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  <title>Collection</title>
</head>

<body bgcolor="#FFFFFF" link="#0000EE" text="#000000" vlink="#551A8B" alink=
"#FF0000">
  <h1><img src="../../boost.png" alt="boost logo" width="277" align="middle"
  height="86"><br>
  Collection</h1>

  <h3>Description</h3>

  <p>A Collection is a <i>concept</i> similar to the STL <a href=
  "http://www.sgi.com/tech/stl/Container.html">Container</a> concept. A
  Collection provides iterators for accessing a range of elements and
  provides information about the number of elements in the Collection.
  However, a Collection has fewer requirements than a Container. The
  motivation for the Collection concept is that there are many useful
  Container-like types that do not meet the full requirements of Container,
  and many algorithms that can be written with this reduced set of
  requirements. To summarize the reduction in requirements:</p>

  <ul>
    <li>It is not required to "own" its elements: the lifetime of an element
    in a Collection does not have to match the lifetime of the Collection
    object, though the lifetime of the element should cover the lifetime of
    the Collection object.</li>

    <li>The semantics of copying a Collection object is not defined (it could
    be a deep or shallow copy or not even support copying).</li>

    <li>The associated reference type of a Collection does not have to be a
    real C++ reference.</li>
  </ul>Because of the reduced requirements, some care must be taken when
  writing code that is meant to be generic for all Collection types. In
  particular, a Collection object should be passed by-reference since
  assumptions can not be made about the behaviour of the copy constructor.

  <h3>Associated types</h3>

  <table border summary="">
    <tr>
      <td valign="top">Value type</td>

      <td valign="top"><tt>X::value_type</tt></td>

      <td valign="top">The type of the object stored in a Collection. If the
      Collection is <i>mutable</i> then the value type must be <a href=
      "http://www.sgi.com/tech/stl/Assignable.html">Assignable</a>. Otherwise
      the value type must be <a href=
      "./CopyConstructible.html">CopyConstructible</a>.</td>
    </tr>

    <tr>
      <td valign="top">Iterator type</td>

      <td valign="top"><tt>X::iterator</tt></td>

      <td valign="top">The type of iterator used to iterate through a
      Collection's elements. The iterator's value type is expected to be the
      Collection's value type. A conversion from the iterator type to the
      const iterator type must exist. The iterator type must be an <a href=
      "http://www.sgi.com/tech/stl/InputIterator.html">InputIterator</a>.</td>
    </tr>

    <tr>
      <td valign="top">Const iterator type</td>

      <td valign="top"><tt>X::const_iterator</tt></td>

      <td valign="top">A type of iterator that may be used to examine, but
      not to modify, a Collection's elements.</td>
    </tr>

    <tr>
      <td valign="top">Reference type</td>

      <td valign="top"><tt>X::reference</tt></td>

      <td valign="top">A type that behaves like a reference to the
      Collection's value type. <a href="#n1">[1]</a></td>
    </tr>

    <tr>
      <td valign="top">Const reference type</td>

      <td valign="top"><tt>X::const_reference</tt></td>

      <td valign="top">A type that behaves like a const reference to the
      Collection's value type.</td>
    </tr>

    <tr>
      <td valign="top">Pointer type</td>

      <td valign="top"><tt>X::pointer</tt></td>

      <td valign="top">A type that behaves as a pointer to the Collection's
      value type.</td>
    </tr>

    <tr>
      <td valign="top">Distance type</td>

      <td valign="top"><tt>X::difference_type</tt></td>

      <td valign="top">A signed integral type used to represent the distance
      between two of the Collection's iterators. This type must be the same
      as the iterator's distance type.</td>
    </tr>

    <tr>
      <td valign="top">Size type</td>

      <td valign="top"><tt>X::size_type</tt></td>

      <td valign="top">An unsigned integral type that can represent any
      nonnegative value of the Collection's distance type.</td>
    </tr>
  </table>

  <h3>Notation</h3>

  <table summary="">
    <tr>
      <td valign="top"><tt>X</tt></td>

      <td valign="top">A type that is a model of Collection.</td>
    </tr>

    <tr>
      <td valign="top"><tt>a</tt>, <tt>b</tt></td>

      <td valign="top">Object of type <tt>X</tt>.</td>
    </tr>

    <tr>
      <td valign="top"><tt>T</tt></td>

      <td valign="top">The value type of <tt>X</tt>.</td>
    </tr>
  </table>

  <h3>Valid expressions</h3>

  <p>The following expressions must be valid.</p>

  <table border summary="">
    <tr>
      <th>Name</th>

      <th>Expression</th>

      <th>Return type</th>
    </tr>

    <tr>
      <td valign="top">Beginning of range</td>

      <td valign="top"><tt>a.begin()</tt></td>

      <td valign="top"><tt>iterator</tt> if <tt>a</tt> is mutable,
      <tt>const_iterator</tt> otherwise</td>
    </tr>

    <tr>
      <td valign="top">End of range</td>

      <td valign="top"><tt>a.end()</tt></td>

      <td valign="top"><tt>iterator</tt> if <tt>a</tt> is mutable,
      <tt>const_iterator</tt> otherwise</td>
    </tr>

    <tr>
      <td valign="top">Size</td>

      <td valign="top"><tt>a.size()</tt></td>

      <td valign="top"><tt>size_type</tt></td>
    </tr><!--
<TR>
<TD VAlign=top>
Maximum size
</TD>
<TD VAlign=top>
<tt>a.max_size()</tt>
</TD>
<TD VAlign=top>
<tt>size_type</tt>
</TD>
</TR>
-->

    <tr>
      <td valign="top">Empty Collection</td>

      <td valign="top"><tt>a.empty()</tt></td>

      <td valign="top">Convertible to <tt>bool</tt></td>
    </tr>

    <tr>
      <td valign="top">Swap</td>

      <td valign="top"><tt>a.swap(b)</tt></td>

      <td valign="top"><tt>void</tt></td>
    </tr>
  </table>

  <h3>Expression semantics</h3>

  <table border summary="">
    <tr>
      <th>Name</th>

      <th>Expression</th>

      <th>Semantics</th>

      <th>Postcondition</th>
    </tr>

    <tr>
      <td valign="top">Beginning of range</td>

      <td valign="top"><tt>a.begin()</tt></td>

      <td valign="top">Returns an iterator pointing to the first element in
      the Collection.</td>

      <td valign="top"><tt>a.begin()</tt> is either dereferenceable or
      past-the-end. It is past-the-end if and only if <tt>a.size() ==
      0</tt>.</td>
    </tr>

    <tr>
      <td valign="top">End of range</td>

      <td valign="top"><tt>a.end()</tt></td>

      <td valign="top">Returns an iterator pointing one past the last element
      in the Collection.</td>

      <td valign="top"><tt>a.end()</tt> is past-the-end.</td>
    </tr>

    <tr>
      <td valign="top">Size</td>

      <td valign="top"><tt>a.size()</tt></td>

      <td valign="top">Returns the size of the Collection, that is, its
      number of elements.</td>

      <td valign="top"><tt>a.size() &gt;= 0</tt></td>
    </tr><!--
<TR>
<TD VAlign=top>
Maximum size
</TD>
<TD VAlign=top>
<tt>a.max_size()</tt>
</TD>
<TD VAlign=top>
&nbsp;
</TD>
<TD VAlign=top>
Returns the largest size that this Collection can ever have. <A href="#8">[8]</A>
</TD>
<TD VAlign=top>
<tt>a.max_size() &gt;= 0 &amp;&amp; a.max_size() &gt;= a.size()</tt>
</TD>
</TR>
 -->

    <tr>
      <td valign="top">Empty Collection</td>

      <td valign="top"><tt>a.empty()</tt></td>

      <td valign="top">Equivalent to <tt>a.size() == 0</tt>. (But possibly
      faster.)</td>

      <td valign="top">&nbsp;</td>
    </tr>

    <tr>
      <td valign="top">Swap</td>

      <td valign="top"><tt>a.swap(b)</tt></td>

      <td valign="top">Equivalent to <tt>swap(a,b)</tt></td>

      <td valign="top">&nbsp;</td>
    </tr>
  </table>

  <h3>Complexity guarantees</h3>

  <p><tt>begin()</tt> and <tt>end()</tt> are amortized constant time.</p>

  <p><tt>size()</tt> is at most linear in the Collection's size.
  <tt>empty()</tt> is amortized constant time.</p>

  <p><tt>swap()</tt> is at most linear in the size of the two
  collections.</p>

  <h3>Invariants</h3>

  <table border summary="">
    <tr>
      <td valign="top">Valid range</td>

      <td valign="top">For any Collection <tt>a</tt>, <tt>[a.begin(),
      a.end())</tt> is a valid range.</td>
    </tr>

    <tr>
      <td valign="top">Range size</td>

      <td valign="top"><tt>a.size()</tt> is equal to the distance from
      <tt>a.begin()</tt> to <tt>a.end()</tt>.</td>
    </tr>

    <tr>
      <td valign="top">Completeness</td>

      <td valign="top">An algorithm that iterates through the range
      <tt>[a.begin(), a.end())</tt> will pass through every element of
      <tt>a</tt>.</td>
    </tr>
  </table>

  <h3>Models</h3>

  <ul>
    <li><tt>array</tt></li>

    <li><tt>array_ptr</tt></li>

    <li><tt>vector&lt;bool&gt;</tt></li>
  </ul>

  <h3>Collection Refinements</h3>

  <p>There are quite a few concepts that refine the Collection concept,
  similar to the concepts that refine the Container concept. Here is a brief
  overview of the refining concepts.</p>

  <h4>ForwardCollection</h4>

  <p>The elements are arranged in some order that does not change
  spontaneously from one iteration to the next. As a result, a
  ForwardCollection is <a href=
  "http://www.sgi.com/tech/stl/EqualityComparable.html">EqualityComparable</a>
  and <a href=
  "http://www.sgi.com/tech/stl/LessThanComparable.html">LessThanComparable</a>.
  In addition, the iterator type of a ForwardCollection is a
  MultiPassInputIterator which is just an InputIterator with the added
  requirements that the iterator can be used to make multiple passes through
  a range, and that if <tt>it1 == it2</tt> and <tt>it1</tt> is
  dereferenceable then <tt>++it1 == ++it2</tt>. The ForwardCollection also
  has a <tt>front()</tt> method.</p>

  <table border summary="">
    <tr>
      <th>Name</th>

      <th>Expression</th>

      <th>Return type</th>

      <th>Semantics</th>
    </tr>

    <tr>
      <td valign="top">Front</td>

      <td valign="top"><tt>a.front()</tt></td>

      <td valign="top"><tt>reference</tt> if <tt>a</tt> is mutable,<br>
      <tt>const_reference</tt> otherwise.</td>

      <td valign="top">Equivalent to <tt>*(a.begin())</tt>.</td>
    </tr>
  </table>

  <h4>ReversibleCollection</h4>

  <p>The container provides access to iterators that traverse in both
  directions (forward and reverse). The iterator type must meet all of the
  requirements of <a href=
  "http://www.sgi.com/tech/stl/BidirectionalIterator.html">BidirectionalIterator</a>
  except that the reference type does not have to be a real C++ reference.
  The ReversibleCollection adds the following requirements to those of
  ForwardCollection.</p>

  <table border summary="">
    <tr>
      <th>Name</th>

      <th>Expression</th>

      <th>Return type</th>

      <th>Semantics</th>
    </tr>

    <tr>
      <td valign="top">Beginning of range</td>

      <td valign="top"><tt>a.rbegin()</tt></td>

      <td valign="top"><tt>reverse_iterator</tt> if <tt>a</tt> is mutable,
      <tt>const_reverse_iterator</tt> otherwise.</td>

      <td valign="top">Equivalent to
      <tt>X::reverse_iterator(a.end())</tt>.</td>
    </tr>

    <tr>
      <td valign="top">End of range</td>

      <td valign="top"><tt>a.rend()</tt></td>

      <td valign="top"><tt>reverse_iterator</tt> if <tt>a</tt> is mutable,
      <tt>const_reverse_iterator</tt> otherwise.</td>

      <td valign="top">Equivalent to
      <tt>X::reverse_iterator(a.begin())</tt>.</td>
    </tr>

    <tr>
      <td valign="top">Back</td>

      <td valign="top"><tt>a.back()</tt></td>

      <td valign="top"><tt>reference</tt> if <tt>a</tt> is mutable,<br>
      <tt>const_reference</tt> otherwise.</td>

      <td valign="top">Equivalent to <tt>*(--a.end())</tt>.</td>
    </tr>
  </table>

  <h4>SequentialCollection</h4>

  <p>The elements are arranged in a strict linear order. No extra methods are
  required.</p>

  <h4>RandomAccessCollection</h4>

  <p>The iterators of a RandomAccessCollection satisfy all of the
  requirements of <a href=
  "http://www.sgi.com/tech/stl/RandomAccessIterator.html">RandomAccessIterator</a>
  except that the reference type does not have to be a real C++ reference. In
  addition, a RandomAccessCollection provides an element access operator.</p>

  <table border summary="">
    <tr>
      <th>Name</th>

      <th>Expression</th>

      <th>Return type</th>

      <th>Semantics</th>
    </tr>

    <tr>
      <td valign="top">Element Access</td>

      <td valign="top"><tt>a[n]</tt></td>

      <td valign="top"><tt>reference</tt> if <tt>a</tt> is mutable,
      <tt>const_reference</tt> otherwise.</td>

      <td valign="top">Returns the nth element of the Collection. <tt>n</tt>
      must be convertible to <tt>size_type</tt>. Precondition: <tt>0 &lt;= n
      &lt; a.size()</tt>.</td>
    </tr>
  </table>

  <h3>Notes</h3>

  <p><a name="n1" id="n1">[1]</a> The reference type does not have to be a
  real C++ reference. The requirements of the reference type depend on the
  context within which the Collection is being used. Specifically it depends
  on the requirements the context places on the value type of the Collection.
  The reference type of the Collection must meet the same requirements as the
  value type. In addition, the reference objects must be equivalent to the
  value type objects in the collection (which is trivially true if they are
  the same object). Also, in a mutable Collection, an assignment to the
  reference object must result in an assignment to the object in the
  Collection (again, which is trivially true if they are the same object, but
  non-trivial if the reference type is a proxy class).</p>

  <h3>See also</h3>

  <p><a href=
  "http://www.sgi.com/tech/stl/Container.html">Container</a><br></p>
  <hr>

  <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
  "../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional"
  height="31" width="88"></a></p>

  <p>Revised 
  <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->05
  December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38516" --></p>

  <table summary="">
    <tr valign="top">
      <td nowrap><i>Copyright &copy; 2000</i></td>

      <td><i><a href="http://www.boost.org/people/jeremy_siek.htm">Jeremy
      Siek</a>, Univ.of Notre Dame and C++ Library &amp; Compiler Group/SGI
      (<a href="mailto:jsiek@engr.sgi.com">jsiek@engr.sgi.com</a>)</i></td>
    </tr>
  </table>

  <p><i>Distributed under the Boost Software License, Version 1.0. (See
  accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
  copy at <a href=
  "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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