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<HTML><!-- -- Copyright (c) 1996-1999 -- Silicon Graphics Computer Systems, Inc. -- -- Permission to use, copy, modify, distribute and sell this software -- and its documentation for any purpose is hereby granted without fee, -- provided that the above copyright notice appears in all copies and -- that both that copyright notice and this permission notice appear -- in supporting documentation. Silicon Graphics makes no -- representations about the suitability of this software for any -- purpose. It is provided "as is" without express or implied warranty. -- -- Copyright (c) 1994 -- Hewlett-Packard Company -- -- Permission to use, copy, modify, distribute and sell this software -- and its documentation for any purpose is hereby granted without fee, -- provided that the above copyright notice appears in all copies and -- that both that copyright notice and this permission notice appear -- in supporting documentation. Hewlett-Packard Company makes no -- representations about the suitability of this software for any -- purpose. It is provided "as is" without express or implied warranty. -- --><Head><Title>Output Iterator</Title><!-- Generated by htmldoc --></HEAD><BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" ALINK="#ff0000"> <IMG SRC="CorpID.gif" ALT="SGI" HEIGHT="43" WIDTH="151"> <!--end header--><BR Clear><H1>Output 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 = "concept.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>: concept</TD></TR></Table><h3>Description</h3>An Output Iterator is a type that provides a mechanism for storing(but not necessarily accessing) a sequence of values. OutputIterators are in some sense the converse of <A href="InputIterator.html">Input Iterators</A>, butthey have a far more restrictive interface: they do not necessarily supportmember access or equality, and they do not necessarily have either anassociated distance type or even a value type <A href="#1">[1]</A>. Intuitively, onepicture of an Output Iterator is a tape: you can write a value to thecurrent location and you can advance to the next location, but youcannot read values and you cannot back up or rewind.<h3>Refinement of</h3><A href="Assignable.html">Assignable</A>, <A href="DefaultConstructible.html">DefaultConstructible</A><h3>Associated types</h3>None. <A href="#1">[1]</A><h3>Notation</h3><Table><TR><TD VAlign=top><tt>X</tt></TD><TD VAlign=top>A type that is a model of Output Iterator</TD></TR><TR><TD VAlign=top><tt>x</tt>, <tt>y</tt></TD><TD VAlign=top>Object of type <tt>X</tt></TD></tr></table><h3>Definitions</h3>If <tt>x</tt> is an Output Iterator of type <tt>X</tt>, then the expression <tt>*x =t;</tt> stores the value <tt>t</tt> into <tt>x</tt>. Note that <tt>operator=</tt>, like otherC++ functions, may be overloaded; it may, in fact, even be a templatefunction. In general, then, <tt>t</tt> may be any of several differenttypes. A type <tt>T</tt> belongs to the <i>set of value types</i> of <tt>X</tt> if, for an object <tt>t</tt> of type <tt>T</tt>, <tt>*x = t;</tt> is well-defined anddoes not require performing any non-trivial conversions on <tt>t</tt>. <A href="#1">[1]</A><P>An Output Iterator may be <i>singular</i>, meaning that the results ofmost operations, including copying and dereference assignment, areundefined. The only operation that is guaranteed to be supported isassigning a nonsingular iterator to a singular iterator.<P>An Output Iterator may be <i>dereferenceable</i>, meaning that assignment through it is defined. Dereferenceable iterators are alwaysnonsingular, but nonsingular iterators are not necessarily dereferenceable.<h3>Valid expressions</h3><Table border><TR><TH>Name</TH><TH>Expression</TH><TH>Type requirements</TH><TH>Return type</TH></TR><TR><TD VAlign=top>Default constructor</TD><TD VAlign=top><pre>X x;X()</pre></TD><TD VAlign=top> </TD><TD VAlign=top> </TD></TR><TR><TD VAlign=top>Copy constructor</TD><TD VAlign=top><tt>X(x)</tt></TD><TD VAlign=top> </TD><TD VAlign=top><tt>X</tt></TD></TR><TR><TD VAlign=top>Copy constructor</TD><TD VAlign=top><tt>X y(x);</tt> or <tt>X y = x;</tt></TD><TD VAlign=top> </TD><TD VAlign=top> </TD></TR><TR><TD VAlign=top>Dereference assignment</TD><TD VAlign=top><tt>*x = t</tt></TD><TD VAlign=top><tt>t</tt> is convertible to a type in the set of value types of <tt>X</tt>. <A href="#1">[1]</A></TD><TD VAlign=top>Result is not used</TD></TR><TR><TD VAlign=top>Preincrement</TD><TD VAlign=top><tt>++x</tt></TD><TD VAlign=top> </TD><TD VAlign=top><tt>X&</tt></TD></TR><TR><TD VAlign=top>Postincrement</TD><TD VAlign=top><tt>(void) x++</tt></TD><TD VAlign=top> </TD><TD VAlign=top><tt>void</tt></TD></TR><TR><TD VAlign=top>Postincrement and assign</TD><TD VAlign=top><tt>*x++ = t;</tt></TD><TD VAlign=top> </TD><TD VAlign=top>Result is not used</TD></tr></table><h3>Expression semantics</h3><Table border><TR><TH>Name</TH><TH>Expression</TH><TH>Precondition</TH><TH>Semantics</TH><TH>Postcondition</TH></TR><TR><TD VAlign=top>Default constructor</TD><TD VAlign=top><pre>X x;X()</pre></TD><TD VAlign=top> </TD><TD VAlign=top> </TD><TD VAlign=top><tt>x</tt> may be singular</TD></TR><TR><TD VAlign=top>Copy constructor</TD><TD VAlign=top><tt>X(x)</tt></TD><TD VAlign=top><tt>x</tt> is nonsingular</TD><TD VAlign=top> </TD><TD VAlign=top><tt>*X(x) = t</tt> is equivalent to <tt>*x = t</tt> <A href="#2">[2]</A></TD></TR><TR><TD VAlign=top>Copy constructor</TD><TD VAlign=top><tt>X x(y);</tt> or <tt>X x = y;</tt></TD><TD VAlign=top><tt>y</tt> is nonsingular</TD><TD VAlign=top> </TD><TD VAlign=top><tt>*y = t</tt> is equivalent to <tt>*x = t</tt> <A href="#2">[2]</A></TD></TR><TR><TD VAlign=top>Dereference assignment</TD><TD VAlign=top><tt>*x = t</tt></TD><TD VAlign=top><tt>x</tt> is dereferenceable. If there has been a previous assignment through <tt>x</tt>, then there has been an intervening increment. <A href="#3">[3]</A></TD><TD VAlign=top> </TD><TD VAlign=top> </TD></TR><TR><TD VAlign=top>Preincrement</TD><TD VAlign=top><tt>++x</tt></TD><TD VAlign=top><tt>x</tt> is dereferenceable. <tt>x</tt> has previously been assigned through. If <tt>x</tt> has previously been incremented, then there has been an intervening assignment through <tt>x</tt> <A href="#3">[3]</A> <A href="#4">[4]</A></TD><TD VAlign=top> </TD><TD VAlign=top><tt>x</tt> points to the next location into which a value may be stored</TD></TR><TR><TD VAlign=top>Postincrement</TD><TD VAlign=top><tt>(void) x++</tt></TD><TD VAlign=top><tt>x</tt> is dereferenceable. <tt>x</tt> has previously been assigned through.</TD><TD VAlign=top>Equivalent to <tt>(void) ++x</tt></TD><TD VAlign=top><tt>x</tt> points to the next location into which a value may be stored</TD></TR><TR><TD VAlign=top>Postincrement and assign</TD><TD VAlign=top><tt>*x++ = t;</tt></TD><TD VAlign=top><tt>x</tt> is dereferenceable. If there has been a previous assignment through <tt>x</tt>, then there has been an intervening increment. <A href="#3">[3]</A> <A href="#4">[4]</A></TD><TD VAlign=top>Equivalent to <tt>{*x = t; ++x; }</tt></TD><TD VAlign=top><tt>x</tt> points to the next location into which a value may be stored</TD></tr></table><h3>Complexity guarantees</h3>The complexity of operations on output iterators isguaranteed to be amortized constant time.<h3>Invariants</h3><h3>Models</h3><UL><LI> <A href="ostream_iterator.html">ostream_iterator</A><LI> <A href="insert_iterator.html">insert_iterator</A><LI> <A href="front_insert_iterator.html">front_insert_iterator</A><LI> <A href="back_insert_iterator.html">back_insert_iterator</A></UL><h3>Notes</h3><P><A name="1">[1]</A>Other iterator types, including <A href="trivial.html">Trivial Iterator</A> and<A href="InputIterator.html">Input Iterator</A>, define the notion of a <i>value type</i>, the type returnedwhen an iterator is dereferenced. This notion does not apply toOutput Iterators, however, since the dereference operator (unary<tt>operator*</tt>) does not return a usable value for Output Iterators. Theonly context in which the dereference operator may be used isassignment through an output iterator: <tt>*x = t</tt>. Although <A href="InputIterator.html">Input Iterators</A> and output iterators are roughly symmetrical concepts,there is an important sense in which accessing and storing values arenot symmetrical: for an <A href="InputIterator.html">Input Iterator</A> <tt>operator*</tt> must return aunique type, but, for an Output Iterator, in the expression <tt>*x = t</tt>,there is no reason why <tt>operator=</tt> must take a unique type. <A href="#5">[5]</A>Consequently, there need not be any unique "value type" for Output Iterators.<P><A name="2">[2]</A>There should be only one active copy of a single Output Iterator atany one time. That is: after creating and using a copy <tt>x</tt> of an Output Iterator <tt>y</tt>, the original output iterator <tt>y</tt> should nolonger be used.<P><A name="3">[3]</A>Assignment through an Output Iterator <tt>x</tt> is expected to alternate withincrementing <tt>x</tt>, and there must be an assignment through <tt>x</tt> before<tt>x</tt> is ever incremented. Any other order of operations results inundefined behavior. That is: <tt>{*x = t</tt>; ++x; *x = t2; ++x}<tt> is acceptable, but </tt>{*x = t<tt>; ++x; ++x; *x = t2;}</tt> is not.<P><A name="4">[4]</A>Note that an Output Iterator need not define comparison for equality.Even if an <tt>operator==</tt> is defined, <tt>x == y</tt> need not imply<tt>++x == ++y</tt>.<P><A name="5">[5]</A>If you are implementing an Output Iterator class <tt>X</tt>, onesensible way to define <tt>*x = t</tt> is to define <tt>X::operator*()</tt> toreturn an object of some private class <tt>X_proxy</tt>, and then to define<tt>X_proxy::operator=</tt>. Note that you may overload<tt>X_proxy::operator=</tt>, or even define it as a member template; thisallows assignment of more than one type through Output Iterators ofclass <tt>X</tt>.<h3>See also</h3><A href="trivial.html">Trivial Iterator</A>, <A href="InputIterator.html">Input Iterator</A>, <A href="Iterators.html">Iterator overview</A><!--start footer--> <HR SIZE="6"><A href="http://www.sgi.com/"><IMG SRC="surf.gif" HEIGHT="54" WIDTH="54" ALT="[Silicon Surf]"></A><A HREF="index.html"><IMG SRC="stl_home.gif" HEIGHT="54" WIDTH="54" ALT="[STL Home]"></A><BR><FONT SIZE="-2"><A href="http://www.sgi.com/Misc/sgi_info.html" TARGET="_top">Copyright © 1999 Silicon Graphics, Inc.</A> All Rights Reserved.</FONT><FONT SIZE="-3"><a href="http://www.sgi.com/Misc/external.list.html" TARGET="_top">TrademarkInformation</A></FONT><P></BODY></HTML> ⌨️ 快捷键说明
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