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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. 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It is provided "as is" without express or implied warranty. -- --><Head><Title>Relational Operators</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>Relational Operators</H1><Table CellPadding=0 CellSpacing=0 width=100%><TR><TD Align=left><Img src = "utilities.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>: utilities</TD><TD Align=right VAlign=top><b>Component type</b>: function</TD></TR></Table><h3>Prototype</h3><pre>template <class T> bool operator!=(const T& x, const T& y);template <class T> bool operator>(const T& x, const T& y);template <class T> bool operator<=(const T& x, const T& y);template <class T> bool operator>=(const T& x, const T& y);</pre> <h3>Description</h3>The <A href="EqualityComparable.html">Equality Comparable</A> requirements specify that it must bepossible to compare objects using <tt>operator!=</tt> as well as <tt>operator==</tt>;similarly, the <A href="LessThanComparable.html">LessThan Comparable</A> requirements include<tt>operator></tt>, <tt>operator<=</tt> and <tt>operator>=</tt> as well as <tt>operator<</tt>.Logically, however, most of these operators are redundant: all ofthem can be defined in terms of <tt>operator==</tt> and <tt>operator<</tt>.<P>These four templates use <tt>operator==</tt> and <tt>operator<</tt> to define the otherfour relational operators. They exist purely for the sake of convenience:they make it possible to write algorithms in terms of the operators<tt>!=</tt>, <tt>></tt>, <tt><=</tt>, and <tt>>=</tt>, without requiring that those operators beexplicitly defined for every type.<P>As specified in the <A href="EqualityComparable.html">Equality Comparable</A> requirements, <tt>x != y</tt> isequivalent to <tt>!(x == y)</tt>. As specified in the <A href="LessThanComparable.html">LessThan Comparable</A>requirements, <tt>x > y</tt> is equivalent to <tt>y < x</tt>, <tt>x >= y</tt> is equivalentto <tt>!(x < y)</tt>, and <tt>x <= y</tt> is equivalent to <tt>!(y < x)</tt>.<h3>Definition</h3>Defined in the standard header <A href="utility">utility</A>, and in the nonstandardbackward-compatibility header <A href="function.h">function.h</A>.<h3>Requirements on types</h3>The requirement for <tt>operator!=</tt> is that <tt>x == y</tt> is a valid expressionfor objects <tt>x</tt> and <tt>y</tt> of type <tt>T</tt>. <P>The requirement for <tt>operator></tt> is that <tt>y < x</tt> is a valid expressionfor objects <tt>x</tt> and <tt>y</tt> of type <tt>T</tt>. <P>The requirement for <tt>operator<=</tt> is that <tt>y < x</tt> is a valid expressionfor objects <tt>x</tt> and <tt>y</tt> of type <tt>T</tt>. <P>The requirement for <tt>operator>=</tt> is that <tt>x < y</tt> is a valid expressionfor objects <tt>x</tt> and <tt>y</tt> of type <tt>T</tt>. <h3>Preconditions</h3>The precondition for <tt>operator!=</tt> is that <tt>x</tt> and <tt>y</tt> are in thedomain of <tt>operator==</tt>.<P>The precondition for <tt>operator></tt>, <tt>operator<=</tt>, and <tt>operator>=</tt> isthat <tt>x</tt> and <tt>y</tt> are in the domain of <tt>operator<</tt>.<h3>Complexity</h3><h3>Example</h3><pre>template <class T> void relations(T x, T y){ if (x == y) assert(!(x != y)); else assert(x != y); if (x < y) { assert(x <= y); assert(y > x); assert(y >= x); } else if (y < x) { assert(y <= x); assert(x < y); assert(x <= y); } else { assert(x <= y); assert(x >= y); }} </pre><h3>Notes</h3><h3>See also</h3><A href="EqualityComparable.html">Equality Comparable</A>, <A href="LessThanComparable.html">LessThan Comparable</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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