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<H2>priority_queue</H2>
<P><B>Library:</B>&nbsp;&nbsp;<A HREF="2-7.html">Containers</A></P>

<PRE><HR><B><I>Does not inherit</I></B><HR></PRE>

<UL>
<LI><A HREF="#sec1">Local Index</A></LI>
<LI><A HREF="#sec2">Summary</A></LI>
<LI><A HREF="#sec3">Synopsis</A></LI>
<LI><A HREF="#sec4">Description</A></LI>
<LI><A HREF="#sec5">Interface</A></LI>
<LI><A HREF="#sec6">Constructors</A></LI>
<LI><A HREF="#sec7">Member Functions</A></LI>
<LI><A HREF="#sec8">Example</A></LI>
<LI><A HREF="#sec9">Warnings</A></LI>
<LI><A HREF="#sec10">See Also</A></LI>
<LI><A HREF="#sec11">Standards Conformance</A></LI>
</UL>
<A NAME="sec1"><H3>Local Index</H3></A>
<H4>Members</H4>
<UL><TABLE CELLPADDING=3>
<TR><TD VALIGN=top>
<A HREF="#idx1106">empty()</A><BR>
<A HREF="#idx1107">pop()</A><BR>
</TD>
<TD VALIGN=top><A HREF="#idx1104">priority_queue()</A><BR>
<A HREF="#idx1108">push()</A><BR>
</TD>
<TD VALIGN=top><A HREF="#idx1109">size()</A><BR>
<A HREF="#idx1110">top()</A><BR>
</TD>
<TD VALIGN=top></TD></TR>
</TABLE></UL>

<A NAME="sec2"><H3>Summary</H3></A>
<P>A container adapter that behaves like a priority queue. Items popped from the queue are in order with respect to a priority.</P>
<A NAME="sec3"><H3>Synopsis</H3></A>

<PRE>#include &lt;queue&gt;

namespace std {
  template &lt;class T,
            class Container = vector&lt;T&gt;,
            class Compare = less&lt;Container::value_type&gt; &gt;
  class priority_queue;
}
</PRE>
<A NAME="sec4"><H3>Description</H3></A>
<P><B><I>priority_queue</I></B> is a container adaptor that allows a container to act as a priority queue. This means that the item with the highest priority, as determined by either <SAMP>operator&lt;()</SAMP> or the comparison <SAMP>Compare</SAMP>, is brought to the front of the queue whenever anything is pushed onto or popped off the queue. </P>
<P><B><I>priority_queue</I></B> adapts any container that supports <SAMP>front()</SAMP>, <SAMP>push_back()</SAMP>,  <SAMP>pop_back()</SAMP>, and has a random access iterator. In particular, <B><I><A HREF="deque.html">deque</A></I></B> and <B><I><A HREF="vector.html">vector</A></I></B> can be used.  To instantiate a <B><I>priority_queue</I></B>, a comparison function object must be supplied.</P>
<A NAME="sec5"><H3>Interface</H3></A>

<UL><PRE>namespace std {

  template &lt;class T, class Container = vector&lt;T&gt;,
            class Compare = less&lt;typename
                            Container::value_type&gt; &gt;
class priority_queue {

public:

 // typedefs

    typedef typename Container::value_type value_type;
    typedef typename Container::size_type size_type;
    typedef Container container_type;

//  Construct

    explicit priority_queue(const Compare&amp; = Compare(),
                            const Container&amp; = Container());
    template &lt;class InputIterator&gt;
    priority_queue(InputIterator start,
                   InputIterator finish,
                   const Compare&amp; = Compare(), 
                   const Container&amp; = Container());

//  Accessors

    bool empty() const;
    size_type size ) const;
    const value_type&amp; top() const;
    void push(const value_type&amp;);
    void pop();
  };
}
</PRE></UL>
<A NAME="sec6"><H3>Constructors</H3></A>

<A NAME="idx1104"></A><PRE>explicit <B>priority_queue</B>(const Compare&amp; x = Compare(),
                          const Container&amp; = Container());</PRE>
<UL>
<P>Constructs a priority queue that uses <SAMP>Container</SAMP> for its underlying implementation, <SAMP>x </SAMP>as its standard for determining priority.</P>
</UL>


<A NAME="idx1105"></A><PRE>template &lt;class InputIterator&gt;
<B>priority_queue</B>(InputIterator start, InputIterator finish,
                 const Compare&amp; x = Compare(),
                 const allocator_type&amp; alloc =
                 allocator_type());</PRE>
<UL>
<P>Constructs a new priority queue and places into it every entity in the range <SAMP>[start, finish)</SAMP>. The priority queue uses <SAMP>x</SAMP> for determining the priority, and the allocator <SAMP>alloc</SAMP> for all storage management.</P>
</UL>

<A NAME="sec7"><H3>Member Functions</H3></A>

<A NAME="idx1106"></A><PRE>bool 
<B>empty</B>() const;</PRE>
<UL>
<P>Returns <SAMP>true</SAMP> if the priority queue is empty, <SAMP>false</SAMP> otherwise.</P>
</UL>


<A NAME="idx1107"></A><PRE>void 
<B>pop</B>();</PRE>
<UL>
<P>Removes the item with the highest priority from the queue.</P>
</UL>


<A NAME="idx1108"></A><PRE>void 
<B>push</B>(const value_type&amp; x);</PRE>
<UL>
<P>Adds <SAMP>x</SAMP> to the queue.</P>
</UL>


<A NAME="idx1109"></A><PRE>size_type 
<B>size</B>() const;</PRE>
<UL>
<P>Returns the number of elements in the priority queue.</P>
</UL>


<A NAME="idx1110"></A><PRE>const value_type&amp; 
<B>top</B>() const;</PRE>
<UL>
<P>Returns a constant reference to the element in the queue with the highest priority.</P>
</UL>

<A NAME="sec8"><H3>Example</H3></A>

<UL><PRE>//
//  p_queue.cpp
//
 
#include &lt;deque&gt;      // for deque
#include &lt;iostream&gt;   // for cout, endl
#include &lt;queue&gt;      // for priority_queue
#include &lt;string&gt;     // for string
#include &lt;vector&gt;     // for vector


int main ()
{
    // Make a priority queue of int  using a vector container.
    std::priority_queue&lt;int,
                        std::vector&lt;int,
                                    std::allocator&lt;int&gt; &gt;,
                        std::less&lt;int&gt; &gt; pq;
    // Push a couple of values.
    pq.push (1);
    pq.push (2);

    // Pop a couple of values and examine the ends.
    std::cout &lt;&lt; pq.top () &lt;&lt; std::endl;
    pq.pop ();
    std::cout &lt;&lt; pq.top () &lt;&lt; std::endl;
    pq.pop ();

    // Make a priority queue of strings.
    std::priority_queue&lt;std::string,
                        std::deque&lt;std::string,
                                std::allocator&lt;std::string&gt; &gt;,
                        std::less&lt;std::string&gt; &gt; pqs;

    // Push on a few strings then pop them back off.
    for (int i = 0; i &lt; 10; i++) {
        pqs.push (std::string (i + 1, 'a'));
        std::cout &lt;&lt; pqs.top () &lt;&lt; std::endl;
    }

    for (int j = 0; j &lt; 10; j++) {
        std::cout &lt;&lt; pqs.top () &lt;&lt; std::endl;
        pqs.pop ();
    }

    // Make a priority queue of strings using greater.
    std::priority_queue&lt;std::string,
                        std::deque&lt;std::string,
                                std::allocator&lt;std::string&gt; &gt;, 
                        std::greater&lt;std::string&gt; &gt; pgqs;

    // Push on a few strings then pop them back off.
    for (int k = 0; k &lt; 10; k++)  {
        pgqs.push (std::string (k + 1, 'a'));
        std::cout &lt;&lt; pgqs.top () &lt;&lt; std::endl;
    }

    for (int m = 0; m &lt; 10; m++) {
        std::cout &lt;&lt; pgqs.top () &lt;&lt; std::endl;
        pgqs.pop ();
    }

    return 0;
}


Program Output:
</PRE></UL>
<UL><PRE>2
1
a
aa
aaa
aaaa
aaaaa
aaaaaa
aaaaaaa
aaaaaaaa
aaaaaaaaa
aaaaaaaaaa
aaaaaaaaaa
aaaaaaaaa
aaaaaaaa
aaaaaaa
aaaaaa
aaaaa
aaaa
aaa
aa
a
a
a
a
a
a
a
a
a
a
a
a
aa
aaa
aaaa
aaaaa
aaaaaa
aaaaaaa
aaaaaaaa
aaaaaaaaa
aaaaaaaaaa
</PRE></UL>
<A NAME="sec9"><H3>Warnings</H3></A>
<P>If your compiler does not support default template parameters, you must always include a <SAMP>Container</SAMP> template parameter and a <SAMP>Compare</SAMP> template parameter when declaring an instance of <B><I>priority_queue</I></B>. For example, you must write:</P>

<UL><PRE>priority_queue&lt;int, vector&lt;int&gt;, 
  less&lt;typename vector&lt;int&gt;::value_type&gt; &gt; var;
</PRE></UL>
<P>instead of:</P>

<UL><PRE>priority_queue&lt;int&gt; var;
</PRE></UL>
<A NAME="sec10"><H3>See Also</H3></A>
<P><A HREF="containers.html">Containers</A>, <B><I><A HREF="queue.html">queue</A></I></B></P>
<A NAME="sec11"><H3>Standards Conformance</H3></A>
<P><I>ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 23.2.3.2</I></P>

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