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<title>Generic Programming Techniques</title>
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<img src="../boost.png" alt="boost.png (6897 bytes)" align="center"
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<h1>Generic Programming Techniques</h1>
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<p>This is an incomplete survey of some of the generic programming
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techniques used in the <a href="../index.htm">boost</a> libraries.</p>
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<h2>Table of Contents</h2>
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<ul>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#concept">The Anatomy of a Concept</a></li>
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<li><a href="#traits">Traits</a></li>
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<li><a href="#tag_dispatching">Tag Dispatching</a></li>
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<li><a href="#adaptors">Adaptors</a></li>
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<li><a href="#type_generator">Type Generators</a></li>
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<li><a href="#object_generator">Object Generators</a></li>
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<li><a href="#policy">Policy Classes</a></li>
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</ul>
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<h2><a name="introduction">Introduction</a></h2>
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<p>Generic programming is about generalizing software components so that
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they can be easily reused in a wide variety of situations. In C++, class
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and function templates are particularly effective mechanisms for generic
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programming because they make the generalization possible without
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sacrificing efficiency.</p>
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<p>As a simple example of generic programming, we will look at how one
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might generalize the <tt>memcpy()</tt> function of the C standard
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library. An implementation of <tt>memcpy()</tt> might look like the
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following:<br>
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<br>
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</p>
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<blockquote>
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<pre>
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void* memcpy(void* region1, const void* region2, size_t n)
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{
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const char* first = (const char*)region2;
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const char* last = ((const char*)region2) + n;
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char* result = (char*)region1;
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while (first != last)
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*result++ = *first++;
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return result;
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}
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</pre>
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</blockquote>
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The <tt>memcpy()</tt> function is already generalized to some extent by
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the use of <tt>void*</tt> so that the function can be used to copy arrays
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of different kinds of data. But what if the data we would like to copy is
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not in an array? Perhaps it is in a linked list. Can we generalize the
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notion of copy to any sequence of elements? Looking at the body of
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<tt>memcpy()</tt>, the function's <b><i>minimal requirements</i></b> are
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that it needs to <i>traverse</i> through the sequence using some sort
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of pointer, <i>access</i> elements pointed to, <i>write</i> the elements
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to the destination, and <i>compare</i> pointers to know when to stop. The
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C++ standard library groups requirements such as these into
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<b><i>concepts</i></b>, in this case the <a href=
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"http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>
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concept (for <tt>region2</tt>) and the <a href=
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"http://www.sgi.com/tech/stl/OutputIterator.html">Output Iterator</a>
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concept (for <tt>region1</tt>).
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<p>If we rewrite the <tt>memcpy()</tt> as a function template, and use
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the <a href="http://www.sgi.com/tech/stl/InputIterator.html">Input
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Iterator</a> and <a href=
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"http://www.sgi.com/tech/stl/OutputIterator.html">Output Iterator</a>
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concepts to describe the requirements on the template parameters, we can
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implement a highly reusable <tt>copy()</tt> function in the following
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way:<br>
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<br>
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</p>
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<blockquote>
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<pre>
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template <typename InputIterator, typename OutputIterator>
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OutputIterator
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copy(InputIterator first, InputIterator last, OutputIterator result)
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{
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while (first != last)
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*result++ = *first++;
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return result;
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}
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</pre>
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</blockquote>
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<p>Using the generic <tt>copy()</tt> function, we can now copy elements
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from any kind of sequence, including a linked list that exports iterators
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such as <tt>std::<a href=
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"http://www.sgi.com/tech/stl/List.html">list</a></tt>.<br>
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<br>
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</p>
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<blockquote>
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<pre>
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#include <list>
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#include <vector>
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#include <iostream>
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int main()
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{
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const int N = 3;
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std::vector<int> region1(N);
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std::list<int> region2;
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region2.push_back(1);
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region2.push_back(0);
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region2.push_back(3);
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std::copy(region2.begin(), region2.end(), region1.begin());
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for (int i = 0; i < N; ++i)
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std::cout << region1[i] << " ";
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std::cout << std::endl;
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}
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</pre>
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</blockquote>
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<h2><a name="concept">Anatomy of a Concept</a></h2>
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A <b><i>concept</i></b> is a set of requirements
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consisting of valid expressions, associated types, invariants, and
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complexity guarantees. A type that satisfies the requirements is
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said to <b><i>model</i></b> the concept. A concept can extend the
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requirements of another concept, which is called
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<b><i>refinement</i></b>.
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<ul>
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<li><a name="valid_expression"><b>Valid Expressions</b></a> are C++
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expressions which must compile successfully for the objects involved in
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the expression to be considered <i>models</i> of the concept.</li>
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<li><a name="associated_type"><b>Associated Types</b></a> are types
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that are related to the modeling type in that they participate in one
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or more of the valid expressions. Typically associated types can be
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accessed either through typedefs nested within a class definition for
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the modeling type, or they are accessed through a <a href=
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"#traits">traits class</a>.</li>
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<li><b>Invariants</b> are run-time characteristics of the objects that
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must always be true, that is, the functions involving the objects must
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preserve these characteristics. The invariants often take the form of
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pre-conditions and post-conditions.</li>
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<li><b>Complexity Guarantees</b> are maximum limits on how long the
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execution of one of the valid expressions will take, or how much of
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various resources its computation will use.</li>
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</ul>
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<p>The concepts used in the C++ Standard Library are documented at the <a
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href="http://www.sgi.com/tech/stl/table_of_contents.html">SGI STL
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site</a>.</p>
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<h2><a name="traits">Traits</a></h2>
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<p>A traits class provides a way of associating information with a
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compile-time entity (a type, integral constant, or address). For example,
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the class template <tt><a href=
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"http://www.sgi.com/tech/stl/iterator_traits.html">std::iterator_traits<T></a></tt>
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looks something like this:</p>
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<blockquote>
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<pre>
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template <class Iterator>
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struct iterator_traits {
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typedef ... iterator_category;
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typedef ... value_type;
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typedef ... difference_type;
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typedef ... pointer;
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typedef ... reference;
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};
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</pre>
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</blockquote>
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The traits' <tt>value_type</tt> gives generic code the type which the
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iterator is "pointing at", while the <tt>iterator_category</tt> can be
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used to select more efficient algorithms depending on the iterator's
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capabilities.
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<p>A key feature of traits templates is that they're
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<i>non-intrusive</i>: they allow us to associate information with
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arbitrary types, including built-in types and types defined in
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third-party libraries, Normally, traits are specified for a particular
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type by (partially) specializing the traits template.</p>
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<p>For an in-depth description of <tt>std::iterator_traits</tt>, see <a
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href="http://www.sgi.com/tech/stl/iterator_traits.html">this page</a>
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provided by SGI. Another very different expression of the traits idiom in
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the standard is <tt>std::numeric_limits<T></tt> which provides
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constants describing the range and capabilities of numeric types.</p>
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<h2><a name="tag_dispatching">Tag Dispatching</a></h2>
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<p>Tag dispatching is a way of using function overloading to
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dispatch based on properties of a type, and is often used hand in
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hand with traits classes. A good example of this synergy is the
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implementation of the <a href=
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"http://www.sgi.com/tech/stl/advance.html"><tt>std::advance()</tt></a>
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function in the C++ Standard Library, which increments an iterator
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<tt>n</tt> times. Depending on the kind of iterator, there are different
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optimizations that can be applied in the implementation. If the iterator
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is <a href="http://www.sgi.com/tech/stl/RandomAccessIterator.html">random
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access</a> (can jump forward and backward arbitrary distances), then the
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<tt>advance()</tt> function can simply be implemented with <tt>i +=
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n</tt>, and is very efficient: constant time. Other iterators must be
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<tt>advance</tt>d in steps, making the operation linear in n. If the
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iterator is <a href=
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"http://www.sgi.com/tech/stl/BidirectionalIterator.html">bidirectional</a>,
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then it makes sense for <tt>n</tt> to be negative, so we must decide
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whether to increment or decrement the iterator.</p>
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<p>The relation between tag dispatching and traits classes is that the
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property used for dispatching (in this case the
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<tt>iterator_category</tt>) is often accessed through a traits class. The
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main <tt>advance()</tt> function uses the <a href=
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"http://www.sgi.com/tech/stl/iterator_traits.html"><tt>iterator_traits</tt></a>
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class to get the <tt>iterator_category</tt>. It then makes a call the the
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overloaded <tt>advance_dispatch()</tt> function. The appropriate
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<tt>advance_dispatch()</tt> is selected by the compiler based on whatever
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type the <tt>iterator_category</tt> resolves to, either <a href=
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"http://www.sgi.com/tech/stl/input_iterator_tag.html"><tt>input_iterator_tag</tt></a>,
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<a href=
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"http://www.sgi.com/tech/stl/bidirectional_iterator_tag.html"><tt>bidirectional_iterator_tag</tt></a>,
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or <a href=
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"http://www.sgi.com/tech/stl/random_access_iterator_tag.html"><tt>random_access_iterator_tag</tt></a>.
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A <b><i>tag</i></b> is simply a class whose only purpose is to convey
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some property for use in tag dispatching and similar techniques. Refer to
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<a href="http://www.sgi.com/tech/stl/iterator_tags.html">this page</a>
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for a more detailed description of iterator tags.</p>
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<blockquote>
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<pre>
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namespace std {
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struct input_iterator_tag { };
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struct bidirectional_iterator_tag { };
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struct random_access_iterator_tag { };
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namespace detail {
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template <class InputIterator, class Distance>
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void advance_dispatch(InputIterator& i, Distance n, <b>input_iterator_tag</b>) {
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while (n--) ++i;
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}
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template <class BidirectionalIterator, class Distance>
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void advance_dispatch(BidirectionalIterator& i, Distance n,
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<b>bidirectional_iterator_tag</b>) {
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if (n >= 0)
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while (n--) ++i;
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else
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while (n++) --i;
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}
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template <class RandomAccessIterator, class Distance>
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void advance_dispatch(RandomAccessIterator& i, Distance n,
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<b>random_access_iterator_tag</b>) {
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i += n;
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}
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}
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template <class InputIterator, class Distance>
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void advance(InputIterator& i, Distance n) {
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typename <b>iterator_traits<InputIterator>::iterator_category</b> category;
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detail::advance_dispatch(i, n, <b>category</b>);
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}
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}
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</pre>
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</blockquote>
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<h2><a name="adaptors">Adaptors</a></h2>
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<p>An <i>adaptor</i> is a class template which builds on another type or
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types to provide a new interface or behavioral variant. Examples of
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standard adaptors are <a href=
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"http://www.sgi.com/tech/stl/ReverseIterator.html">std::reverse_iterator</a>,
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which adapts an iterator type by reversing its motion upon
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increment/decrement, and <a href=
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"http://www.sgi.com/tech/stl/stack.html">std::stack</a>, which adapts a
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container to provide a simple stack interface.</p>
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<p>A more comprehensive review of the adaptors in the standard can be
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found <a href="http://portal.acm.org/citation.cfm?id=249118.249120">
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here</a>.</p>
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<h2><a name="type_generator">Type Generators</a></h2>
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<p><b>Note:</b> The <i>type generator</i> concept has largely been
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superseded by the more refined notion of a <a href=
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"../libs/mpl/doc/refmanual/metafunction.html"><i>metafunction</i></a>. See
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<i><a href="http://www.boost-consulting.com/mplbook">C++ Template
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Metaprogramming</a></i> for an in-depth discussion of metafunctions.</p>
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<p>A <i>type generator</i> is a template whose only purpose is to
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synthesize a new type or types based on its template argument(s)<a href=
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"#1">[1]</a>. The generated type is usually expressed as a nested typedef
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named, appropriately <tt>type</tt>. A type generator is usually used to
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consolidate a complicated type expression into a simple one. This example
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uses an old version of <tt><a href=
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"../libs/iterator/doc/iterator_adaptor.html">iterator_adaptor</a></tt>
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whose design didn't allow derived iterator types. As a result, every
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adapted iterator had to be a specialization of <tt>iterator_adaptor</tt>
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itself and generators were a convenient way to produce those types.</p>
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<blockquote>
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<pre>
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template <class Predicate, class Iterator,
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class Value = <i>complicated default</i>,
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class Reference = <i>complicated default</i>,
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class Pointer = <i>complicated default</i>,
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class Category = <i>complicated default</i>,
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class Distance = <i>complicated default</i>
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>
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struct filter_iterator_generator {
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typedef iterator_adaptor<
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Iterator,filter_iterator_policies<Predicate,Iterator>,
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Value,Reference,Pointer,Category,Distance> <b>type</b>;
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};
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</pre>
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</blockquote>
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<p>Now, that's complicated, but producing an adapted filter iterator
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using the generator is much easier. You can usually just write:</p>
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<blockquote>
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<pre>
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boost::filter_iterator_generator<my_predicate,my_base_iterator>::type
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</pre>
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</blockquote>
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<h2><a name="object_generator">Object Generators</a></h2>
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<p>An <i>object generator</i> is a function template whose only purpose
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is to construct a new object out of its arguments. Think of it as a kind
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of generic constructor. An object generator may be more useful than a
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plain constructor when the exact type to be generated is difficult or
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impossible to express and the result of the generator can be passed
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directly to a function rather than stored in a variable. Most Boost
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object generators are named with the prefix "<tt>make_</tt>", after
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<tt>std::<a href=
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"http://www.sgi.com/tech/stl/pair.html">make_pair</a>(const T&, const U&)</tt>.</p>
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<p>For example, given:</p>
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<blockquote>
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<pre>
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struct widget {
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void tweak(int);
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};
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std::vector<widget *> widget_ptrs;
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</pre>
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</blockquote>
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By chaining two standard object generators, <tt>std::<a href=
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"http://www.dinkumware.com/htm_cpl/functio2.html#bind2nd">bind2nd</a>()</tt>
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and <tt>std::<a href=
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"http://www.dinkumware.com/htm_cpl/functio2.html#mem_fun">mem_fun</a>()</tt>,
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we can easily tweak all widgets:
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<blockquote>
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<pre>
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void tweak_all_widgets1(int arg)
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{
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for_each(widget_ptrs.begin(), widget_ptrs.end(),
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<b>bind2nd</b>(std::<b>mem_fun</b>(&widget::tweak), arg));
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}
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</pre>
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</blockquote>
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<p>Without using object generators the example above would look like
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this:</p>
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<blockquote>
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<pre>
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void tweak_all_widgets2(int arg)
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{
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for_each(struct_ptrs.begin(), struct_ptrs.end(),
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<b>std::binder2nd<std::mem_fun1_t<void, widget, int> ></b>(
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std::<b>mem_fun1_t<void, widget, int></b>(&widget::tweak), arg));
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}
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</pre>
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</blockquote>
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<p>As expressions get more complicated the need to reduce the verbosity
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of type specification gets more compelling.</p>
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<h2><a name="policy">Policy Classes</a></h2>
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<p>A policy class is a template parameter used to transmit behavior. An
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example from the standard library is <tt>std::<a href=
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"http://www.dinkumware.com/htm_cpl/memory.html#allocator">allocator</a></tt>,
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which supplies memory management behaviors to standard <a href=
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"http://www.sgi.com/tech/stl/Container.html">containers</a>.</p>
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<p>Policy classes have been explored in detail by <a href=
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"http://www.moderncppdesign.com/">Andrei Alexandrescu</a> in <a href=
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"http://www.informit.com/articles/article.asp?p=167842">this chapter</a>
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of his book, <i>Modern C++ Design</i>. He writes:</p>
|
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<blockquote>
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<p>In brief, policy-based class design fosters assembling a class with
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complex behavior out of many little classes (called policies), each of
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which takes care of only one behavioral or structural aspect. As the
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name suggests, a policy establishes an interface pertaining to a
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specific issue. You can implement policies in various ways as long as
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you respect the policy interface.</p>
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<p>Because you can mix and match policies, you can achieve a
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combinatorial set of behaviors by using a small core of elementary
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components.</p>
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</blockquote>
|
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<p>Andrei's description of policy classes suggests that their power is
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derived from granularity and orthogonality. Less-granular policy
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interfaces have been shown to work well in practice, though. <a href=
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|
"http://cvs.sourceforge.net/viewcvs.py/*checkout*/boost/boost/libs/utility/Attic/iterator_adaptors.pdf">
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This paper</a> describes an old version of <tt><a href=
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"../libs/iterator/doc/iterator_adaptor.html">iterator_adaptor</a></tt>
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that used non-orthogonal policies. There is also precedent in the
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standard library: <tt><a href=
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"http://www.dinkumware.com/htm_cpl/string2.html#char_traits">std::char_traits</a></tt>,
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despite its name, acts as a policies class that determines the behaviors
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of <a href=
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"http://www.dinkumware.com/htm_cpl/string2.html#basic_string">std::basic_string</a>.</p>
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<h2>Notes</h2>
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<a name="1">[1]</a> Type generators are sometimes viewed as a workaround
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for the lack of ``templated typedefs'' in C++.
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<hr>
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<p>Revised
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %b %Y" startspan -->18
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August 2004<!--webbot bot="Timestamp" endspan i-checksum="14885" -->
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</p>
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<p>© Copyright David Abrahams 2001. Permission to copy, use, modify,
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sell and distribute this document is granted provided this copyright
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notice appears in all copies. This document is provided "as is" without
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express or implied warranty, and with no claim as to its suitability for
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any purpose.
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<!-- LocalWords: HTML html charset gif alt htm struct SGI namespace std libs
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<!-- LocalWords: InputIterator BidirectionalIterator RandomAccessIterator pdf
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<!-- LocalWords: typename Alexandrescu templated Andrei's Abrahams memcpy int
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-->
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<!-- LocalWords: const OutputIterator iostream pre cpl
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</p>
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</body>
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</html>
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