queue.html

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See below.
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<tt>const value_type&amp; back() const</tt>
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<tt>queue</tt>
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See below.
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<tt>void push(const value_type&amp;)</tt>
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<tt>queue</tt>
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See below.
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<tt>void pop()</tt> <A href="#3">[3]</A>
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<tt>queue</tt>
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See below.
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<tt>bool operator==(const queue&amp;, const queue&amp;)</tt>
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<tt>queue</tt>
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See below.
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<tt>bool operator&lt;(const queue&amp;, const queue&amp;)</tt>
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<tt>queue</tt>
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See below.
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<h3>New members</h3>
These members are not defined in the 
<A href="Assignable.html">Assignable</A> and <A href="DefaultConstructible.html" tppabs="http://www.sgi.com/Technology/STL/DefaultConstructible.shtml">Default Constructible</A>
requirements, but are specific to 
<tt>queue</tt>.
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Member
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Description
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<tt>value_type</tt>
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The type of object stored in the <tt>queue</tt>.  This is the same as
   <tt>T</tt> and <tt>Sequence::value_type</tt>.
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<tt>size_type</tt>
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An unsigned integral type.  This is the same as <tt>Sequence::size_type</tt>.
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<tt>bool empty() const</tt>
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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>.
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<tt>size_type size() const</tt>
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Returns the number of elements contained in the <tt>queue</tt>.
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<tt>value_type&amp; front()</tt>
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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>.
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<tt>const value_type&amp; front() const</tt>
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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>.
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<tt>value_type&amp; back()</tt>
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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>.
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<tt>const value_type&amp; back() const</tt>
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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>.
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<tt>void push(const value_type&amp; x)</tt>
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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>.
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<tt>void pop()</tt>
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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>.
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<tt>bool operator==(const queue&amp;, const queue&amp;)</tt>
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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.
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<tt>bool operator&lt;(const queue&amp;, const queue&amp;)</tt>
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Lexicographical ordering of two queues.
  This is a global function, not a member function.
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<h3>Notes</h3>
<P><A name="1">[1]</A>
Queues are a standard data structure, and are discussed in all
algorithm books.  See, for example, section 2.2.1 of Knuth.
(D. E. Knuth, <i>The Art of Computer
Programming.  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" tppabs="http://www.sgi.com/Technology/STL/Deque.shtml">deque</A></tt>, for
example, 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 the
container adaptor <tt>queue</tt> instead of the container <tt><A href="Deque.html">deque</A></tt> is to
make it clear that you are performing only queue operations, and no
other 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> to
examine and remove the element at the front of the <tt>queue</tt>, instead of
combining the two in a single member function?  In fact, there is a
good reason for this design.  If <tt>pop()</tt> returned the front element, it
would have to return by value rather than by reference: return by
reference would create a dangling pointer.  Return by value, however,
is inefficient: it involves at least one redundant copy constructor
call.  Since it is impossible for <tt>pop()</tt> to return a value in such a
way as to be both efficient and correct, it is more sensible for it to
return no value at all and to require clients to use <tt>front()</tt> to
inspect 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" tppabs="http://www.sgi.com/Technology/STL/priority_queue.shtml">priority_queue</A></tt>, <tt><A href="Deque.html" tppabs="http://www.sgi.com/Technology/STL/Deque.shtml">deque</A></tt>, <A href="Container.html" tppabs="http://www.sgi.com/Technology/STL/Container.shtml">Container</A>, <A href="Sequence.html" tppabs="http://www.sgi.com/Technology/STL/Sequence.shtml">Sequence</A>

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