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<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() >= 0</tt></td>
</tr><!--
<TR>
<TD VAlign=top>
Maximum size
</TD>
<TD VAlign=top>
<tt>a.max_size()</tt>
</TD>
<TD VAlign=top>
</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() >= 0 && a.max_size() >= 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"> </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"> </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<bool></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 <= n
< 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>
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<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 © 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 & 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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