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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>Front Insertion Sequence</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>Front Insertion Sequence</H1><Table CellPadding=0 CellSpacing=0 width=100%><TR><TD Align=left><Img src = "containers.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>: containers</TD><TD Align=right VAlign=top><b>Component type</b>: concept</TD></TR></Table><h3>Description</h3>A Front Insertion Sequence is a <A href="Sequence.html">Sequence</A>where it is possible to insert an element at the beginning, or toaccess the first element, in amortized constant time. Front InsertionSequences have special member functions as a shorthand for thoseoperations.<h3>Refinement of</h3><A href="Sequence.html">Sequence</A><h3>Associated types</h3>None, except for those of <A href="Sequence.html">Sequence</A>.<h3>Notation</h3><Table><TR><TD VAlign=top><tt>X</tt></TD><TD VAlign=top>A type that is a model of Front Insertion Sequence</TD></TR><TR><TD VAlign=top><tt>a</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><TR><TD VAlign=top><tt>t</tt></TD><TD VAlign=top>Object of type <tt>T</tt></TD></tr></table><h3>Definitions</h3><h3>Valid expressions</h3>In addition to the expressions defined in<A href="Sequence.html">Sequence</A>, the following expressions must bevalid.<Table border><TR><TH>Name</TH><TH>Expression</TH><TH>Type requirements</TH><TH>Return type</TH></TR><TR><TD VAlign=top>Front</TD><TD VAlign=top><tt>a.front()</tt> <A href="#1">[1]</A></TD><TD VAlign=top> </TD><TD VAlign=top><tt>reference</tt> if <tt>a</tt> is mutable, otherwise <tt>const_reference</tt>.</TD></TR><TR><TD VAlign=top>Push front</TD><TD VAlign=top><tt>a.push_front(t)</tt></TD><TD VAlign=top><tt>a</tt> is mutable.</TD><TD VAlign=top><tt>void</tt></TD></TR><TR><TD VAlign=top>Pop front</TD><TD VAlign=top><tt>a.pop_front()</tt></TD><TD VAlign=top><tt>a</tt> is mutable.</TD><TD VAlign=top><tt>void</tt></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>Front</TD><TD VAlign=top><tt>a.front()</tt> <A href="#1">[1]</A></TD><TD VAlign=top><tt>!a.empty()</tt></TD><TD VAlign=top>Equivalent to <tt>*(a.begin())</tt>.</TD><TD VAlign=top> </TD></TR><TR><TD VAlign=top>Push front</TD><TD VAlign=top><tt>a.push_front(t)</tt></TD><TD VAlign=top> </TD><TD VAlign=top>Equivalent to <tt>a.insert(a.begin(), t)</tt></TD><TD VAlign=top><tt>a.size</tt> is incremented by 1. <tt>a.front()</tt> is a copy of <tt>t</tt>.</TD></TR><TR><TD VAlign=top>Pop front</TD><TD VAlign=top><tt>a.pop_front()</tt></TD><TD VAlign=top><tt>!a.empty()</tt></TD><TD VAlign=top>Equivalent to <tt>a.erase(a.begin())</tt></TD><TD VAlign=top><tt>a.size()</tt> is decremented by 1.</TD></tr></table><h3>Complexity guarantees</h3>Front, push front, and pop front are amortized constant time.<A href="#2">[2]</A><h3>Invariants</h3><Table border><TR><TD VAlign=top>Symmetry of push and pop</TD><TD VAlign=top><tt>push_front()</tt> followed by <tt>pop_front()</tt> is a null operation.</TD></tr></table><h3>Models</h3><UL><LI> <A href="List.html">list</A><LI> <A href="Deque.html">deque</A></UL><h3>Notes</h3><P><A name="1">[1]</A>Front is actually defined in <A href="Sequence.html">Sequence</A>,since it is always possible to implement it in amortized constanttime. Its definition is repeated here, along with push front and popfront, in the interest of clarity.<P><A name="2">[2]</A>This complexity guarantee is the only reason that <tt>front()</tt>,<tt>push_front()</tt>, and <tt>pop_front()</tt> are defined: theyprovide no additional functionality. Not every sequence must definethese operations, but it is guaranteed that they are efficient if theyexist at all.<h3>See also</h3><A href="Container.html">Container</A>,<A href="Sequence.html">Sequence</A>,<A href="BackInsertionSequence.html">Back Insertion Sequence</A>,<tt><A href="Deque.html">deque</A></tt>,<tt><A href="List.html">list</A></tt>,<tt><A href="Slist.html">slist</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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