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<HTML>
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<Head>
<Title>queue<T, Sequence></Title>
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<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 = "39" ></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> functionality
A <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 removed
from 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 on
top of some underlying container type. By default that underlying
type 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 nonstandard
backward-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><b>queue</b>()</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><b>queue</b>(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& <b>operator=</b>(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 <b>empty</b>() const</tt>
</TD>
<TD VAlign=top>
<tt>queue</tt>
</TD>
<TD VAlign=top>
See below.
</TD>
</TR>
<TR>
<TD VAlign=top>
<tt>size_type <B>size</b>() const</tt>
</TD>
<TD VAlign=top>
<tt>queue</tt>
</TD>
<TD VAlign=top>
See below.
</TD>
</TR>
<TR>
<TD VAlign=top>
<tt>value_type& <b>front</b>()</tt>
</TD>
<TD VAlign=top>
<tt>queue</tt>
</TD>
<TD VAlign=top>
See below.
</TD>
</TR>
<TR>
<TD VAlign=top>
<tt>const value_type& <b>front</b>() const</tt>
</TD>
<TD VAlign=top>
<tt>queue</tt>
</TD>
<TD VAlign=top>
See below.
</TD>
</TR>
<TR>
<TD VAlign=top>
<tt>value_type& <b>back</b>()</tt>
</TD>
<TD VAlign=top>
<tt>queue</tt>
</TD>
<TD VAlign=top>
See below.
</TD>
</TR>
<TR>
<TD VAlign=top>
<tt>const value_type& <b>back</b>() const</tt>
</TD>
<TD VAlign=top>
<tt>queue</tt>
</TD>
<TD VAlign=top>
See below.
</TD>
</TR>
<TR>
<TD VAlign=top>
<tt>void <b>push</b>(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 <b>pop</b>()</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 <b>operator==</b>(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 <b>operator<</b>(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 <b>empty</b>() 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 <b>size</b>() 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& <b>front</b>()</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& <b>front</b>() 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& <b>back</b>()</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& <b>back</b>() 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 <b>push</b>(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 <b>pop</b>()</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 <b>operator==</b>(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 <b>operator<</b>(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 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">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">priority_queue</A></tt>, <tt><A href="Deque.html">deque</A></tt>, <A href="Container.html">Container</A>, <A href="Sequence.html">Sequence</A>
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