## queue

    Container Adaptor

- Summary
- Data Type and Member Function Indexes
- Synopsis
- Description
- Interface
- Constructors
- Member Functions
- Non-member Operators
- Example
- Warnings
- See Also

### Summary

A container adaptor that behaves like a queue (first in, first out).

### Data Type and Member Function Indexes
(exclusive of constructors and destructors)

Member Functions
back() empty() front() operator!=() operator>() operator<() | operator==() pop() push() size()

### Synopsis

    #include <queue>
    template <class T, class Container = deque<T> >
      class queue ;

### Description

The queue container adaptor lets a container act as a queue. In a queue, items are pushed into the back of the container and removed from the front. The first items pushed into the queue are the first items to be popped off of the queue (first in, first out, or "FIFO").

queue can adapt any container that supports the `front()`, `back()`, `push_back()`, and `pop_front()` operations. In particular, deque and list can be used.

### Interface

    template <class T, class Container = deque<T> >
     class queue {

    public:

    // typedefs

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

    // Construct/Copy/Destroy
       explicit queue (const Container& = Container());

    // Accessors

       bool empty () const;
       size_type size () const;
       value_type& front ();
       const value_type& front () const;
       value_type& back ();
       const value_type& back () const;
       void push (const value_type&);
       void pop ();
    };

    // Non-member Operators

    template <class T, class Container>
     bool operator== (const queue<T, Container>&,
                      const queue<T, Container>&);

    template <class T, class Container>
     bool operator!= (const queue<T, Container>&,
                      const queue<T, Container>&);

    template <class T, class Container>
     bool operator< (const queue<T, Container>&,
                     const queue<T, Container>&);

    template <class T, class Container>
     bool operator> (const queue<T, Container>&,
                     const queue<T, Container>&);

    template <class T, class Container>
     bool operator<= (const queue<T, Container>&,
                     const queue<T, Container>&);

    template <class T, class Container>
     bool operator>= (const queue<T, Container>&,
                     const queue<T, Container>&);

### Constructors

    explicit queue (const Container& = Container());

Creates a queue of zero elements. The queue uses the allocator `Allocator()` for all storage management.

### Member Functions

    value_type&
    back ();

Returns a reference to the item at the back of the queue (the last item pushed into the queue).

    const value_type&
    back() const;

Returns a constant reference to the item at the back of the queue as a `const_value_type`.

    bool
    empty () const;

Returns `true` if the queue is empty, otherwise `false`.

    value_type&
    front ();

Returns a reference to the item at the front of the queue. This is the first item pushed onto the queue unless `pop()` has been called since then.

    const value_type&
    front () const;

Returns a constant reference to the item at the front of the queue as a `const_value_type`.

    void
    pop ();

Removes the item at the front of the queue.

    void
    push (const value_type& x);

Pushes `x` onto the back of the queue.

    size_type
    size () const;

Returns the number of elements on the queue.

### Non-member Operators

    template <class T, class Container>
      bool operator== (const queue<T, Container>& x,
                       const queue<T, Container>& y);

Returns `true` if `x` is the same as `y`.

    template <class T, class Container>
      bool operator!= (const queue<T, Container>& x,
                       const queue<T, Container>& y);

Returns `!(x==y)`.

    template <class T, class Container>
      bool operator< (const queue<T, Container>& x,
                      const queue<T, Container>& y);

Returns `true` if the queue defined by the elements contained in `x` is lexicographically less than the queue defined by the elements contained in `y`.

    template <class T, class Container>
      bool operator> (const queue<T, Container>& x,
                      const queue<T, Container>& y);

Returns `y < x`.

    template <class T, class Container>
      bool operator< (const queue<T, Container>& x,
                      const queue<T, Container>& y);

Returns `!(y < x)`.

    template <class T, class Container>
      bool operator< (const queue<T, Container>& x,
                      const queue<T, Container>& y);

Returns `!(x < y)`.

### Example

    //
    // queue.cpp
    //
     #include <queue>
     #include <string>
     #include <deque>
     #include <list>
     #include <iostream>
     using namespace std;

     int main(void)
     {
       // Make a queue using a list container
       queue<int, list<int>> q;

       // Push a couple of values on then pop them off
       q.push(1);
       q.push(2);
       cout << q.front() << endl;
       q.pop();
       cout << q.front() << endl;
       q.pop();

       // Make a queue of strings using a deque container
       queue<string,deque<string>> qs;

       // Push on a few strings then pop them back off
       int i;
       for (i = 0; i < 10; i++)
       {
         qs.push(string(i+1,'a'));
         cout << qs.front() << endl;
       }
       for (i = 0; i < 10; i++)
       {
         cout << qs.front() << endl;
         qs.pop();
       }

       return 0;
     }

Program Output

    1
    2
    a
    a
    a
    a
    a
    a
    a
    a
    a
    a
    a
    aa
    aaa
    aaaa
    aaaaa
    aaaaaa
    aaaaaaa
    aaaaaaaa
    aaaaaaaaa
    aaaaaaaaaa

### Warnings

If your compiler does not support default template parameters, you must always include a `Container` template parameter. For example you would not be able to write:

`queue<int> var;`

rather, you would have to write,

`queue<int, deque<int> > var;`

If your compiler does not support namespaces, then you do not need the using declaration for `std`.

### See Also

allocator, Containers, priority_queue

Copyright (c) 1998, Rogue Wave Software, Inc.
Send mail to report errors or comment on the documentation.
OEM Release, June 1998