`queue` Class

Article
01/25/2022

A template container adaptor class that provides a restriction of functionality for some underlying container type, limiting access to the front and back elements. Elements can be added at the back or removed from the front, and elements can be inspected at either end of the queue.

Syntax

template <class Type, class Container = deque <Type>>
class queue

Parameters

Type
The element data type to be stored in the queue.

Container
The type of the underlying container used to implement the queue.

Remarks

The elements of class Type stipulated in the first template parameter of a queue object are synonymous with value_type and must match the type of element in the underlying container class Container stipulated by the second template parameter. The Type must be assignable, so that it is possible to copy objects of that type and to assign values to variables of that type.

Suitable underlying container classes for queue include deque and list, or any other sequence container that supports the operations of front, back, push_back, and pop_front. The underlying container class is encapsulated within the container adaptor, which exposes only the limited set of the sequence container member functions as a public interface.

The queue objects are equality comparable if and only if the elements of class Type are equality comparable, and are less-than comparable if and only if the elements of class Type are less-than comparable.

There are three types of container adaptors defined by the C++ Standard Library: stack, queue, and priority_queue. Each restricts the functionality of some underlying container class to provide a precisely controlled interface to a standard data structure.

The stack class supports a last-in, first-out (LIFO) data structure. A good analog to keep in mind would be a stack of plates. Elements (plates) may be inserted, inspected, or removed only from the top of the stack, which is the last element at the end of the base container. The restriction to accessing only the top element is the reason for using the stack class.
The queue class supports a first-in, first-out (FIFO) data structure. A good analog to keep in mind would be people lining up for a bank teller. Elements (people) may be added to the back of the line and are removed from the front of the line. Both the front and the back of a line may be inspected. The restriction to accessing only the front and back elements in this way is the reason for using the queue class.
The priority_queue class orders its elements so that the largest element is always at the top position. It supports insertion of an element and the inspection and removal of the top element. A good analog to keep in mind would be people lining up where they are arranged by age, height, or some other criterion.

Members

Constructors

Name	Description
`queue`	Constructs a `queue` that is empty or that is a copy of a base container object.

Typedefs

Name	Description
`container_type`	A type that provides the base container to be adapted by the `queue`.
`size_type`	An unsigned integer type that can represent the number of elements in a `queue`.
`value_type`	A type that represents the type of object stored as an element in a `queue`.

Functions

Name	Description
`back`	Returns a reference to the last and most recently added element at the back of the `queue`.
`empty`	Tests if the `queue` is empty.
`front`	Returns a reference to the first element at the front of the `queue`.
`pop`	Removes an element from the front of the `queue`.
`push`	Adds an element to the back of the `queue`.
`size`	Returns the number of elements in the `queue`.

`back`

Returns a reference to the last and most recently added element at the back of the queue.

reference back();

const_reference back() const;

Return Value

The last element of the queue. If the queue is empty, the return value is undefined.

Remarks

If the return value of back is assigned to a const_reference, the queue object cannot be modified. If the return value of back is assigned to a reference, the queue object can be modified.

When compiled by using _ITERATOR_DEBUG_LEVEL defined as 1 or 2, a runtime error will occur if you attempt to access an element in an empty queue. See Checked Iterators for more information.

Example

// queue_back.cpp
// compile with: /EHsc
#include <queue>
#include <iostream>

int main( )
{
   using namespace std;
   queue <int> q1;

   q1.push( 10 );
   q1.push( 11 );

   int& i = q1.back( );
   const int& ii = q1.front( );

   cout << "The integer at the back of queue q1 is " << i
        << "." << endl;
   cout << "The integer at the front of queue q1 is " << ii
        << "." << endl;
}

`container_type`

A type that provides the base container to be adapted.

typedef Container container_type;

Remarks

The type is a synonym for the template parameter Container. Two C++ Standard Library sequence container classes—the list class and the default deque class—meet the requirements to be used as the base container for a queue object. User-defined types satisfying the requirements may also be used.

For more information on Container, see the Remarks section of the queue Class topic.

Example

See the example for queue for an example of how to declare and use container_type.

`empty`

Tests if a queue is empty.

bool empty() const;

Return Value

true if the queue is empty; false if the queue is nonempty.

Example

// queue_empty.cpp
// compile with: /EHsc
#include <queue>
#include <iostream>

int main( )
{
using namespace std;

   // Declares queues with default deque base container
   queue <int> q1, q2;

   q1.push( 1 );

   if ( q1.empty( ) )
      cout << "The queue q1 is empty." << endl;
   else
      cout << "The queue q1 is not empty." << endl;

   if ( q2.empty( ) )
      cout << "The queue q2 is empty." << endl;
   else
      cout << "The queue q2 is not empty." << endl;
}

The queue q1 is not empty.
The queue q2 is empty.

`front`

Returns a reference to the first element at the front of the queue.

reference front();

const_reference front() const;

Return Value

The first element of the queue. If the queue is empty, the return value is undefined.

Remarks

If the return value of front is assigned to a const_reference, the queue object cannot be modified. If the return value of front is assigned to a reference, the queue object can be modified.

The member function returns a reference to the first element of the controlled sequence, which must be nonempty.

When compiled by using _ITERATOR_DEBUG_LEVEL defined as 1 or 2, a runtime error will occur if you attempt to access an element in an empty queue. See Checked Iterators for more information.

Example

// queue_front.cpp
// compile with: /EHsc
#include <queue>
#include <iostream>

int main() {
   using namespace std;
   queue <int> q1;

   q1.push( 10 );
   q1.push( 20 );
   q1.push( 30 );

   queue <int>::size_type i;
   i = q1.size( );
   cout << "The queue length is " << i << "." << endl;

   int& ii = q1.back( );
   int& iii = q1.front( );

   cout << "The integer at the back of queue q1 is " << ii
        << "." << endl;
   cout << "The integer at the front of queue q1 is " << iii
        << "." << endl;
}

`pop`

Removes an element from the front of the queue.

void pop();

Remarks

The queue must be nonempty to apply the member function. The top of the queue is the position occupied by the most recently added element and is the last element at the end of the container.

Example

// queue_pop.cpp
// compile with: /EHsc
#include <queue>
#include <iostream>

int main( )
{
   using namespace std;
   queue <int> q1, s2;

   q1.push( 10 );
   q1.push( 20 );
   q1.push( 30 );

   queue <int>::size_type i;
   i = q1.size( );
   cout << "The queue length is " << i << "." << endl;

   i = q1.front( );
   cout << "The element at the front of the queue is "
        << i << "." << endl;

   q1.pop( );

   i = q1.size( );
   cout << "After a pop the queue length is "
        << i << "." << endl;

   i = q1. front ( );
   cout << "After a pop, the element at the front of the queue is "
        << i << "." << endl;
}

The queue length is 3.
The element at the front of the queue is 10.
After a pop the queue length is 2.
After a pop, the element at the front of the queue is 20.

`push`

Adds an element to the back of the queue.

void push(const Type& val);

Parameters

val
The element added to the back of the queue.

Remarks

The back of the queue is the position occupied by the most recently added element and is the last element at the end of the container.

Example

// queue_push.cpp
// compile with: /EHsc
#include <queue>
#include <iostream>

int main( )
{
   using namespace std;
   queue <int> q1;

   q1.push( 10 );
   q1.push( 20 );
   q1.push( 30 );

   queue <int>::size_type i;
   i = q1.size( );
   cout << "The queue length is " << i << "." << endl;

   i = q1.front( );
   cout << "The element at the front of the queue is "
        << i << "." << endl;
}

The queue length is 3.
The element at the front of the queue is 10.

`queue`

Constructs a queue that is empty or that is a copy of a base container object.

queue();

explicit queue(const container_type& right);

Parameters

right
The const container of which the constructed queue is to be a copy.

Remarks

The default base container for queue is deque. You can also specify list as a base container, but you cannot specify vector, because it lacks the required pop_front member function.

Example

// queue_queue.cpp
// compile with: /EHsc
#include <queue>
#include <vector>
#include <list>
#include <iostream>

int main( )
{
   using namespace std;

   // Declares queue with default deque base container
   queue <char> q1;

   // Explicitly declares a queue with deque base container
   queue <char, deque<char> > q2;

   // These lines don't cause an error, even though they
   // declares a queue with a vector base container
   queue <int, vector<int> > q3;
   q3.push( 10 );
   // but the following would cause an error because vector has
   // no pop_front member function
   // q3.pop( );

   // Declares a queue with list base container
   queue <int, list<int> > q4;

   // The second member function copies elements from a container
   list<int> li1;
   li1.push_back( 1 );
   li1.push_back( 2 );
   queue <int, list<int> > q5( li1 );
   cout << "The element at the front of queue q5 is "
        << q5.front( ) << "." << endl;
   cout << "The element at the back of queue q5 is "
        << q5.back( ) << "." << endl;
}

The element at the front of queue q5 is 1.
The element at the back of queue q5 is 2.

`size`

Returns the number of elements in the queue.

size_type size() const;

Return Value

The current length of the queue.

Example

// queue_size.cpp
// compile with: /EHsc
#include <queue>
#include <iostream>

int main( )
{
   using namespace std;
   queue <int> q1, q2;
   queue <int>::size_type i;

   q1.push( 1 );
   i = q1.size( );
   cout << "The queue length is " << i << "." << endl;

   q1.push( 2 );
   i = q1.size( );
   cout << "The queue length is now " << i << "." << endl;
}

The queue length is 1.
The queue length is now 2.

`size_type`

An unsigned integer type that can represent the number of elements in a queue.

typedef typename Container::size_type size_type;

Remarks

The type is a synonym for the size_type of the base container adapted by the queue.

Example

See the example for queue::front for an example of how to declare and use size_type.

`value_type`

A type that represents the type of object stored as an element in a queue.

typedef typename Container::value_type value_type;

Remarks

The type is a synonym for the value_type of the base container adapted by the queue.

Example

// queue_value_type.cpp
// compile with: /EHsc
#include <queue>
#include <iostream>

int main( )
{
using namespace std;

   // Declares queues with default deque base container
   queue<int>::value_type AnInt;

   AnInt = 69;
   cout << "The value_type is AnInt = " << AnInt << endl;

   queue<int> q1;
   q1.push(AnInt);
   cout << "The element at the front of the queue is "
        << q1.front( ) << "." << endl;
}

The value_type is AnInt = 69
The element at the front of the queue is 69.

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queue Class

Syntax

Parameters

Remarks

Members

Constructors

Typedefs

Functions

back

Return Value

Remarks

Example

container_type

Remarks

Example

empty

Return Value

Example

front

Return Value

Remarks

Example

pop

Remarks

Example

push

Parameters

Remarks

Example

queue

Parameters

Remarks

Example

size

Return Value

Example

size_type

Remarks

Example

value_type

Remarks

Example

See also

Feedback

Additional resources

`queue` Class

`back`

`container_type`

`empty`

`front`

`pop`

`push`

`queue`

`size`

`size_type`

`value_type`