cppreference.com > C++ Queues back Syntax:
      #include <queue>
      TYPE& back();
      const TYPE& back() const;
The back() function returns a reference to the last element in the queue. For example:
     vector<int> v;
     for( int i = 0; i < 5; i++ ) {
       v.push_back(i);
     }
     cout << "The first element is " << v.front()
          << " and the last element is " << v.back() << endl;
This code produces the following output:
     The first element is 0 and the last element is 4
The back() function runs in constant time. Related topics: front (C++ Lists) pop_back

empty
Syntax:

      #include <queue>
      bool empty() const;

The empty() function returns true if the queue has no elements, false otherwise.

For example, the following code uses empty() as the stopping condition on a (C/C++ Keywords) while loop to clear a queue and display its contents in reverse order:

     vector<int> v;
     for( int i = 0; i < 5; i++ ) {
       v.push_back(i);
     }
     while( !v.empty() ) {
       cout << v.back() << endl;
       v.pop_back();
     }

Related topics:
size

front
Syntax:

      #include <queue>
      TYPE& front();
      const TYPE& front() const;

The front() function returns a reference to the first element of the queue, and runs in constant time.

Related topics:
back
(C++ Lists) pop_front
(C++ Lists) push_front

pop
Syntax:

      #include <queue>
      void pop();

The function pop() removes the top element of the queue and discards it.

Related topics:
push
(C++ Priority Queues) top

push
Syntax:

      #include <queue>
      void push( const TYPE& val );

The function push() adds val to the end of the current queue.

For example, the following code uses the push() function to add ten integers to the end of a queue:

       queue<int> q;
       for( int i=0; i < 10; i++ )
         q.push(i);

size
Syntax:

      #include <queue>
      size_type size() const;

The size() function returns the number of elements in the current queue.

Related topics:
(C++ Strings) capacity
empty
(C++ Strings) length
(C++ Multimaps) max_size
(C++ Strings) resize

Queue constructor
Syntax:

      #include <queue>
      queue();
      queue( const Container& con );

Queues have an empty constructor and a constructor that can be used to specify a container type.