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It is provided "as is" without express or implied warranty. -- --><Head><Title>iterator_traits<Iterator></Title><!-- Generated by htmldoc --></HEAD><BODY TEXT="#000000" LINK="#006600" ALINK="#003300" VLINK="#7C7F87" BGCOLOR="#FFFFFF"><A HREF="/"><IMG SRC="/images/common/sgilogo_small.gif" ALT="SGI Logo" WIDTH="80" HEIGHT="72" BORDER="0"></A><P><!--end header--><BR Clear><H1>iterator_traits<Iterator></H1><Table CellPadding=0 CellSpacing=0 width=100%><TR><TD Align=left><Img src = "iterators.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD><TD Align=right><Img src = "type.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD></TR><TR><TD Align=left VAlign=top><b>Category</b>: iterators</TD><TD Align=right VAlign=top><b>Component type</b>: type</TD></TR></Table><h3>Description</h3>As described in the <A href="Iterators.html">Iterator Overview</A>, one of the most importantfacts about iterators is that they have associated types. An iteratortype, for example, has an associated <i>value type</i>: the type ofobject that the iterator points to. It also has an associated<i>distance type</i>, or <i>difference type</i>, a signed integral type thatcan be used to represent the distance between two iterators.<P>(Pointers, for example, are iterators; the value type of <tt>int*</tt> is <tt>int</tt>. Its distance type is <tt>ptrdiff_t</tt>, because, if<tt>p1</tt> and <tt>p2</tt> are pointers, the expression <tt>p1 - p2</tt> has type<tt>ptrdiff_t</tt>.)<P>Generic algorithms often need to have access to these associatedtypes; an algorithm that takes a range of iterators, for example,might need to declare a temporary variable whose type is theiterators' value type. The class <tt>iterator_traits</tt> is amechanism that allows such declarations.<P>The most obvious way to allow declarations of that sort wouldbe to require that all iterators declare nested types; an iterator<tt>I</tt>'s value type, for example, would be <tt>I::value_type</tt>. Thatcan't possibly work, though. Pointers are iterators, and pointers aren't classes; if <tt>I</tt> is (say) <tt>int*</tt>, then it'simpossible to define <tt>I::value_type</tt> to be <tt>int</tt>.Instead, <tt>I</tt>'s value type is written <tt>iterator_traits<I>::value_type</tt>.<tt>iterator_traits</tt> is a template class that contains nothing butnested <tt>typedef</tt>s; in addition to <tt>value_type</tt>, <tt>iterator_traits</tt> defines the nested types <tt>iterator_category</tt>, <tt>difference_type</tt>,<tt>pointer</tt>, and <tt>reference</tt>.<P>The library contains two definitions of <tt>iterator_traits</tt>:a fully generic one, and a specialization that is used wheneverthe template argument is a pointer type. The fully generic version defines <tt>iterator_traits<I>::value_type</tt> asa synonym for <tt>I::value_type</tt>, <tt>iterator_traits<I>::difference_type</tt> as a synonym for<tt>I::difference_type</tt>, and so on. Since pointers don't havenested types, <tt>iterator_traits<T*></tt> has a different definition.<P>The implementation of <tt>iterator_traits</tt> is actually simpler than this discussion.<pre> template <class Iterator> struct iterator_traits { typedef typename Iterator::iterator_category iterator_category; typedef typename Iterator::value_type value_type; typedef typename Iterator::difference_type difference_type; typedef typename Iterator::pointer pointer; typedef typename Iterator::reference reference; }; template <class T> struct iterator_traits<T*> { typedef random_access_iterator_tag iterator_category; typedef T value_type; typedef ptrdiff_t difference_type; typedef T* pointer; typedef T& reference; };</pre><P>If you are defining a new iterator type <tt>I</tt>, then you must ensurethat <tt>iterator_traits<I></tt> is defined properly. There are two waysto do this. First, you can define your iterator so that it hasnested types <tt>I::value_type</tt>, <tt>I::difference_type</tt>, and so on.Second, you can explicitly specialize <tt>iterator_traits</tt> for yourtype. The first way is almost always more convenient, however,especially since you can easily ensure that your iterator hasthe appropriate nested types just by inheriting from one of thebase classes <tt><A href="input_iterator.html">input_iterator</A></tt>, <tt><A href="output_iterator.html">output_iterator</A></tt>, <tt><A href="forward_iterator.html">forward_iterator</A></tt>, <tt><A href="bidirectional_iterator.html">bidirectional_iterator</A></tt>, or<tt><A href="random_access_iterator.html">random_access_iterator</A></tt>.<P>Note that <tt>iterator_traits</tt> is new; it was added to the draft C++standard relatively recently. Both the old <A href="iterator_tags.html">iterator tags</A>mechanism and the new <tt>iterator_traits</tt> mechanism are currentlysupported, but the old iterator tag functions are no longerpart of the standard C++ library and they will eventually beremoved.<h3>Example</h3>This generic function returns the last element in a non-empty range.Note that there is no way to define a function with this interfacein terms of the old <tt><A href="value_type.html">value_type</A></tt> function, because the function'sreturn type must be declared to be the iterator's value type.<pre>template <class InputIterator>iterator_traits<InputIterator>::value_typelast_value(InputIterator first, InputIterator last) { iterator_traits<InputIterator>::value_type result = *first; for (++first; first != last; ++first) result = *first; return result;} </pre><P>(Note: this is an example of how to use <tt>iterator_traits</tt>; it is notan example of good code. There are better ways of finding the last element in a range of <A href="BidirectionalIterator.html">bidirectional iterators</A>, or even<A href="ForwardIterator.html">forward iterators</A>.)<h3>Definition</h3>Defined in the standard header <A href="iterator">iterator</A>, and in thenonstandard backward-compatibility header <A href="iterator.h">iterator.h</A>.<h3>Template parameters</h3><Table border><TR><TH>Parameter</TH><TH>Description</TH><TH>Default</TH></TR><TR><TD VAlign=top><tt>Iterator</tt></TD><TD VAlign=top>The iterator type whose associated types are being accessed.</TD><TD VAlign=top> </TD></tr></table><h3>Model of</h3><A href="DefaultConstructible.html">Default Constructible</A>, <A href="Assignable.html">Assignable</A><h3>Type requirements</h3><UL><LI><tt>Iterator</tt> is a model of one of the iterator concepts. (<A href="InputIterator.html">Input Iterator</A>, <A href="OutputIterator.html">Output Iterator</A>, <A href="ForwardIterator.html">Forward Iterator</A>, <A href="BidirectionalIterator.html">Bidirectional Iterator</A>, or <A href="RandomAccessIterator.html">Random Access Iterator</A>.)</UL><h3>Public base classes</h3>None.<h3>Members</h3>None, except for nested types. <Table border><TR><TH>Member</TH><TH>Description</TH></TR><TR><TD VAlign=top><tt>iterator_category</tt></TD><TD VAlign=top>One of the types <tt>input_iterator_tag</tt>, <tt>output_iterator_tag</tt>, <tt>forward_iterator_tag</tt>, <tt>bidirectional_iterator_tag</tt>, or <tt>random_access_iterator_tag</tt>. An iterator's category is the <i>most specific</i> iterator concept that it is a model of.</TD></TR><TR><TD VAlign=top><tt>value_type</tt></TD><TD VAlign=top><tt>Iterator</tt>'s value type, as defined in the <A href="trivial.html">Trivial Iterator</A> requirements.</TD></TR><TR><TD VAlign=top><tt>difference_type</tt></TD><TD VAlign=top><tt>Iterator</tt>'s distance type, as defined in the <A href="InputIterator.html">Input Iterator</A> requirements.</TD></TR><TR><TD VAlign=top><tt>pointer</tt></TD><TD VAlign=top><tt>Iterator</tt>'s pointer type: a pointer to its value type.</TD></TR><TR><TD VAlign=top><tt>reference</tt></TD><TD VAlign=top><tt>Iterator</tt>'s reference type: a reference to its value type.</TD></tr></table><h3>Notes</h3><P><A name="3">[3]</A>The <tt>iterator_traits</tt> class relies on a C++ feature known as<i>partial specialization</i>. Many of today's compilers don't implementthe complete standard; in particular, many compilers do not supportpartial specialization. If your compiler does not support partialspecialization, then you will not be able to use<tt>iterator_traits</tt>, and you will have to continue using the olditerator tag functions <tt><A href="iterator_category.html">iterator_category</A></tt>, <tt><A href="distance_type.html">distance_type</A></tt>, and<tt><A href="value_type.html">value_type</A></tt>. This is one reason that those functions have notyet been removed.<h3>See also</h3>The <A href="Iterators.html">iterator overview</A>,<A href="iterator_tags.html">iterator tags</A>,<tt><A href="input_iterator_tag.html">input_iterator_tag</A>,</tt><A href="output_iterator_tag.html">output_iterator_tag</A>,<tt><A href="forward_iterator_tag.html">forward_iterator_tag</A>,</tt><A href="bidirectional_iterator_tag.html">bidirectional_iterator_tag</A>,<tt><A href="random_access_iterator_tag.html">random_access_iterator_tag</A>,</tt><A href="input_iterator.html">input_iterator</A>,<tt><A href="output_iterator.html">output_iterator</A>,</tt><A href="forward_iterator.html">forward_iterator</A>,<tt><A href="bidirectional_iterator.html">bidirectional_iterator</A>,</tt><A href="random_access_iterator.html">random_access_iterator</A><!-- start footer --><!-- Footer Begins --><STYLE TYPE="text/css"><!--TD.footer, TD.footer A{ font-family: Arial, helvetica, sans-serif; font-size: 8pt;}A.home {font-family: Arial, helvetica, sans-serif;}--></STYLE><P><A CLASS="home" HREF="index.html">STL Home</A><P><TABLE WIDTH="600" CELLPADDING="0" CELLPADDING="0" BORDER="0"> <TR> <TD ALIGN="RIGHT" CLASS="footer"><A HREF="/company_info/terms.html" TARGET="_top">terms of use</A> | <A HREF="/company_info/privacy.html" TARGET="_top">privacy policy</A></TD> <TD ALIGN="CENTER" CLASS="footer"> | </TD> <TD ALIGN="LEFT" CLASS="footer"><A HREF="/cgi-bin/feedback/" TARGET="_top">contact us</A></TD> </TR><TR> <TD ALIGN="RIGHT" CLASS="footer">Copyright © 1993-2003 Silicon Graphics, Inc. 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