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    <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="#introduction">Introduction</a>

      <li><a href="#concept">The Anatomy of a Concept</a>

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

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

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

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

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

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


    <h2><a name="introduction">Introduction</a></h2>

    <p>Generic programming is about generalizing software components
    so that they can be easily reused in a wide variety of situations.
    In C++, class and function templates are particularly effective
    mechanisms for generic programming because they make the
    generalization possible without sacrificing efficiency. 
   
    <p>As a simple example of generic programming, we will look at how
    one might generalize the <tt>memcpy()</tt> function of the 
    C standard library. An implementation of <tt>memcpy()</tt>
    might look like the following: 
<p>
<blockquote>
<pre>
void* memcpy(void* region1, const void* region2, size_t n)
{
  const char* first = (const char*)region2;
  const char* last = ((const char*)region2) + n;
  char* result = (char*)region1;
  while (first != last)
    *result++ = *first++;
  return result;
}
</pre>
</blockquote>
    The <tt>memcpy()</tt> function is already generalized to some
    extent by the use of <tt>void*</tt> so that the function can be
    used to copy arrays of different kinds of data. But what if the
    data we would like to copy is not in an array? Perhaps it is in a
    linked list. Can we generalize the notion of copy to any sequence
    of elements? Looking at the body of <tt>memcpy()</tt>, the
    function's <b><i>minimal requirements</i></b> are that it needs to
    to <i>traverse</i> through the sequence using some sort of
    pointer, <i>access</i> elements pointed to, <i>write</i> the
    elements to the destination, and <i>compare</i> pointers to know
    when to stop.  The C++ standard library groups requirements such
    as these into <b><i>concepts</i></b>, in this case the <a
    href="http://www.sgi.com/tech/stl/InputIterator.html"> Input
    Iterator</a> concept (for <tt>region2</tt>) and the <a
    href="http://www.sgi.com/tech/stl/OutputIterator.html"> Output
    Iterator</a> concept (for <tt>region1</tt>).

    <p>If we rewrite the <tt>memcpy()</tt> as a function template, and
    use the <a href="http://www.sgi.com/tech/stl/InputIterator.html">
    Input Iterator</a> and <a
    href="http://www.sgi.com/tech/stl/OutputIterator.html"> Output
    Iterator</a> concepts to describe the requirements on the template
    parameters, we can implement a highly reusable <tt>copy()</tt>
    function in the following way:
<p>
<blockquote>
<pre>
template &lt;typename InputIterator, typename OutputIterator&gt;
OutputIterator
copy(InputIterator first, InputIterator last, OutputIterator result)
{
  while (first != last)
    *result++ = *first++;
  return result;
}
</pre>
</blockquote>

    <p>Using the generic <tt>copy()</tt> function, we can now copy
    elements from any kind of sequence, including a linked list that
    exports iterators such as <tt>std::<a
    href="http://www.sgi.com/tech/stl/List.html">list</a></tt>.
<p>
<blockquote>
<pre>
#include &lt;list&gt;
#include &lt;vector&gt;
#include &lt;iostream&gt;

int main()
{
  const int N = 3;
  std::vector&lt;int&gt; region1(N);
  std::list&lt;int&gt; region2;

  region2.push_back(1);
  region2.push_back(0);
  region2.push_back(3);
  
  std::copy(region2.begin(), region2.end(), region1.begin());

  for (int i = 0; i &lt; N; ++i)
    std::cout &lt;&lt; region1[i] &lt;&lt; " ";
  std::cout &lt;&lt; std::endl;
}
</pre>
</blockquote>


    <h2><a name="concept">Anatomy of a Concept</a></h2>

    A <b><i>concept</i></b> is a set requirements, where the
    requirements consist of valid expressions, associated types,
    invariants, and complexity guarantees. A type that satisfies the
    set of requirements is said to <b><i>model</i></b> the concept.  A
    concept can extend the requirements of another concept, which is
    called <b><i>refinement</i></b>.

    <ul>
       <li><a name="valid_expression"><b>Valid Expressions</b></a> are
	 C++ expressions which must compile successfully for the
	 objects involved in the expression to be considered
	 <i>models</i> of the concept.

      <li><a name="associated_type"><b>Associated Types</b></a> are
	types that are related to the modeling type in that they
	participate in one or more of the valid expressions. Typically
	associated types can be accessed either through typedefs
	nested within a class definition for the modeling type, or
	they are accessed through a <a href="#traits">traits
	class</a>.

      <li><b>Invariants</b> are run-time characteristics of the
	objects that must always be true, that is, the functions involving
	the objects must preserve these characteristics. The invariants
	often take the form of pre-conditions and post-conditions.

      <li><b>Complexity Guarantees</b> are maximum limits on how long
	the execution of one of the valid expressions will take, or how
	much of various resources its computation will use.
    </ul>

    <p>The concepts used in the C++ Standard Library are documented at
    the <a href="http://www.sgi.com/tech/stl/table_of_contents.html">
    SGI STL site</a>.

    <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="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>.

    <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)<a
    href="#1">[1]</a>. 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">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>Notes</h2>

    <a name="1">[1]</a> Type generators are a workaround for the lack
    of ``templated typedefs'' in C++.

    <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.

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