event-queue.h

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/*

Copyright (C) 2012-2017 John W. Eaton

This file is part of Octave.

Octave is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 3 of the License, or (at your
option) any later version.

Octave is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License
along with Octave; see the file COPYING.  If not, see
<http://www.gnu.org/licenses/>.

*/

#if ! defined (octave_event_queue_h)
#define octave_event_queue_h 1

#include "octave-config.h"

#include <queue>
#include <memory>

#include "action-container.h"

class
event_queue : public action_container
{
public:

  event_queue (void) : fifo () { }

  // Destructor should not raise an exception, so all actions
  // registered should be exception-safe.  If you're not sure, see
  // event_queue_safe.

  ~event_queue (void) { run (); }

  void add (elem *new_elem)
  {
    fifo.push (new_elem);
  }

  void run_first (void)
  {
    if (! empty ())
      {
        // No leak on exception!
        std::unique_ptr<elem> ptr (fifo.front ());
        fifo.pop ();
        ptr->run ();
      }
  }

  void discard_first (void)
  {
    if (! empty ())
      {
        elem *ptr = fifo.front ();
        fifo.pop ();
        delete ptr;
      }
  }

  size_t size (void) const { return fifo.size (); }

protected:

  std::queue<elem *> fifo;

private:

  // No copying!

  event_queue (const event_queue&);

  event_queue& operator = (const event_queue&);
};

// Like event_queue, but this one will guard against the
// possibility of seeing an exception (or interrupt) in the cleanup actions.
// Not that we can do much about it, but at least we won't crash.

class
event_queue_safe : public event_queue
{
private:

  void warn_unhandled_exception (void) const;

public:

  event_queue_safe (void) : event_queue () { }

  ~event_queue_safe (void)
  {
    while (! empty ())
      {
        try
          {
            run_first ();
          }
        catch (...) // Yes, the black hole.  Remember we're in a dtor.
          {
            warn_unhandled_exception ();
          }
      }
  }

private:

  // No copying!

  event_queue_safe (const event_queue_safe&);

  event_queue_safe& operator = (const event_queue_safe&);
};

#endif