Access c++ queue elements like an array

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Can queue elements be accessed like an array? If not, then what containers similar to a queue can?

This is a task ideal for std::deque. Its optimized for adding/removing onto the end but also provides random access to elements in the middle. To quote the linked article:

A deque is very much like a vector: like vector, it is a sequence that supports random access to elements, constant time insertion and removal of elements at the end of the sequence, and linear time insertion and removal of elements in the middle.

... deque also supports constant time insertion and removal of elements at the beginning of the sequence

So because it can efficiently add/remove from both ends, deque can be used efficiently as a queue with its push_back and pop_front methods:

std::deque<int> aDeque;

// enqueue
aDeque.push_back(1);
aDeque.push_back(2);

// dequeue
int top = aDeque.front();
aDeque.pop_front();

Accessing elements like an array means using the subscript operator

deque also support the random access through the subscript operator:

std::cout << aDeque[0];

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Can queue elements be accessed like an array?

Sure! As long as the underlying container (which defaults to deque) does, though you might want to call the code bad names...

template<class T, class C=std::deque<T> >
struct pubqueue : std::queue<T, C> {
  using std::queue<T, C>::c;

  static C& get_c(std::queue<T, C> &s) {
    return s.*&pubqueue::c;
  }
  static C const& get_c(std::queue<T, C> const &s) {
    return s.*&pubqueue::c;
  }
};

template<class T, class C>
C& get_c(std::queue<T, C> &a) {
  return pubqueue<T, C>::get_c(a);
}
template<class T, class C>
C& get_c(std::queue<T, C> const &a) {
  return pubqueue<T, C>::get_c(a);
}

int main() {
  std::queue<int> q;
  q.push(42);
  std::cout << get_c(q)[0] << '\n';

  pubqueue<int> p;
  p.push(3);
  std::cout << p.c[0] << '\n';

  return 0;
}

Notice the trick that allows you to change your std::queue variables to pubqueue variables and just access the container member directly. This lets you keep the push/pop (instead of push_back/pop_front, etc.) interface of std::queue.

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Since you've clarified that you want subscript operator access, the answer is no. Queues are not a data structure that would ever require random element access. If you need random element access, use a vector, or an actual array.

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The answer, it depends on the implementation of the queue. The queue provided by the Standard Template Library, doesn't give you random access to elements via the subscript operator, because the implementation of random access defeats the point of a queue.

Recall that a Queue is a datastructure that provides first-in-first-out behavior. This means you need to really concern yourself with the head-element, and thats it. Once you need access to elements beside the head, you no longer have a queue.

Now that doesn't mean you can't implement your own queue on top of an array/vector class, however it's not going to be efficient, as both arrays and vectors aren't ideal for adding and removing elements dynamically.

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Instead of a queue, use a vector. Queue doesn't use the [] operator.

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Comments
  • Define "like an array". Do you mean "using the subscript operator", "in O(1) time", or what?
  • Accessing elements like an array means using the subscript operator.
  • are you referring to STL's queue? IE std::queue?
  • The link to std::deque is broken :(
  • (std::queue uses a std::deque as its underlying container, FWIW.)
  • I wouldn't implement a queue with anything but an array-like class.
  • @Dennis: Why? What benefit do you get by using an array over a list?
  • @Alan: Note he said array-like, which could include a deque, which has efficient front and back manipulations. In any case, linked-lists are possibly the worse container there is, slow in every regard (compared to other containers) except splicing and reversing, and the benefits of those are only noticeable when the primary operation is splicing or reversing.
  • @GMan: A deque isn't necessarily "Array-like." You can implement one with a growing array, but that's an implementation choice. Array-like, means the characteristics of an array (constant time access, contiguous memory allocation) etc. A deque doesn't guarantee either. A deque implemented via an dynamic array, has an amortized cost for inserts, where as and implementation using a list, has a fixed cost of O(1) for inserts at front and back. You add much more complexity which increases the likelihood of it being error prone if you use an array to implement your queue.
  • Well vector's interface only lets you push/pop on the back. There's a performance hit when pushing or popping onto the front since everything needs to move down in the array. Vector would be better optimized for a stack with random access.