How do I clear the std queue efficiently

Question

I am using std  queue for implementing JobQueue class    Basically this class process each job in FIFO manner   In one scenario  I want to clear the queue in one shot  delete all jobs from the queue   I don t see any clear method available in std  queue class   How do I efficiently implement the clear method for JobQueue class    I have one simple solution of popping in a loop but I am looking for better ways     Clears the job queue void JobQueue   clearJobs           I want to avoid pop in a loop     while   m Queue empty                  m Queue pop

User · Answer

I d rather not rely on swap   or setting the queue to a newly created queue object  because the queue elements are not properly destroyed  Calling pop  invokes the destructor for the respective element object  This might not be an issue in  lt int gt  queues but might very well have side effects on queues containing objects   Therefore a loop with while  queue empty    queue pop   seems unfortunately to be the most efficient solution at least for queues containing objects if you want to prevent possible side effects

User · Answer

Assuming your m Queue contains integers   std  queue lt int gt    swap m Queue    Otherwise  if it contains e g  pointers to Job objects  then   std  queue lt Job  gt    swap m Queue    This way you swap an empty queue with your m Queue  thus m Queue becomes empty

User · Answer

Author of the topic asked how to clear the queue  efficiently   so I assume he wants better complexity than linear O queue size   Methods served by David Rodriguez  anon have the same complexity  according to STL reference  operator   has complexity O queue size    IMHO it s because each element of queue is reserved separately and it isn t allocated in one big memory block  like in vector  So to clear all memory  we have to delete every element separately  So the straightest way to clear std  queue is one line   while  Q empty    Q pop

User · Answer

You could create a class that inherits from queue and clear the underlying container directly  This is very efficient   template lt class T gt  class queue clearable   public std  queue lt T gt    public      void clear                 c clear               Maybe your a implementation also allows your Queue object  here JobQueue  to inherit std  queue lt Job gt  instead of having the queue as a member variable  This way you would have direct access to c clear   in your member functions

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I do this  Using C  14    std  queue lt int gt  myqueue  myqueue   decltype myqueue       This way is useful if you have a non-trivial queue type that you don t want to build an alias typedef for  I always make sure to leave a comment around this usage  though  to explain to unsuspecting   maintenance programmers that this isn t crazy  and done in lieu of an actual clear   method

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A common idiom for clearing standard containers is swapping with an empty version of the container   void clear  std  queue lt int gt   amp q        std  queue lt int gt  empty     std  swap  q  empty        It is also the only way of actually clearing the memory held inside some containers  std  vector

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Apparently  there are two most obvious ways to clear std  queue  swapping with empty object and assignment to empty object   I would suggest using assignment because it simply faster  more readable  and unambiguous   I measured performance using following simple code and I found that swapping in C  03 version works 70-80  slower than assignment to an empty object  In C  11 there is no difference in performance  however  Anyway  I would go with assignment    include  lt algorithm gt   include  lt ctime gt   include  lt iostream gt   include  lt queue gt   include  lt vector gt   int main         std  cout  lt  lt   Started   lt  lt  std  endl       std  queue lt int gt  q       for  int i   0  i  lt  10000    i                q push i              std  vector lt std  queue lt int gt   gt  queues 10000  q        const std  clock t begin   std  clock         for  std  vector lt int gt   size type i   0  i  lt  queues size      i                   OK in all versions         queues i    std  queue lt int gt                 OK since C  11            std  queue lt int gt    swap queues i                OK before C  11 but slow            std  queue lt int gt  empty             std  swap empty  queues i               const double elapsed   double clock   - begin    CLOCKS PER SEC       std  cout  lt  lt  elapsed  lt  lt  std  endl       return 0

User · Answer

In C  11 you can clear the queue by doing this   std  queue lt int gt  queue         queue

User · Answer

Yes - a bit of a misfeature of the queue class  IMHO  This is what I do    include  lt queue gt  using namespace std    int main         queue  lt int gt  q1         stuff     q1   queue lt int gt

User · Answer

Another option is to use a simple hack to get the underlying container std  queue  c and call clear on it  This member must be present in std  queue as per the standard  but is unfortunately protected  The hack here was taken from this answer    include  lt queue gt   template lt class ADAPTER gt  typename ADAPTER  container type amp  get container ADAPTER amp  a        struct hack   ADAPTER               static typename ADAPTER  container type amp  get ADAPTER amp  a                        return a     amp hack  c                       return hack  get a      template lt typename T  typename C gt  void clear std  queue lt T C gt  amp  q        get container q  clear        include  lt iostream gt  int main         std  queue lt int gt  q      q push 3       q push 5       std  cout  lt  lt  q size    lt  lt    n       clear q       std  cout  lt  lt  q size    lt  lt    n

User · Answer

Using a unique ptr might be OK  You then reset it to obtain an empty queue and release the memory of the first queue  As to the complexity   I m not sure - but guess it s O 1    Possible code   typedef queue lt int gt  quint   unique ptr lt quint gt  p new quint            p reset new quint       the old queue has been destroyed and you start afresh with an empty queue

[c++] How do I clear the std::queue efficiently?

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