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It is provided "as is" without express or implied warranty. -- --><Head><Title>queue<T, 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>queue<T, 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 = "type.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD></TR><TR><TD Align=left><Img src = "adaptors.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD><TD Align=right></TD></TR><TR><TD Align=left VAlign=top><b>Categories</b>: containers, adaptors</TD><TD Align=right VAlign=top><b>Component type</b>: type</TD></TR></Table><h3>Description</h3>A <tt>queue</tt> is an adaptor that provides a restricted subset of<A href="Container.html">Container</A> functionalityA <tt>queue</tt> is a "first in first out" (FIFO) data structure. <A href="#1">[1]</A> That is,elements are added to the back of the <tt>queue</tt> and may be removedfrom the front;<tt>Q.front()</tt> is the element that was added to the <tt>queue</tt> least recently.<tt>Queue</tt> does not allow iteration through its elements. <A href="#2">[2]</A><P><tt>Queue</tt> is a container adaptor, meaning that it is implemented ontop of some underlying container type. By default that underlyingtype is <tt><A href="Deque.html">deque</A></tt>, but a different type may be selected explicitly.<h3>Example</h3><pre>int main() { queue<int> Q; Q.push(8); Q.push(7); Q.push(6); Q.push(2); assert(Q.size() == 4); assert(Q.back() == 2); assert(Q.front() == 8); Q.pop(); assert(Q.front() == 7); Q.pop(); assert(Q.front() == 6); Q.pop(); assert(Q.front() == 2); Q.pop(); assert(Q.empty());}</pre><h3>Definition</h3>Defined in the standard header <A href="queue">queue</A>, and in the nonstandardbackward-compatibility header <A href="stack.h">stack.h</A>.<h3>Template parameters</h3><Table border><TR><TH>Parameter</TH><TH>Description</TH><TH>Default</TH></TR><TR><TD VAlign=top><tt>T</tt></TD><TD VAlign=top>The type of object stored in the queue.</TD><TD VAlign=top> </TD></TR><TR><TD VAlign=top><tt>Sequence</tt></TD><TD VAlign=top>The type of the underlying container used to implement the queue.</TD><TD VAlign=top><tt><A href="Deque.html">deque</A><T></tt></TD></tr></table><h3>Model of</h3><A href="Assignable.html">Assignable</A>, <A href="DefaultConstructible.html">Default Constructible</A><h3>Type requirements</h3><UL><LI><tt>T</tt> is a model of <A href="Assignable.html">Assignable</A>.<LI><tt>Sequence</tt> is a model of <A href="FrontInsertionSequence.html">Front Insertion Sequence</A>.<LI><tt>Sequence</tt> is a model of <A href="BackInsertionSequence.html">Back Insertion Sequence</A>.<LI><tt>Sequence::value_type</tt> is the same type as <tt>T</tt>.<LI>If <tt>operator==</tt> is used, then <tt>T</tt> is a model of <A href="EqualityComparable.html">Equality Comparable</A><LI>If <tt>operator<</tt> is used, then <tt>T</tt> is a model of <A href="LessThanComparable.html">LessThan Comparable</A>.</UL><h3>Public base classes</h3>None.<h3>Members</h3><Table border><TR><TH>Member</TH><TH>Where defined</TH><TH>Description</TH></TR><TR><TD VAlign=top><tt>value_type</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>size_type</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>queue()</tt></TD><TD VAlign=top> <A href="DefaultConstructible.html">Default Constructible</A></TD><TD VAlign=top>The default constructor. Creates an empty <tt>queue</tt>.</TD></TR><TR><TD VAlign=top><tt>queue(const queue&)</tt></TD><TD VAlign=top> <A href="Assignable.html">Assignable</A></TD><TD VAlign=top>The copy constructor.</TD></TR><TR><TD VAlign=top><tt>queue& operator=(const queue&)</tt></TD><TD VAlign=top> <A href="Assignable.html">Assignable</A></TD><TD VAlign=top>The assignment operator.</TD></TR><TR><TD VAlign=top><tt>bool empty() const</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>size_type size() const</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>value_type& front()</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>const value_type& front() const</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>value_type& back()</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>const value_type& back() const</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>void push(const value_type&)</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>void pop()</tt> <A href="#3">[3]</A></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>bool operator==(const queue&, const queue&)</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>bool operator<(const queue&, const queue&)</tt></TD><TD VAlign=top><tt>queue</tt></TD><TD VAlign=top>See below.</TD></tr></table><h3>New members</h3>These members are not defined in the <A href="Assignable.html">Assignable</A> and <A href="DefaultConstructible.html">Default Constructible</A>requirements, but are specific to <tt>queue</tt>.<Table border><TR><TH>Member</TH><TH>Description</TH></TR><TR><TD VAlign=top><tt>value_type</tt></TD><TD VAlign=top>The type of object stored in the <tt>queue</tt>. This is the same as <tt>T</tt> and <tt>Sequence::value_type</tt>.</TD></TR><TR><TD VAlign=top><tt>size_type</tt></TD><TD VAlign=top>An unsigned integral type. This is the same as <tt>Sequence::size_type</tt>.</TD></TR><TR><TD VAlign=top><tt>bool empty() const</tt></TD><TD VAlign=top>Returns <tt>true</tt> if the <tt>queue</tt> contains no elements, and <tt>false</tt> otherwise. <tt>Q.empty()</tt> is equivalent to <tt>Q.size() == 0</tt>.</TD></TR><TR><TD VAlign=top><tt>size_type size() const</tt></TD><TD VAlign=top>Returns the number of elements contained in the <tt>queue</tt>.</TD></TR><TR><TD VAlign=top><tt>value_type& front()</tt></TD><TD VAlign=top>Returns a mutable reference to the element at the front of the queue, that is, the element least recently inserted. Precondition: <tt>empty()</tt> is <tt>false</tt>.</TD></TR><TR><TD VAlign=top><tt>const value_type& front() const</tt></TD><TD VAlign=top>Returns a const reference to the element at the front of the queue, that is, the element least recently inserted. Precondition: <tt>empty()</tt> is <tt>false</tt>.</TD></TR><TR><TD VAlign=top><tt>value_type& back()</tt></TD><TD VAlign=top>Returns a mutable reference to the element at the back of the queue, that is, the element most recently inserted. Precondition: <tt>empty()</tt> is <tt>false</tt>.</TD></TR><TR><TD VAlign=top><tt>const value_type& back() const</tt></TD><TD VAlign=top>Returns a const reference to the element at the back of the queue, that is, the element most recently inserted. Precondition: <tt>empty()</tt> is <tt>false</tt>.</TD></TR><TR><TD VAlign=top><tt>void push(const value_type& x)</tt></TD><TD VAlign=top>Inserts <tt>x</tt> at the back of the queue. Postconditions: <tt>size()</tt> will be incremented by <tt>1</tt>, and <tt>back()</tt> will be equal to <tt>x</tt>.</TD></TR><TR><TD VAlign=top><tt>void pop()</tt></TD><TD VAlign=top>Removes the element at the front of the queue. <A href="#3">[3]</A> Precondition: <tt>empty()</tt> is <tt>false</tt>. Postcondition: <tt>size()</tt> will be decremented by <tt>1</tt>.</TD></TR><TR><TD VAlign=top><tt>bool operator==(const queue&, const queue&)</tt></TD><TD VAlign=top>Compares two queues for equality. Two queues are equal if they contain the same number of elements and if they are equal element-by-element. This is a global function, not a member function.</TD></TR><TR><TD VAlign=top><tt>bool operator<(const queue&, const queue&)</tt></TD><TD VAlign=top>Lexicographical ordering of two queues. This is a global function, not a member function.</TD></tr></table><h3>Notes</h3><P><A name="1">[1]</A>Queues are a standard data structure, and are discussed in allalgorithm books. See, for example, section 2.2.1 of Knuth.(D. E. Knuth, <i>The Art of ComputerProgramming. Volume 1: Fundamental Algorithms</i>, second edition. Addison-Wesley, 1973.)<P><A name="2">[2]</A>This restriction is the only reason for <tt>queue</tt> to exist at all.Any container that is both a <A href="FrontInsertionSequence.html">front insertion sequence</A> and a<A href="BackInsertionSequence.html">back insertion sequence</A> can be used as a queue; <tt><A href="Deque.html">deque</A></tt>, forexample, has member functions <tt>front</tt>, <tt>back</tt>, <tt>push_front</tt>,<tt>push_back</tt>, <tt>pop_front</tt>, and <tt>pop_back</tt> The only reason to use thecontainer adaptor <tt>queue</tt> instead of the container <tt><A href="Deque.html">deque</A></tt> is tomake it clear that you are performing only queue operations, and noother operations.<P><A name="3">[3]</A>One might wonder why <tt>pop()</tt> returns <tt>void</tt>, instead of<tt>value_type</tt>. That is, why must one use <tt>front()</tt> and <tt>pop()</tt> toexamine and remove the element at the front of the <tt>queue</tt>, instead ofcombining the two in a single member function? In fact, there is agood reason for this design. If <tt>pop()</tt> returned the front element, itwould have to return by value rather than by reference: return byreference would create a dangling pointer. Return by value, however,is inefficient: it involves at least one redundant copy constructorcall. Since it is impossible for <tt>pop()</tt> to return a value in such away as to be both efficient and correct, it is more sensible for it toreturn no value at all and to require clients to use <tt>front()</tt> toinspect the value at the front of the <tt>queue</tt>.<h3>See also</h3><tt><A href="stack.html">stack</A></tt>, <tt><A href="priority_queue.html">priority_queue</A></tt>, <tt><A href="Deque.html">deque</A></tt>, <A href="Container.html">Container</A>, <A href="Sequence.html">Sequence</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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