15.7.2. queue Adapter

A queue is similar to a waiting line. The item that has been in the queue the longest is the next one removed—so a queue is referred to as a first-in, first-out (FIFO) data structure. Class queue (from header <queue> ) enables insertions at the back of the underlying data structure and deletions from the front. A queue can store its elements in objects of the Standard Library’s list or deque containers. By default, a queue is implemented with a deque. The common queue operations are push to insert an element at the back of the queue (implemented by calling function push_back of the underlying container), pop to remove the element at the front of the queue (implemented by calling function pop_front of the underlying container), front to get a reference to the first element in the queue (implemented by calling function front of the underlying container), back to get a reference to the last element in the queue (implemented by calling function back of the underlying container), empty to determine whether the queue is empty (implemented by calling function empty of the underlying container) and size to get the number of elements in the queue (implemented by calling function size of the underlying container).

Figure 15.20 demonstrates the queue adapter class. Line 9 instantiates a queue of doubles. Lines 12–14 use function push to add elements to the queue. The while statement in lines 19–23 uses function empty (available in all containers) to determine whether the queue is empty (line 19). While there are more elements in the queue, line 21 uses queue function front to read (but not remove) the first element in the queue for output. Line 22 removes the first element in the queue with function pop (available in all adapter classes).

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 1   // Fig. 15.20: fig15_20.cpp
 2   // Standard Library queue adapter class template.
 3   #include <iostream>
 4   #include <queue> // queue adapter definition
 5   using namespace std;
 6
 7   int main()
 8   {
 9      queue< double > values; // queue with doubles
10
11      // push elements onto queue values
12      values.push( 3.2 );               
13      values.push( 9.8 );               
14      values.push( 5.4 );               
15
16      cout << "Popping from values: ";
17
18      // pop elements from queue
19      while ( !values.empty() )
20      {
21         cout << values.front() << ' '; // view front element
22         values.pop(); // remove element                     
23      } // end while
24
25      cout << endl;
26   } // end main

Popping from values: 3.2 9.8 5.4

Fig. 15.20. Standard Library queue adapter class templates.

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