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It is provided "as is" without express or implied warranty. -- --><Head><Title>iterator_category</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_category</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 = "function.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>: function</TD></TR></Table><h3>Prototype</h3><tt>Iterator_category</tt> is overloaded; it is in fact six different functions.<pre>inline output_iterator_tag iterator_category(const output_iterator&);template <class T, class Distance> inline <A href="input_iterator_tag.html">input_iterator_tag</A> iterator_category(const <A href="input_iterator.html">input_iterator</A><T, Distance>&);template <class T, class Distance> inline <A href="forward_iterator_tag.html">forward_iterator_tag</A> iterator_category(const <A href="forward_iterator.html">forward_iterator</A><T, Distance>&);template <class T, class Distance> inline <A href="bidirectional_iterator_tag.html">bidirectional_iterator_tag</A> iterator_category(const <A href="bidirectional_iterator.html">bidirectional_iterator</A><T, Distance>&);template <class T, class Distance> inline <A href="random_access_iterator_tag.html">random_access_iterator_tag</A> iterator_category(const <A href="random_access_iterator.html">random_access_iterator</A><T, Distance>&);template <class T>inline <A href="random_access_iterator_tag.html">random_access_iterator_tag</A> iterator_category(const T*);</pre> <h3>Description</h3><tt>Iterator_category</tt> is an <A href="iterator_tags.html">iterator tag</A> function: it is used todetermine the category to which an iterator belongs. Specifically,every iterator must belong to a type that is a model of the concept<A href="OutputIterator.html">Output Iterator</A>, <A href="InputIterator.html">Input 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>. <A href="#1">[1]</A><tt>Iterator_category</tt> returns an object of class<tt><A href="output_iterator_tag.html">output_iterator_tag</A></tt>, <tt><A href="input_iterator_tag.html">input_iterator_tag</A></tt>,<tt><A href="forward_iterator_tag.html">forward_iterator_tag</A></tt>, or <tt><A href="random_access_iterator_tag.html">random_access_iterator_tag</A></tt>,depending on which concept the type of <tt>iterator_category</tt>'s argument is a model of. <A href="#2">[2]</A> This information is useful in the case ofan algorithm that has a sensible definition for more than one categoryof iterator, but whose definition is different depending on the category.<P>Although <tt>iterator_category</tt> looks like a single function whosereturn type depends on its argument type, in reality it is a setof functions; the name <tt>iterator_category</tt> is overloaded. The function <tt>iterator_category</tt> mustbe overloaded for every iterator type.<P>In practice, ensuring that <tt>iterator_category</tt> is defined requiresessentially no work at all. It is already defined for pointers, andfor the base 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>, and<tt><A href="random_access_iterator.html">random_access_iterator</A></tt>. If you are implementing a new type offorward iterator, for example, you can simply derive it from the baseclass <tt><A href="forward_iterator.html">forward_iterator</A></tt>; this means that <tt>iterator_category</tt>(along with <tt><A href="distance_type.html">distance_type</A></tt> and <tt><A href="value_type.html">value_type</A></tt>) will automatically be defined for your iterator. These base classes areempty: they contain no member functions or member variables, butonly type information. Using them should therefore incur no overhead.<P>Note that, while the function <tt>iterator_category</tt> was present in theoriginal STL, it is no longer present in the most recent draft C++standard: it has been replaced by the <tt><A href="iterator_traits.html">iterator_traits</A></tt> class.At present both mechanisms are supported <A href="#3">[3]</A>, but eventually<tt>iterator_category</tt> will be removed.<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>.This function is no longer part of the C++ standard, althoughit was present in early drafts of the standard. It is retainedin this implementation for backward compatibility.<h3>Requirements on types</h3>The argument of <tt>iterator_category</tt> must be an iterator.<h3>Preconditions</h3>None. <tt>Iterator_category</tt>'s argument is even permitted to bea singular iterator.<h3>Complexity</h3>At most amortized constant time. In many cases, a compiler should beable to optimize away <tt>iterator_category</tt> entirely.<h3>Example</h3><tt><A href="reverse.html">Reverse</A></tt> can be implemented for either<A href="BidirectionalIterator.html">Bidirectional Iterators</A> or for <A href="RandomAccessIterator.html">Random Access Iterators</A>,but the algorithm for <A href="RandomAccessIterator.html">Random Access Iterators</A> is more efficient.Consequently, <tt><A href="reverse.html">reverse</A></tt> uses <tt>iterator_category</tt> to select whichever algorithm is appropriate for the iterator type.This dispatch takes place at compile time, and should notincur any run-time penalty.<pre>template <class <A href="BidirectionalIterator.html">BidirectionalIterator</A>>void __reverse(BidirectionalIterator first, BidirectionalIterator last, <A href="bidirectional_iterator_tag.html">bidirectional_iterator_tag</A>) { while (true) if (first == last || first == --last) return; else iter_swap(first++, last);}template <class <A href="RandomAccessIterator.html">RandomAccessIterator</A>>void __reverse(RandomAccessIterator first, RandomAccessIterator last, <A href="random_access_iterator_tag.html">random_access_iterator_tag</A>) { while (first < last) iter_swap(first++, --last);}template <class <A href="BidirectionalIterator.html">BidirectionalIterator</A>>inline void <A href="reverse.html">reverse</A>(BidirectionalIterator first, BidirectionalIterator last) { __reverse(first, last, iterator_category(first));}</pre><h3>Notes</h3><P><A name="1">[1]</A>The STL also defines one other concept, <A href="trivial.html">Trivial Iterator</A>.This concept is introduced only for conceptual clarity, however,in order to separate the axioms related to an object that refersto another object from those related to iteration over a range.In fact, the STL does not define any types that are <A href="trivial.html">Trivial Iterators</A>.Although built-in C pointers may be <A href="trivial.html">Trivial Iterators</A>, the Ctype system does not allow a distinction between pointers that are<A href="trivial.html">Trivial Iterators</A> and pointers that are <A href="RandomAccessIterator.html">Random Access Iterators</A>into C arrays. Consequently, there is no <A href="trivial.html">Trivial Iterator</A> categorytag.<P><A name="2">[2]</A>Any type that is a model of <A href="ForwardIterator.html">Forward Iterator</A> is also a modelof <A href="InputIterator.html">Input Iterator</A>, any type that is a model of <A href="BidirectionalIterator.html">Bidirectional Iterator</A> is also a model of <A href="ForwardIterator.html">Forward Iterator</A>,and any type that is a model of <A href="RandomAccessIterator.html">Random Access Iterator</A> is alsoa model of <A href="BidirectionalIterator.html">Bidirectional Iterator</A>. <tt>Iterator_category</tt> mustreturn a tag representing the <i>most specific</i> concept that its argument is a model of. If its argument is a <tt><A href="Vector.html">vector</A>::iterator</tt>,for example, then it must return <tt><A href="random_access_iterator_tag.html">random_access_iterator_tag</A></tt>.<P><A name="3">[3]</A>The <tt><A href="iterator_traits.html">iterator_traits</A></tt> classrelies on a C++ feature known as <i>partial specialization</i>. Many oftoday's compilers don't implement the complete standard; inparticular, many compilers do not support partial specialization. Ifyour compiler does not support partial specialization, then you willnot be able to use <tt><A href="iterator_traits.html">iterator_traits</A></tt>, and you will have to continue using the functions <tt>iterator_category</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 havenot yet been removed.<h3>See also</h3>The <A href="iterator_tags.html">Iterator Tags</A> overview, <tt><A href="iterator_traits.html">iterator_traits</A></tt>,<tt><A href="distance_type.html">distance_type</A></tt>, <tt><A href="value_type.html">value_type</A></tt>,<tt><A href="output_iterator_tag.html">output_iterator_tag</A></tt>, <tt><A href="input_iterator_tag.html">input_iterator_tag</A></tt>,<tt><A href="forward_iterator_tag.html">forward_iterator_tag</A></tt>, <tt><A href="bidirectional_iterator_tag.html">bidirectional_iterator_tag</A></tt>,<tt><A href="random_access_iterator_tag.html">random_access_iterator_tag</A></tt><!-- 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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