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Synchronized Queues -- EXPERIMENTAL

Introduction
Tutorial
Examples
Reference
Synchronized Queue Model
Queue Operation Status
Queue Base
Queue Adaptor
Queue Views
Synchronized Bounded Queue
Synchronized Unbounded Queue
[Warning] Warning

These features are experimental and subject to change in future versions. There are not too much tests yet, so it is possible that you can find out some trivial bugs :(

[Note] Note

These features are based on the N3533 - C++ Concurrent Queues C++1y proposal from Lawrence Crowl and Chris Mysen and C++ Concurrency in Action from Anthony Williams.

Queues provide a mechanism for communicating data between components of a system.

The existing deque in the standard library is an inherently sequential data structure. Its reference-returning element access operations cannot synchronize access to those elements with other queue operations. So, concurrent pushes and pops on queues require a different interface to the queue structure.

Moreover, concurrency adds a new dimension for performance and semantics. Different queue implementation must trade off uncontended operation cost, contended operation cost, and element order guarantees. Some of these trade-offs will necessarily result in semantics weaker than a serial queue.

Concurrent queues are a well know mechanism for communicating data between different threads.

Concurrent queues have inherently copy/move semantics for the data handling operation. Reference-returning interfaces are forbidden as multiple access to these references can not be thread-safe.

One of the major features of a concurrent queue is whether it has a bounded-unbounded capacity.

Locking queues can by nature block waiting for the queue to be non-empty or non-full.

Lock-free queues will have some trouble waiting for the queue to be non-empty or non-full queues. These queues can not define operations such as push (and pull for bounded queues). That is, it could have blocking operations (presumably emulated with busy wait) but not waiting operations.

Threads using a queue for communication need some mechanism to signal when the queue is no longer needed. The usual approach is add an additional out-of-band signal. However, this approach suffers from the flaw that threads waiting on either full or empty queues need to be woken up when the queue is no longer needed. Rather than require an out-of-band signal, we chose to directly support such a signal in the queue itself, which considerably simplifies coding.

To achieve this signal, a thread may close a queue. Once closed, no new elements may be pushed onto the queue. Push operations on a closed queue will either return queue_op_status::closed (when they have a queue_op_status return type), set the closed parameter if it has one or throw sync_queue::closed (when they do not). Elements already on the queue may be pulled off. When a queue is empty and closed, pull operations will either return queue_op_status::closed (when they have a status return), set the closed parameter if it has one or throw sync_queue::closed (when they do not).

All the functions are defined as if we had in addition to its specific Throw specification the following:

Throws:

Any exception thrown by the internal locking.

All the functions that allocate a resource are defined as if we had in addition to its specific Throw specification the following:

Throws:

Any exception due to allocation errors.

The essential solution to the problem of concurrent queuing is to shift to value-based operations, rather than reference-based operations.

The BasicConcurrentQueue concept models the basic operations of a concurrent queue.

A type Q meets the BasicConcurrentQueue requirements if the following expressions are well-formed and have the specified semantics

  • Q::value_type
  • Q::size_type
  • q.push_back(e);
  • q.push_back(rve);
  • q.pull_front(lre);
  • lre = q.pull_front();
  • b = q.empty();
  • u = q.size();

where

  • q denotes a value of type Q,
  • e denotes a value of type Q::value_type,
  • u denotes a value of type Q::size_type,
  • lve denotes an lvalue reference of type Q::value_type,
  • rve denotes an rvalue reference of type Q::value_type:
  • qs denotes a variable of of type queue_op_status,

Effects:

Waits until the queue is not full (for bounded queues) and then push back e to the queue copying it (this could need an allocation for unbounded queues).

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Postcondition:

! q.empty().

Return type:

void.

Throws:

If the queue was closed, throws sync_queue_is_closed. Any exception thrown by the copy of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

Effects:

Waits until the queue is not full (for bounded queues) and then push e to the queue moving it back in the queue (this could need an allocation for unbounded queues).

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Postcondition:

! q.empty().

Return type:

void.

Throws:

If the queue is closed, throws sync_queue_is_closed. Any exception thrown by the copy of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

Effects:

Waits until the queue is not empty and then pull_front the element from the queue q and moves the pulled element into lve (this could need an allocation for unbounded queues).

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Postcondition:

! q.full().

Return type:

void.

Throws:

Any exception thrown by the move of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

Requires:

Q::value_type is no throw move constructible. This is needed to ensure the exception safety.

Effects:

Waits until the queue is not empty and not closed. If the queue is empty and closed throws sync_queue_is_closed. Otherwise pull the element from the queue q and moves the pulled element.

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Postcondition:

! q.full().

Return type:

Q::value_type.

Return:

The pulled element.

Throws:

Any exception thrown by the copy of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

The ConcurrentQueue concept models a queue with Non-waiting operations.

A type Q meets the ConcurrentQueue requirements if is a model of a BasicConcurrentQueue and the following expressions are well-formed and have the specified semantics

  • s = q.try_push_back(e);
  • s = q.try_push_back(rve);
  • s = q.try_pull_front(lre);

where

  • q denotes a value of type Q,
  • e denotes a value of type Q::value_type,
  • s denotes a value of type queue_status,
  • u denotes a value of type Q::size_type,
  • lve denotes an lvalue reference of type Q::value_type,
  • rve denotes an rvalue reference of type Q::value_type:

Effects:

If the queue q is not full and not closed, push back the e to the queue copying it.

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation when the operation succeeds.

Return type:

queue_op_status.

Return:

- If the queue is closed, returns queue_op_status::closed,

- otherwise if the queue q is full return queue_op_status::full,

- otherwise return queue_op_status::success;

Postcondition:

If the call returns queue_op_status::success, ! q.empty().

Throws:

If the queue is closed, throws sync_queue_is_closed. Any exception thrown by the copy of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

Effects:

If the queue q is not full and not closed, push back the e onto the queue moving it.

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Return type:

queue_op_status.

Return:

- If the queue is closed, returns queue_op_status::closed,

- otherwise if the queue q is full return queue_op_status::full,

- otherwise return queue_op_status::success;

Postcondition:

If the call returns queue_op_status::success, ! q.empty().

Throws:

Any exception thrown by the copy of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

Effects:

If the queue is not empty pulls the element from the queue q and moves the pulled element into lve (this could need an allocation for unbounded queues).

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Postcondition:

! q.full().

Return type:

bool.

Return:

- If the queue q is empty return queue_op_status::empty,

- otherwise return queue_op_status::success;

Throws:

Any exception thrown by the move of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

For cases when blocking for mutual exclusion is undesirable, we have non-blocking operations. The interface is the same as the try operations but is allowed to also return queue_op_status::busy in case the operation is unable to complete without blocking.

Non-blocking operations are provided only for lock based queues

  • s = q.nonblocking_push_back(nb, e);
  • s = q.nonblocking_push_back(nb, rve);
  • s = q.nonblocking_pull_front(nb, lre);

where

  • q denotes a value of type Q,
  • e denotes a value of type Q::value_type,
  • s denotes a value of type queue_status,
  • lve denotes an lvalue reference of type Q::value_type,
  • rve denotes an rvalue reference of type Q::value_type:

Effects:

If the queue q is not full and not closed, push back the e to the queue copying it.

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation when the operation succeeds.

Return type:

queue_op_status.

Return:

- If the operation would block, return queue_op_status::busy,

- otherwise, if the queue is closed, return queue_op_status::closed,

- otherwise, if the queue q is full return queue_op_status::full,

- otherwise return queue_op_status::success;

Postcondition:

If the call returns queue_op_status::success, ! q.empty().

Throws:

If the queue is closed, throws sync_queue_is_closed. Any exception thrown by the copy of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

Effects:

If the queue q is not full and not closed, push back the e onto the queue moving it.

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Return type:

queue_op_status.

Return:

- If the operation would block, return queue_op_status::busy,

- otherwise if the queue is closed, returns queue_op_status::closed,

- otherwise if the queue q is full return queue_op_status::full,

- otherwise return queue_op_status::success;

Postcondition:

If the call returns queue_op_status::success, ! q.empty().

Throws:

Any exception thrown by the copy of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

Effects:

If the queue is not empty pulls the element from the queue q and moves the pulled element into lve (this could need an allocation for unbounded queues).

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Postcondition:

! q.full().

Return type:

bool.

Return:

- If the operation would block, return queue_op_status::busy,

- otherwise if the queue q is empty return queue_op_status::empty,

- otherwise return queue_op_status::success;

Throws:

Any exception thrown by the move of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

Bounded queues add the following valid expressions

  • Q q(u);
  • b = q.full();
  • u = q.capacity();

where

  • q denotes a value of type Q,
  • b denotes a value of type bool,
  • u denotes a value of type Q::size_type,

Return type:

bool.

Return:

Return true iff the queue is full.

Remark:

Not all queues will have a full state, and these would always return false if the function is provided.

Return type:

Q::size_type.

Return:

Return the capacity of queue.

Closed queues add the following valid expressions

  • q.close();
  • b = q.closed();
  • s = q.wait_push_back(e);
  • s = q.wait_push_back(rve);
  • s = q.wait_pull_front(lre);

Effects:

Close the queue.

Return type:

bool.

Return:

Return true iff the queue is closed.

Effects:

Waits until the queue is not full (for bounded queues) and then push back e to the queue copying it (this could need an allocation for unbounded queues).

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Postcondition:

! q.empty().

Return type:

queue_op_status.

Return:

- If the queue is closed return queue_op_status::closed,

- otherwise, return queue_op_status::success if no exception is thrown.

Throws:

Any exception thrown by the copy of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

Effects:

Waits until the queue is not full (for bounded queues) and then push e to the queue moving it back in the queue (this could need an allocation for unbounded queues).

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Postcondition:

! q.empty().

Return type:

queue_op_status.

Return:

- If the queue is closed return queue_op_status::closed,

- otherwise, return queue_op_status::success if no exception is thrown.

.

Throws:

Any exception thrown by the copy of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

Effects:

if the queue is not empty and not closed, waits until the queue is not empty and then pull_front the element from the queue q and moves the pulled element into lve.

Synchronization:

Prior pull-like operations on the same object synchronizes with this operation.

Postcondition:

! q.full().

Return type:

queue_op_status.

Return:

- If the queue is empty and closed, return queue_op_status::closed,

- otherwise, return queue_op_status::success if no exception is thrown.

Throws:

Any exception thrown by the move of e.

Exception safety:

If an exception is thrown then the queue state is unmodified.

#include <boost/thread/concurrent_queues/queue_op_status.hpp>

namespace boost
{
   enum class queue_op_status { success = 0, empty, full, closed, busy }
}
#include <boost/thread/concurrent_queues/queue_base.hpp>

namespace boost
{
  template <typename ValueType, class SizeType=std::size_t>
  class queue_base
  {
  public:
    typedef ValueType value_type;
    typedef SizeType size_type;

    // Constructors/Assignment/Destructors
    virtual ~queue_base() {};

    // Observers
    virtual bool empty() const = 0;
    virtual bool full() const = 0;
    virtual size_type size() const = 0;
    virtual bool closed() const = 0;

    // Modifiers
    virtual void close() = 0;

    virtual void push_back(const value_type& x) = 0;
    virtual void push_back(BOOST_THREAD_RV_REF(value_type) x) = 0;

    virtual void pull_front(value_type&) = 0;
    virtual value_type pull_front() = 0;

    virtual queue_op_status try_push_back(const value_type& x) = 0;
    virtual queue_op_status try_push_back(BOOST_THREAD_RV_REF(value_type) x) = 0;
    virtual queue_op_status try_pull_front(value_type&) = 0;

    virtual queue_op_status nonblocking_push_back(const value_type& x) = 0;
    virtual queue_op_status nonblocking_push_back(BOOST_THREAD_RV_REF(value_type) x) = 0;
    virtual queue_op_status nonblocking_pull_front(value_type&) = 0;

    virtual queue_op_status wait_push_back(const value_type& x) = 0;
    virtual queue_op_status wait_push_back(BOOST_THREAD_RV_REF(value_type) x) = 0;
    virtual queue_op_status wait_pull_front(value_type& elem) = 0;

  };
}
#include <boost/thread/concurrent_queues/queue_adaptor.hpp>

namespace boost
{
  template <typename Queue>
  class queue_adaptor : public queue_base<typename Queue::value_type, typename Queue::size_type>
  {
  public:
    typedef typename Queue::value_type value_type;
    typedef typename Queue::size_type size_type;

    // Constructors/Assignment/Destructors

    queue_adaptor();

    // Observers
    bool empty() const;
    bool full() const;
    size_type size() const { return queue.size(); }
    bool closed() const;

    // Modifiers
    void close();

    void push_back(const value_type& x);
    void push_back(BOOST_THREAD_RV_REF(value_type) x);

    void pull_front(value_type& x);
    value_type pull_front();

    queue_op_status try_push_back(const value_type& x);
    queue_op_status try_push_back(BOOST_THREAD_RV_REF(value_type) x);
    queue_op_status try_pull_front(value_type& x);

    queue_op_status nonblocking_push_back(const value_type& x);
    queue_op_status nonblocking_push_back(BOOST_THREAD_RV_REF(value_type) x);
    queue_op_status nonblocking_pull_front(value_type& x);

    queue_op_status wait_push_back(const value_type& x);
    queue_op_status wait_push_back(BOOST_THREAD_RV_REF(value_type) x);
    queue_op_status wait_pull_front(value_type& x);

  };
}
#include <boost/thread/concurrent_queues/queue_views.hpp>

namespace boost
{
  template <typename Queue>
  class queue_back_view;
  template <typename Queue>
  class queue_front_view

  template <class T>
  using queue_back = queue_back_view<queue_base<T>>;
  template <class T>
  using queue_front = queue_front_view<queue_base<T>>;
}
template <typename Queue>
class queue_back_view
{
public:
  typedef typename Queue::value_type value_type;
  typedef typename Queue::size_type size_type;

  // Constructors/Assignment/Destructors
  queue_back_view(Queue& q) noexcept;

  // Observers
  bool empty() const;
  bool full() const;
  size_type size() const;
  bool closed() const;

  // Modifiers
  void close();

  void push(const value_type& x);
  void push(BOOST_THREAD_RV_REF(value_type) x);

  void pull(value_type& x);
  value_type pull();

  queue_op_status try_push(const value_type& x);
  queue_op_status try_push(BOOST_THREAD_RV_REF(value_type) x);

  queue_op_status try_pull(value_type& x);

  queue_op_status nonblocking_push(const value_type& x);
  queue_op_status nonblocking_push(BOOST_THREAD_RV_REF(value_type) x);

  queue_op_status nonblocking_pull(value_type& x);

  queue_op_status wait_push(const value_type& x);
  queue_op_status wait_push(BOOST_THREAD_RV_REF(value_type) x);
  queue_op_status wait_pull_front(value_type& x);

};
template <typename Queue>
class queue_front_view
{
public:
  typedef typename Queue::value_type value_type;
  typedef typename Queue::size_type size_type;

  // Constructors/Assignment/Destructors
  queue_front_view(Queue& q) BOOST_NOEXCEPT;

  // Observers
  bool empty() const;
  bool full() const;
  size_type size() const;
  bool closed() const;

  // Modifiers
  void close();

  void push(const value_type& x);
  void push(BOOST_THREAD_RV_REF(value_type) x);

  void pull(value_type& x);
  value_type pull();

  queue_op_status try_push(const value_type& x);
  queue_op_status try_push(BOOST_THREAD_RV_REF(value_type) x);

  queue_op_status try_pull(value_type& x);

  queue_op_status nonblocking_push(const value_type& x);
  queue_op_status nonblocking_push(BOOST_THREAD_RV_REF(value_type) x);

  queue_op_status nonblocking_pull(value_type& x);

  queue_op_status wait_push(const value_type& x);
  queue_op_status wait_push(BOOST_THREAD_RV_REF(value_type) x);
  queue_op_status wait_pull(value_type& x);

};
#include <boost/thread/sync_bounded_queue.hpp>

namespace boost
{
  struct sync_queue_is_closed : std::exception {};

  template <typename ValueType>
  class sync_bounded_queue;

  // Stream-like operators
  template <typename ValueType>
  sync_bounded_queue<ValueType>& operator<<(sync_bounded_queue<ValueType>& sbq, ValueType&& elem);
  template <typename ValueType>
  sync_bounded_queue<ValueType>& operator<<(sync_bounded_queue<ValueType>& sbq, ValueType const&elem);
  template <typename ValueType>
  sync_bounded_queue<ValueType>& operator>>(sync_bounded_queue<ValueType>& sbq, ValueType &elem);
}
#include <boost/thread/sync_bounded_queue.hpp>

namespace boost
{
  struct sync_queue_is_closed : std::exception {};
}
#include <boost/thread/sync_bounded_queue.hpp>
namespace boost
{
  template <typename ValueType>
  class sync_bounded_queue
  {
  public:
    typedef ValueType value_type;
    typedef std::size_t size_type;

    sync_bounded_queue(sync_bounded_queue const&) = delete;
    sync_bounded_queue& operator=(sync_bounded_queue const&) = delete;
    explicit sync_bounded_queue(size_type max_elems);
    template <typename Range>
    sync_bounded_queue(size_type max_elems, Range range);
    ~sync_bounded_queue();

    // Observers
    bool empty() const;
    bool full() const;
    size_type capacity() const;
    size_type size() const;
    bool closed() const;

    // Modifiers
    void push_back(const value_type& x);
    void push_back(value_type&& x);

    queue_op_status try_push_back(const value_type& x);
    queue_op_status try_push_back(value_type&&) x);

    queue_op_status nonblocking_push_back(const value_type& x);
    queue_op_status nonblocking_push_back(value_type&& x);

    void pull_front(value_type&);
    value_type pull_front();

    queue_op_status try_pull_front(value_type&);
    queue_op_status nonblocking_pull_front(value_type&);


    void close();
  };
}
explicit sync_bounded_queue(size_type max_elems);

Effects:

Constructs a sync_bounded_queue with a maximum number of elements given by max_elems.

Throws:

any exception that can be throw because of resources unavailable.

template <typename Range>
sync_bounded_queue(size_type max_elems, Range range);

Effects:

Constructs a sync_bounded_queue with a maximum number of elements given by max_elems and push back the elements of the range.

Throws:

any exception that can be throw because of resources unavailable.

#include <boost/thread/sync_bounded_queue.hpp>
namespace boost
{
  template <typename ValueType>
  sync_bounded_queue<ValueType>& operator<<(sync_bounded_queue<ValueType>& sbq, ValueType&& elem);
  template <typename ValueType>
  sync_bounded_queue<ValueType>& operator<<(sync_bounded_queue<ValueType>& sbq, ValueType const&elem);
}
#include <boost/thread/sync_bounded_queue.hpp>
namespace boost
{
  template <typename ValueType>
  sync_bounded_queue<ValueType>& operator>>(sync_bounded_queue<ValueType>& sbq, ValueType &elem);
}
#include <boost/thread/sync_queue.hpp>
namespace boost
{
  template <typename ValueType>
  class sync_queue;

  // Stream-like operators
  template <typename ValueType>
  sync_queue<ValueType>& operator<<(sync_queue<ValueType>& sbq, ValueType&& elem);
  template <typename ValueType>
  sync_queue<ValueType>& operator<<(sync_queue<ValueType>& sbq, ValueType const&elem);
  template <typename ValueType>
  sync_queue<ValueType>& operator>>(sync_queue<ValueType>& sbq, ValueType &elem);
}
#include <boost/thread/sync_queue.hpp>

namespace boost
{
  template <typename ValueType, class Container = csbl::devector<ValueType>>
  class sync_queue
  {
  public:
    typedef ValueType value_type;
    typedef Container underlying_queue_type;
    typedef typename Container::size_type size_type;

    sync_queue(sync_queue const&) = delete;
    sync_queue& operator=(sync_queue const&) = delete;
    sync_queue();
    explicit template <typename Range>
    sync_queue(Range range); // Not yet implemented
    ~sync_queue();

    // Observers
    bool empty() const;
    bool full() const;
    size_type size() const;
    bool closed() const;

    // Modifiers
    void push_back(const value_type& x);
    void push_back(value_type&& x);

    queue_op_status try_push_back(const value_type& x);
    queue_op_status try_push_back(value_type&&) x);

    queue_op_status nonblocking_push_back(const value_type& x);
    queue_op_status nonblocking_push_back(value_type&& x);

    void pull_front(value_type&);
    value_type pull_front();

    queue_op_status try_pull_front(value_type&);
    queue_op_status nonblocking_pull_front(value_type&);

    underlying_queue_type underlying_queue() noexcept;

    void close();
  };
}
explicit sync_queue();

Effects:

Constructs an empty sync_queue.

Throws:

any exception that can be throw because of resources unavailable.

bool full() const;

Returns:

false.

underlying_queue_type underlying_queue() noexcept;

Returns:

Moves internal queue.

#include <boost/thread/sync_queue.hpp>
namespace boost
{
  template <typename ValueType>
  sync_queue<ValueType>& operator<<(sync_queue<ValueType>& sbq, ValueType&& elem);
  template <typename ValueType>
  sync_queue<ValueType>& operator<<(sync_queue<ValueType>& sbq, ValueType const&elem);
}
#include <boost/thread/sync_queue.hpp>
namespace boost
{
  template <typename ValueType>
  sync_queue<ValueType>& operator>>(sync_queue<ValueType>& sbq, ValueType &elem);
}

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