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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>priority_queue<T, Sequence, Compare></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>priority_queue<T, Sequence, Compare></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>priority_queue</tt> is an adaptor that provides a restricted subset of<A href="Container.html">Container</A> functionality: it provides insertion of elements, andinspection and removal of the top element. It is guaranteed that the top element is the largest element in the <tt>priority_queue</tt>,where the function object <tt>Compare</tt> is used for comparisons. <A href="#1">[1]</A><tt>Priority_queue</tt> does not allow iteration through its elements. <A href="#2">[2]</A><P><tt>Priority_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="Vector.html">vector</A></tt>, but a different type may be selected explicitly.<h3>Example</h3><pre>int main() { priority_queue<int> Q; Q.push(1); Q.push(4); Q.push(2); Q.push(8); Q.push(5); Q.push(7); assert(Q.size() == 6); assert(Q.top() == 8); Q.pop(); assert(Q.top() == 7); Q.pop(); assert(Q.top() == 5); Q.pop(); assert(Q.top() == 4); Q.pop(); assert(Q.top() == 2); Q.pop(); assert(Q.top() == 1); 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 priority 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 priority queue.</TD><TD VAlign=top><tt><A href="Vector.html">vector</A><T></tt></TD></TR><TR><TD VAlign=top><tt>Compare</tt></TD><TD VAlign=top>The comparison function used to determine whether one element is smaller than another element. If <tt>Compare(x,y)</tt> is <tt>true</tt>, then <tt>x</tt> is smaller than <tt>y</tt>. The element returned by <tt>Q.top()</tt> is the largest element in the priority queue. That is, it has the property that, for every other element <tt>x</tt> in the priority queue, <tt>Compare(Q.top(), x)</tt> is <tt>false</tt>.</TD><TD VAlign=top><tt><A href="less.html">less</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="Sequence.html">Sequence</A>.<LI><tt>Sequence</tt> is a model of <A href="RandomAccessContainer.html">Random Access Container</A><LI><tt>Sequence::value_type</tt> is the same type as <tt>T</tt>.<LI><tt>Compare</tt> is a model of <A href="BinaryPredicate.html">Binary Predicate</A><LI><tt>Compare</tt> induces a strict weak ordering, as defined in the <A href="LessThanComparable.html">LessThan Comparable</A> requirements, on its argument type.<LI><tt>T</tt> is convertible to <tt>Compare</tt>'s argument type.</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>priority_queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>size_type</tt></TD><TD VAlign=top><tt>priority_queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>priority_queue()</tt></TD><TD VAlign=top> <A href="DefaultConstructible.html">Default Constructible</A></TD><TD VAlign=top>The default constructor. Creates an empty <tt>priority_queue</tt>, using <tt>Compare()</tt> as the comparison function.</TD></TR><TR><TD VAlign=top><tt>priority_queue(const priority_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>priority_queue(const Compare&)</tt></TD><TD VAlign=top><tt>priority_queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><pre>priority_queue(const value_type*, const value_type*)</pre></TD><TD VAlign=top><tt>priority_queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><pre>priority_queue(const value_type*, const value_type*, const Compare&)</pre></TD><TD VAlign=top><tt>priority_queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><pre>priority_queue& operator=(const priority_queue&)</pre></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>priority_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>priority_queue</tt></TD><TD VAlign=top>See below.</TD></TR><TR><TD VAlign=top><tt>const value_type& top() const</tt></TD><TD VAlign=top><tt>priority_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>priority_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>priority_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>priority_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>priority_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>priority_queue(const Compare& comp)</tt></TD><TD VAlign=top>The constructor. Creates an empty <tt>priority_queue</tt>, using <tt>comp</tt> as the comparison function. The default constructor uses <tt>Compare()</tt> as the comparison function.</TD></TR><TR><TD VAlign=top><pre>priority_queue(const value_type* first, const value_type* last)</pre></TD><TD VAlign=top>The constructor. Creates a <tt>priority_queue</tt> initialized to contain the elements in the range <tt>[first, last)</tt>, and using <tt>Compare()</tt> as the comparison function.</TD></TR><TR><TD VAlign=top><pre>priority_queue(const value_type* first, const value_type* last, const Compare& comp)</pre></TD><TD VAlign=top>The constructor. Creates a <tt>priority_queue</tt> initialized to contain the elements in the range <tt>[first, last)</tt>, and using <tt>comp</tt> as the comparison function.</TD></TR><TR><TD VAlign=top><tt>bool empty() const</tt></TD><TD VAlign=top>Returns <tt>true</tt> if the <tt>priority_queue</tt> contains no elements, and <tt>false</tt> otherwise. <tt>S.empty()</tt> is equivalent to <tt>S.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>priority_queue</tt>.</TD></TR><TR><TD VAlign=top><tt>const value_type& top() const</tt></TD><TD VAlign=top>Returns a const reference to the element at the top of the priority_queue. The element at the top is guaranteed to be the largest element in the priority queue, as determined by the comparison function <tt>Compare</tt>. That is, for every other element <tt>x</tt> in the <tt>priority_queue</tt>, <tt>Compare(Q.top(), x)</tt> is <tt>false</tt>. 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> into the priority_queue. Postcondition: <tt>size()</tt> will be incremented by <tt>1</tt>.</TD></TR><TR><TD VAlign=top><tt>void pop()</tt></TD><TD VAlign=top>Removes the element at the top of the priority_queue, that is, the largest element in the priority_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></table><h3>Notes</h3><P><A name="1">[1]</A>Priority queues are a standard concept, and can be implemented inmany different ways; this implementation uses heaps. Priority queuesare discussed in all algorithm books; see, for example, section 5.2.3of Knuth. (D. E. Knuth, <i>The Art of ComputerProgramming. Volume 3: Sorting and Searching</i>.Addison-Wesley, 1975.)<P><A name="2">[2]</A>This restriction is the only reason for <tt>priority_queue</tt> to existat all. If iteration through elements is important, you can eitheruse a <tt><A href="Vector.html">vector</A></tt> that is maintained in sorted order, or a <tt><A href="set.html">set</A></tt>,or a <tt><A href="Vector.html">vector</A></tt> that is maintained as a heap using <tt><A href="make_heap.html">make_heap</A></tt>,<tt><A href="push_heap.html">push_heap</A></tt>, and <tt><A href="pop_heap.html">pop_heap</A></tt>. <tt>Priority_queue</tt> is, in fact,implemented as a <A href="RandomAccessContainer.html">random access container</A> that is maintained as a heap. The only reason to use the containeradaptor <tt>priority_queue</tt>, instead of performing the heap operationsmanually, is to make it clear that you are never performingany operations that might violate the heap invariant.<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>top()</tt> and <tt>pop()</tt> toexamine and remove the element at the top of the <tt>priority_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 top 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>top()</tt> toinspect the value at the top of the <tt>priority_queue</tt>.<h3>See also</h3><tt><A href="stack.html">stack</A></tt>, <tt><A href="queue.html">queue</A></tt>, <tt><A href="set.html">set</A></tt>,<tt><A href="make_heap.html">make_heap</A></tt>, <tt><A href="push_heap.html">push_heap</A></tt>, <tt><A href="pop_heap.html">pop_heap</A></tt>, <tt><A href="is_heap.html">is_heap</A></tt>,<tt><A href="sort.html">sort</A></tt>, <A href="is_sorted.html">is_sorted</A>,<A href="Container.html">Container</A>, <A href="SortedAssociativeContainer.html">Sorted Associative 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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