <a href="../libs/utility/iterator_adaptors.htm">Iterator
    Adaptors</a> library, where we transmit all of an adapted iterator's
    behavior in a single policies class. There is precedent for this, however:
    <tt><a
    href="http://www.dinkumware.com/htm_cpl/string2.html#char_traits">std::char_traits</a></tt>,
    despite its name, acts as a policies class that determines the behaviors of
    <a
    href="http://www.dinkumware.com/htm_cpl/string2.html#basic_string">std::basic_string</a>.

    <h2><a name="adaptors">Adaptors</a></h2>

    <p>An <i>adaptor</i> is a class template which builds on another type or
    types to provide a new interface or behavioral variant. Examples of
    standard adaptors are <a href=
    "http://www.sgi.com/tech/stl/ReverseIterator.html">std::reverse_iterator</a>,
    which adapts an iterator type by reversing its motion upon
    increment/decrement, and <a href=
    "http://www.sgi.com/tech/stl/stack.html">std::stack</a>, which adapts a
    container to provide a simple stack interface.

    <p>A more comprehensive review of the adaptors in the standard can be found
    <a href=
    "http://www.cs.rpi.edu/~wiseb/xrds/ovp2-3b.html#SECTION00015000000000000000">
    here</a>.
    <hr>

    <p>Revised 
    <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %b %Y" startspan -->12 Feb 2001<!--webbot bot="Timestamp" endspan i-checksum="14377" -->


    <p>&copy; Copyright David Abrahams 2001. Permission to copy, use, modify,
    sell and distribute this document is granted provided this copyright notice
    appears in all copies. This document is provided "as is" without express or
    implied warranty, and with no claim as to its suitability for any purpose.

<!--  LocalWords:  HTML html charset gif alt htm struct SGI namespace std libs
 -->
<!--  LocalWords:  InputIterator BidirectionalIterator RandomAccessIterator pdf
 -->
<!--  LocalWords:  typename Alexandrescu templated Andrei's Abrahams
 -->
</body>
</html>