std queue iteration

Question

I need to iterate over std  queue  www cplusplus com says      By default  if no container class is specified for a particular queue class  the standard container class template deque is used    So can I somehow get to the queue s underlying deque and iterate over it

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In short  No   There is a hack  use vector as underlaid container  so queue  front will return valid reference  convert it to pointer an iterate until  lt   queue  back

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std  queue is a container adaptor  and you can specify the container used  it defaults to use a deque    If you need functionality beyond that in the adaptor then just use a deque or another container directly

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One indirect solution can be to use std  deque instead  It supports all operations of queue and you can iterate over it just by using for auto amp  x qu   It s much more efficient than using a temporary copy of queue for iteration

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you can save the original queue to a temporary queue  Then you simply do your normal pop on the temporary queue to go through the original one  for example   queue tmp q   original q    copy the original queue to the temporary queue  while   tmp q empty          q element   tmp q front        std  cout  lt  lt  q element  lt  lt   n       tmp q pop         At the end  the tmp q will be empty but the original queue is untouched

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I use something like this  Not very sophisticated but should work        queue lt int gt  tem        while  q1 empty       q1 is your initial queue                 int u   q1 front                 do what you need to do with this value             q1 pop             tem push u                while  tem empty                  int u   tem front             tem pop             q1 push u      putting it back in our original queue           It will work because when you pop something from q1  and push it into tem  it becomes the first element of tem  So  in the end tem becomes a replica of q1

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While I agree with others that direct use of an iterable container is a preferred solution  I want to point out that the C   standard guarantees enough support for a do-it-yourself solution in case you want it for whatever reason   Namely  you can inherit from std  queue and use its protected member Container c  to access begin   and end   of the underlying container  provided that such methods exist there   Here is an example that works in VS 2010 and tested with ideone    include  lt queue gt   include  lt deque gt   include  lt iostream gt   template lt typename T  typename Container std  deque lt T gt   gt  class iterable queue   public std  queue lt T Container gt    public      typedef typename Container  iterator iterator      typedef typename Container  const iterator const iterator       iterator begin     return this- gt c begin          iterator end     return this- gt c end          const iterator begin   const   return this- gt c begin          const iterator end   const   return this- gt c end          int main         iterable queue lt int gt  int queue      for int i 0  i lt 10    i          int queue push i       for auto it int queue begin    it  int queue end     it          std  cout  lt  lt   it  lt  lt    n       return 0

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If you need to iterate a queue     queue isn t the container you need  Why did you pick a queue    Why don t you take a container that you can iterate over     1 if you pick a queue then you say you want to wrap a container into a  queue  interface      - front      - back     - push      - pop     -      if you also want to iterate  a queue has an incorrect interface   A queue is an adaptor that provides a restricted subset of the original container  2 The definition of a queue is a FIFO and by definition a FIFO is not iterable

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Why not just make a copy of the queue that you want to iterate over  and remove items one at a time  printing them as you go  If you want to do more with the elements as you iterate  then a queue is the wrong data structure

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while Alexey Kukanov s answer may be more efficient  you can also iterate through a queue in a very natural manner  by popping each element from the front of the queue  then pushing it to the back    include  lt iostream gt   include  lt queue gt   using namespace std   int main           populate queue     queue lt int gt  q      for  int i   0  i  lt  10    i  q push i           iterate through queue     for  size t i   0  i  lt  q size      i            int elem   std  move q front             q pop            elem    elem          q push std  move elem                 print queue     while   q empty              cout  lt  lt  q front    lt  lt               q pop              output   0 1 4 9 16 25 36 49 64 81

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If you need to iterate over a queue then you need something more than a queue  The point of the standard container adapters is to provide a minimal interface  If you need to do iteration as well  why not just use a deque  or list  instead

[c++] std::queue iteration

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