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## Synchronization

### Mutex Concepts

A mutex object facilitates protection against data races and allows thread-safe synchronization of data between threads. A thread obtains ownership of a mutex object by calling one of the lock functions and relinquishes ownership by calling the corresponding unlock function. Mutexes may be either recursive or non-recursive, and may grant simultaneous ownership to one or many threads. Boost.Thread supplies recursive and non-recursive mutexes with exclusive ownership semantics, along with a shared ownership (multiple-reader / single-writer) mutex.

Boost.Thread supports four basic concepts for lockable objects: `Lockable`, `TimedLockable`, `SharedLockable` and `UpgradeLockable`. Each mutex type implements one or more of these concepts, as do the various lock types.

#### `Lockable` Concept

The `Lockable` concept models exclusive ownership. A type that implements the `Lockable` concept shall provide the following member functions:

- `void lock();`
- `bool try_lock();`
- `void unlock();`

Lock ownership acquired through a call to `lock()` or `try_lock()` must be released through a call to `unlock()`.

##### `void lock()`

Effects:

  The current thread blocks until ownership can be obtained for the current thread.

Postcondition:

  The current thread owns `*this`.

Throws:

  `boost::thread_resource_error` if an error occurs.

##### `bool try_lock()`

Effects:

  Attempt to obtain ownership for the current thread without blocking.

Returns:

  `true` if ownership was obtained for the current thread, `false` otherwise.

Postcondition:

  If the call returns `true`, the current thread owns the `*this`.

Throws:

  `boost::thread_resource_error` if an error occurs.

##### `void unlock()`

Precondition:

  The current thread owns `*this`.

Effects:

  Releases ownership by the current thread.

Postcondition:

  The current thread no longer owns `*this`.

Throws:

  Nothing

#### `TimedLockable` Concept

The `TimedLockable` concept refines the `Lockable` concept to add support for timeouts when trying to acquire the lock.

A type that implements the `TimedLockable` concept shall meet the requirements of the `Lockable` concept. In addition, the following member functions must be provided:

- `bool timed_lock(boost::system_time const& abs_time);`
- `template<typename DurationType> bool timed_lock(DurationType const& rel_time);`

Lock ownership acquired through a call to `timed_lock()` must be released through a call to `unlock()`.

##### `bool timed_lock(boost::system_time const& abs_time)`

Effects:

  Attempt to obtain ownership for the current thread. Blocks until ownership can be obtained, or the specified time is reached. If the specified time has already passed, behaves as `try_lock()`.

Returns:

  `true` if ownership was obtained for the current thread, `false` otherwise.

Postcondition:

  If the call returns `true`, the current thread owns `*this`.

Throws:

  `boost::thread_resource_error` if an error occurs.

##### `template<typename DurationType> bool timed_lock(DurationType const& rel_time)`

Effects:

  As-if `timed_lock(boost::get_system_time()+rel_time)`.

#### `SharedLockable` Concept

The `SharedLockable` concept is a refinement of the `TimedLockable` concept that allows for shared ownership as well as exclusive ownership. This is the standard multiple-reader / single-write model: at most one thread can have exclusive ownership, and if any thread does have exclusive ownership, no other threads can have shared or exclusive ownership. Alternatively, many threads may have shared ownership.

For a type to implement the `SharedLockable` concept, as well as meeting the requirements of the `TimedLockable` concept, it must also provide the following member functions:

- `void lock_shared();`
- `bool try_lock_shared();`
- `bool unlock_shared();`
- `bool timed_lock_shared(boost::system_time const& abs_time);`

Lock ownership acquired through a call to `lock_shared()`, `try_lock_shared()` or `timed_lock_shared()` must be released through a call to `unlock_shared()`.

##### `void lock_shared()`

Effects:

  The current thread blocks until shared ownership can be obtained for the current thread.

Postcondition:

  The current thread has shared ownership of `*this`.

Throws:

  `boost::thread_resource_error` if an error occurs.

##### `bool try_lock_shared()`

Effects:

  Attempt to obtain shared ownership for the current thread without blocking.

Returns:

  `true` if shared ownership was obtained for the current thread, `false` otherwise.

Postcondition:

  If the call returns `true`, the current thread has shared ownership of `*this`.

Throws:

  `boost::thread_resource_error` if an error occurs.

##### `bool timed_lock_shared(boost::system_time const& abs_time)`

Effects:

  Attempt to obtain shared ownership for the current thread. Blocks until shared ownership can be obtained, or the specified time is reached. If the specified time has already passed, behaves as `try_lock_shared()`.

Returns:

  `true` if shared ownership was acquired for the current thread, `false` otherwise.

Postcondition:

  If the call returns `true`, the current thread has shared ownership of `*this`.

Throws:

  `boost::thread_resource_error` if an error occurs.

##### `void unlock_shared()`

Precondition:

  The current thread has shared ownership of `*this`.

Effects:

  Releases shared ownership of `*this` by the current thread.

Postcondition:

  The current thread no longer has shared ownership of `*this`.

Throws:

  Nothing

#### `UpgradeLockable` Concept

The `UpgradeLockable` concept is a refinement of the `SharedLockable` concept that allows for upgradable ownership as well as shared ownership and exclusive ownership. This is an extension to the multiple-reader / single-write model provided by the `SharedLockable` concept: a single thread may have upgradable ownership at the same time as others have shared ownership. The thread with upgradable ownership may at any time attempt to upgrade that ownership to exclusive ownership. If no other threads have shared ownership, the upgrade is completed immediately, and the thread now has exclusive ownership, which must be relinquished by a call to `unlock()`, just as if it had been acquired by a call to `lock()`.

If a thread with upgradable ownership tries to upgrade whilst other threads have shared ownership, the attempt will fail and the thread will block until exclusive ownership can be acquired.

Ownership can also be downgraded as well as upgraded: exclusive ownership of an implementation of the `UpgradeLockable` concept can be downgraded to upgradable ownership or shared ownership, and upgradable ownership can be downgraded to plain shared ownership.

For a type to implement the `UpgradeLockable` concept, as well as meeting the requirements of the `SharedLockable` concept, it must also provide the following member functions:

- `void lock_upgrade();`
- `bool unlock_upgrade();`
- `void unlock_upgrade_and_lock();`
- `void unlock_and_lock_upgrade();`
- `void unlock_upgrade_and_lock_shared();`

Lock ownership acquired through a call to `lock_upgrade()` must be released through a call to `unlock_upgrade()`. If the ownership type is changed through a call to one of the `unlock_xxx_and_lock_yyy()` functions, ownership must be released through a call to the unlock function corresponding to the new level of ownership.

##### `void lock_upgrade()`

Effects:

  The current thread blocks until upgrade ownership can be obtained for the current thread.

Postcondition:

  The current thread has upgrade ownership of `*this`.

Throws:

  `boost::thread_resource_error` if an error occurs.

##### `void unlock_upgrade()`

Precondition:

  The current thread has upgrade ownership of `*this`.

Effects:

  Releases upgrade ownership of `*this` by the current thread.

Postcondition:

  The current thread no longer has upgrade ownership of `*this`.

Throws:

  Nothing

##### `void unlock_upgrade_and_lock()`

Precondition:

  The current thread has upgrade ownership of `*this`.

Effects:

  Atomically releases upgrade ownership of `*this` by the current thread and acquires exclusive ownership of `*this`. If any other threads have shared ownership, blocks until exclusive ownership can be acquired.

Postcondition:

  The current thread has exclusive ownership of `*this`.

Throws:

  Nothing

##### `void unlock_upgrade_and_lock_shared()`

Precondition:

  The current thread has upgrade ownership of `*this`.

Effects:

  Atomically releases upgrade ownership of `*this` by the current thread and acquires shared ownership of `*this` without blocking.

Postcondition:

  The current thread has shared ownership of `*this`.

Throws:

  Nothing

##### `void unlock_and_lock_upgrade()`

Precondition:

  The current thread has exclusive ownership of `*this`.

Effects:

  Atomically releases exclusive ownership of `*this` by the current thread and acquires upgrade ownership of `*this` without blocking.

Postcondition:

  The current thread has upgrade ownership of `*this`.

Throws:

  Nothing

### Lock Types

#### Lock option tags

    #include <boost/thread/locks.hpp>

    struct defer_lock_t {};
    struct try_to_lock_t {};
    struct adopt_lock_t {};
    const defer_lock_t defer_lock;
    const try_to_lock_t try_to_lock;
    const adopt_lock_t adopt_lock;

These tags are used in scoped locks constructors to specify a specific behavior.

- `defer_lock_t`: is used to construct the scoped lock without locking it.
- `try_to_lock_t`: is used to construct the scoped lock trying to lock it.
- `adopt_lock_t`: is used to construct the scoped lock without locking it but adopting ownership.

#### Class template `lock_guard`

    #include <boost/thread/locks.hpp>

    template<typename Lockable>
    class lock_guard
    {
    public:
        explicit lock_guard(Lockable& m_);
        lock_guard(Lockable& m_,boost::adopt_lock_t);

        ~lock_guard();
    };

`boost::lock_guard` is very simple: on construction it acquires ownership of the implementation of the `Lockable` concept supplied as the constructor parameter. On destruction, the ownership is released. This provides simple RAII-style locking of a `Lockable` object, to facilitate exception-safe locking and unlocking. In addition, the `lock_guard(Lockable & m,boost::adopt_lock_t)` constructor allows the `boost::lock_guard` object to take ownership of a lock already held by the current thread.

##### `lock_guard(Lockable & m)`

Effects:

  Stores a reference to `m`. Invokes `m.lock()`.

Throws:

  Any exception thrown by the call to `m.lock()`.

##### `lock_guard(Lockable & m,boost::adopt_lock_t)`

Precondition:

  The current thread owns a lock on `m` equivalent to one obtained by a call to `m.lock()`.

Effects:

  Stores a reference to `m`. Takes ownership of the lock state of `m`.

Throws:

  Nothing.

##### `~lock_guard()`

Effects:

  Invokes `m.unlock()` on the `Lockable` object passed to the constructor.

Throws:

  Nothing.

#### Class template `unique_lock`

    #include <boost/thread/locks.hpp>

    template<typename Lockable>
    class unique_lock
    {
    public:
        unique_lock();
        explicit unique_lock(Lockable& m_);
        unique_lock(Lockable& m_,adopt_lock_t);
        unique_lock(Lockable& m_,defer_lock_t);
        unique_lock(Lockable& m_,try_to_lock_t);
        unique_lock(Lockable& m_,system_time const& target_time);

        ~unique_lock();

        unique_lock(detail::thread_move_t<unique_lock<Lockable> > other);
        unique_lock(detail::thread_move_t<upgrade_lock<Lockable> > other);

        operator detail::thread_move_t<unique_lock<Lockable> >();
        detail::thread_move_t<unique_lock<Lockable> > move();
        unique_lock& operator=(detail::thread_move_t<unique_lock<Lockable> > other);
        unique_lock& operator=(detail::thread_move_t<upgrade_lock<Lockable> > other);

        void swap(unique_lock& other);
        void swap(detail::thread_move_t<unique_lock<Lockable> > other);

        void lock();
        bool try_lock();

        template<typename TimeDuration>
        bool timed_lock(TimeDuration const& relative_time);
        bool timed_lock(::boost::system_time const& absolute_time);

        void unlock();

        bool owns_lock() const;
        operator unspecified-bool-type() const;
        bool operator!() const;

        Lockable* mutex() const;
        Lockable* release();
    };

`boost::unique_lock` is more complex than `boost::lock_guard`: not only does it provide for RAII-style locking, it also allows for deferring acquiring the lock until the `lock()` member function is called explicitly, or trying to acquire the lock in a non-blocking fashion, or with a timeout. Consequently, `unlock()` is only called in the destructor if the lock object has locked the `Lockable` object, or otherwise adopted a lock on the `Lockable` object.

Specializations of `boost::unique_lock` model the `TimedLockable` concept if the supplied `Lockable` type itself models `TimedLockable` concept (e.g. `boost::unique_lock<boost::timed_mutex>`), or the `Lockable` concept otherwise (e.g. `boost::unique_lock<boost::mutex>`).

An instance of `boost::unique_lock` is said to own the lock state of a `Lockable` `m` if `mutex()` returns a pointer to `m` and `owns_lock()` returns `true`. If an object that owns the lock state of a `Lockable` object is destroyed, then the destructor will invoke `mutex()->unlock()`.

The member functions of `boost::unique_lock` are not thread-safe. In particular, `boost::unique_lock` is intended to model the ownership of a `Lockable` object by a particular thread, and the member functions that release ownership of the lock state (including the destructor) must be called by the same thread that acquired ownership of the lock state.

##### `unique_lock()`

Effects:

  Creates a lock object with no associated mutex.

Postcondition:

  `owns_lock()` returns `false`. `mutex()` returns `NULL`.

Throws:

  Nothing.

##### `unique_lock(Lockable & m)`

Effects:

  Stores a reference to `m`. Invokes `m.lock()`.

Postcondition:

  `owns_lock()` returns `true`. `mutex()` returns `&m`.

Throws:

  Any exception thrown by the call to `m.lock()`.

##### `unique_lock(Lockable & m,boost::adopt_lock_t)`

Precondition:

  The current thread owns an exclusive lock on `m`.

Effects:

  Stores a reference to `m`. Takes ownership of the lock state of `m`.

Postcondition:

  `owns_lock()` returns `true`. `mutex()` returns `&m`.

Throws:

  Nothing.

##### `unique_lock(Lockable & m,boost::defer_lock_t)`

Effects:

  Stores a reference to `m`.

Postcondition:

  `owns_lock()` returns `false`. `mutex()` returns `&m`.

Throws:

  Nothing.

##### `unique_lock(Lockable & m,boost::try_to_lock_t)`

Effects:

  Stores a reference to `m`. Invokes `m.try_lock()`, and takes ownership of the lock state if the call returns `true`.

Postcondition:

  `mutex()` returns `&m`. If the call to `try_lock()` returned `true`, then `owns_lock()` returns `true`, otherwise `owns_lock()` returns `false`.

Throws:

  Nothing.

##### `unique_lock(Lockable & m,boost::system_time const& abs_time)`

Effects:

  Stores a reference to `m`. Invokes `m.timed_lock(abs_time)`, and takes ownership of the lock state if the call returns `true`.

Postcondition:

  `mutex()` returns `&m`. If the call to `timed_lock()` returned `true`, then `owns_lock()` returns `true`, otherwise `owns_lock()` returns `false`.

Throws:

  Any exceptions thrown by the call to `m.timed_lock(abs_time)`.

##### `~unique_lock()`

Effects:

  Invokes `mutex()``->` `unlock()` if `owns_lock()` returns `true`.

Throws:

  Nothing.

##### `bool owns_lock() const`

Returns:

  `true` if the `*this` owns the lock on the `Lockable` object associated with `*this`.

Throws:

  Nothing.

##### `Lockable* mutex() const`

Returns:

  A pointer to the `Lockable` object associated with `*this`, or `NULL` if there is no such object.

Throws:

  Nothing.

##### `operator unspecified-bool-type() const`

Returns:

  If `owns_lock()` would return `true`, a value that evaluates to `true` in boolean contexts, otherwise a value that evaluates to `false` in boolean contexts.

Throws:

  Nothing.

##### `bool operator!() const`

Returns:

  `!` `owns_lock()`.

Throws:

  Nothing.

##### `Lockable* release()`

Effects:

  The association between `*this` and the `Lockable` object is removed, without affecting the lock state of the `Lockable` object. If `owns_lock()` would have returned `true`, it is the responsibility of the calling code to ensure that the `Lockable` is correctly unlocked.

Returns:

  A pointer to the `Lockable` object associated with `*this` at the point of the call, or `NULL` if there is no such object.

Throws:

  Nothing.

Postcondition:

  `*this` is no longer associated with any `Lockable` object. `mutex()` returns `NULL` and `owns_lock()` returns `false`.

#### Class template `shared_lock`

    #include <boost/thread/locks.hpp>

    template<typename Lockable>
    class shared_lock
    {
    public:
        shared_lock();
        explicit shared_lock(Lockable& m_);
        shared_lock(Lockable& m_,adopt_lock_t);
        shared_lock(Lockable& m_,defer_lock_t);
        shared_lock(Lockable& m_,try_to_lock_t);
        shared_lock(Lockable& m_,system_time const& target_time);
        shared_lock(detail::thread_move_t<shared_lock<Lockable> > other);
        shared_lock(detail::thread_move_t<unique_lock<Lockable> > other);
        shared_lock(detail::thread_move_t<upgrade_lock<Lockable> > other);

        ~shared_lock();

        operator detail::thread_move_t<shared_lock<Lockable> >();
        detail::thread_move_t<shared_lock<Lockable> > move();

        shared_lock& operator=(detail::thread_move_t<shared_lock<Lockable> > other);
        shared_lock& operator=(detail::thread_move_t<unique_lock<Lockable> > other);
        shared_lock& operator=(detail::thread_move_t<upgrade_lock<Lockable> > other);
        void swap(shared_lock& other);

        void lock();
        bool try_lock();
        bool timed_lock(boost::system_time const& target_time);
        void unlock();

        operator unspecified-bool-type() const;
        bool operator!() const;
        bool owns_lock() const;
    };

Like `boost::unique_lock`, `boost::shared_lock` models the `Lockable` concept, but rather than acquiring unique ownership of the supplied `Lockable` object, locking an instance of `boost::shared_lock` acquires shared ownership.

Like `boost::unique_lock`, not only does it provide for RAII-style locking, it also allows for deferring acquiring the lock until the `lock()` member function is called explicitly, or trying to acquire the lock in a non-blocking fashion, or with a timeout. Consequently, `unlock()` is only called in the destructor if the lock object has locked the `Lockable` object, or otherwise adopted a lock on the `Lockable` object.

An instance of `boost::shared_lock` is said to own the lock state of a `Lockable` `m` if `mutex()` returns a pointer to `m` and `owns_lock()` returns `true`. If an object that owns the lock state of a `Lockable` object is destroyed, then the destructor will invoke `mutex()->unlock_shared()`.

The member functions of `boost::shared_lock` are not thread-safe. In particular, `boost::shared_lock` is intended to model the shared ownership of a `Lockable` object by a particular thread, and the member functions that release ownership of the lock state (including the destructor) must be called by the same thread that acquired ownership of the lock state.

##### `shared_lock()`

Effects:

  Creates a lock object with no associated mutex.

Postcondition:

  `owns_lock()` returns `false`. `mutex()` returns `NULL`.

Throws:

  Nothing.

##### `shared_lock(Lockable & m)`

Effects:

  Stores a reference to `m`. Invokes `m.lock_shared()`.

Postcondition:

  `owns_lock()` returns `true`. `mutex()` returns `&m`.

Throws:

  Any exception thrown by the call to `m.lock_shared()`.

##### `shared_lock(Lockable & m,boost::adopt_lock_t)`

Precondition:

  The current thread owns an exclusive lock on `m`.

Effects:

  Stores a reference to `m`. Takes ownership of the lock state of `m`.

Postcondition:

  `owns_lock()` returns `true`. `mutex()` returns `&m`.

Throws:

  Nothing.

##### `shared_lock(Lockable & m,boost::defer_lock_t)`

Effects:

  Stores a reference to `m`.

Postcondition:

  `owns_lock()` returns `false`. `mutex()` returns `&m`.

Throws:

  Nothing.

##### `shared_lock(Lockable & m,boost::try_to_lock_t)`

Effects:

  Stores a reference to `m`. Invokes `m.try_lock_shared()`, and takes ownership of the lock state if the call returns `true`.

Postcondition:

  `mutex()` returns `&m`. If the call to `try_lock_shared()` returned `true`, then `owns_lock()` returns `true`, otherwise `owns_lock()` returns `false`.

Throws:

  Nothing.

##### `shared_lock(Lockable & m,boost::system_time const& abs_time)`

Effects:

  Stores a reference to `m`. Invokes `m.timed_lock(abs_time)`, and takes ownership of the lock state if the call returns `true`.

Postcondition:

  `mutex()` returns `&m`. If the call to `timed_lock_shared()` returned `true`, then `owns_lock()` returns `true`, otherwise `owns_lock()` returns `false`.

Throws:

  Any exceptions thrown by the call to `m.timed_lock(abs_time)`.

##### `~shared_lock()`

Effects:

  Invokes `mutex()``->` `unlock_shared()` if `owns_lock()` returns `true`.

Throws:

  Nothing.

##### `bool owns_lock() const`

Returns:

  `true` if the `*this` owns the lock on the `Lockable` object associated with `*this`.

Throws:

  Nothing.

##### `Lockable* mutex() const`

Returns:

  A pointer to the `Lockable` object associated with `*this`, or `NULL` if there is no such object.

Throws:

  Nothing.

##### `operator unspecified-bool-type() const`

Returns:

  If `owns_lock()` would return `true`, a value that evaluates to `true` in boolean contexts, otherwise a value that evaluates to `false` in boolean contexts.

Throws:

  Nothing.

##### `bool operator!() const`

Returns:

  `!` `owns_lock()`.

Throws:

  Nothing.

##### `Lockable* release()`

Effects:

  The association between `*this` and the `Lockable` object is removed, without affecting the lock state of the `Lockable` object. If `owns_lock()` would have returned `true`, it is the responsibility of the calling code to ensure that the `Lockable` is correctly unlocked.

Returns:

  A pointer to the `Lockable` object associated with `*this` at the point of the call, or `NULL` if there is no such object.

Throws:

  Nothing.

Postcondition:

  `*this` is no longer associated with any `Lockable` object. `mutex()` returns `NULL` and `owns_lock()` returns `false`.

#### Class template `upgrade_lock`

    #include <boost/thread/locks.hpp>

    template<typename Lockable>
    class upgrade_lock
    {
    public:
        explicit upgrade_lock(Lockable& m_);

        upgrade_lock(detail::thread_move_t<upgrade_lock<Lockable> > other);
        upgrade_lock(detail::thread_move_t<unique_lock<Lockable> > other);

        ~upgrade_lock();

        operator detail::thread_move_t<upgrade_lock<Lockable> >();
        detail::thread_move_t<upgrade_lock<Lockable> > move();

        upgrade_lock& operator=(detail::thread_move_t<upgrade_lock<Lockable> > other);
        upgrade_lock& operator=(detail::thread_move_t<unique_lock<Lockable> > other);

        void swap(upgrade_lock& other);

        void lock();
        void unlock();

        operator unspecified-bool-type() const;
        bool operator!() const;
        bool owns_lock() const;
    };

Like `boost::unique_lock`, `boost::upgrade_lock` models the `Lockable` concept, but rather than acquiring unique ownership of the supplied `Lockable` object, locking an instance of `boost::upgrade_lock` acquires upgrade ownership.

Like `boost::unique_lock`, not only does it provide for RAII-style locking, it also allows for deferring acquiring the lock until the `lock()` member function is called explicitly, or trying to acquire the lock in a non-blocking fashion, or with a timeout. Consequently, `unlock()` is only called in the destructor if the lock object has locked the `Lockable` object, or otherwise adopted a lock on the `Lockable` object.

An instance of `boost::upgrade_lock` is said to own the lock state of a `Lockable` `m` if `mutex()` returns a pointer to `m` and `owns_lock()` returns `true`. If an object that owns the lock state of a `Lockable` object is destroyed, then the destructor will invoke `mutex()->unlock_upgrade()`.

The member functions of `boost::upgrade_lock` are not thread-safe. In particular, `boost::upgrade_lock` is intended to model the upgrade ownership of a `Lockable` object by a particular thread, and the member functions that release ownership of the lock state (including the destructor) must be called by the same thread that acquired ownership of the lock state.

#### Class template `upgrade_to_unique_lock`

    #include <boost/thread/locks.hpp>

    template <class Lockable>
    class upgrade_to_unique_lock
    {
    public:
        explicit upgrade_to_unique_lock(upgrade_lock<Lockable>& m_);

        ~upgrade_to_unique_lock();

        upgrade_to_unique_lock(detail::thread_move_t<upgrade_to_unique_lock<Lockable> > other);
        upgrade_to_unique_lock& operator=(detail::thread_move_t<upgrade_to_unique_lock<Lockable> > other);
        void swap(upgrade_to_unique_lock& other);

        operator unspecified-bool-type() const;
        bool operator!() const;
        bool owns_lock() const;
    };

`boost::upgrade_to_unique_lock` allows for a temporary upgrade of an `boost::upgrade_lock` to exclusive ownership. When constructed with a reference to an instance of `boost::upgrade_lock`, if that instance has upgrade ownership on some `Lockable` object, that ownership is upgraded to exclusive ownership. When the `boost::upgrade_to_unique_lock` instance is destroyed, the ownership of the `Lockable` is downgraded back to upgrade ownership.

#### Mutex-specific class `scoped_try_lock`

    class MutexType::scoped_try_lock
    {
    private:
        MutexType::scoped_try_lock(MutexType::scoped_try_lock<MutexType>& other);
        MutexType::scoped_try_lock& operator=(MutexType::scoped_try_lock<MutexType>& other);
    public:
        MutexType::scoped_try_lock();
        explicit MutexType::scoped_try_lock(MutexType& m);
        MutexType::scoped_try_lock(MutexType& m_,adopt_lock_t);
        MutexType::scoped_try_lock(MutexType& m_,defer_lock_t);
        MutexType::scoped_try_lock(MutexType& m_,try_to_lock_t);

        MutexType::scoped_try_lock(MutexType::scoped_try_lock<MutexType>&& other);
        MutexType::scoped_try_lock& operator=(MutexType::scoped_try_lock<MutexType>&& other);

        void swap(MutexType::scoped_try_lock&& other);

        void lock();
        bool try_lock();
        void unlock();
        bool owns_lock() const;

        MutexType* mutex() const;
        MutexType* release();
        bool operator!() const;

        typedef unspecified-bool-type bool_type;
        operator bool_type() const;
    };

The member typedef `scoped_try_lock` is provided for each distinct `MutexType` as a typedef to a class with the preceding definition. The semantics of each constructor and member function are identical to those of `boost::unique_lock<MutexType>` for the same `MutexType`, except that the constructor that takes a single reference to a mutex will call `m.try_lock()` rather than `m.lock()`.

### Lock functions

#### Non-member function `lock(Lockable1,Lockable2,...)`

    template<typename Lockable1,typename Lockable2>
    void lock(Lockable1& l1,Lockable2& l2);

    template<typename Lockable1,typename Lockable2,typename Lockable3>
    void lock(Lockable1& l1,Lockable2& l2,Lockable3& l3);

    template<typename Lockable1,typename Lockable2,typename Lockable3,typename Lockable4>
    void lock(Lockable1& l1,Lockable2& l2,Lockable3& l3,Lockable4& l4);

    template<typename Lockable1,typename Lockable2,typename Lockable3,typename Lockable4,typename Lockable5>
    void lock(Lockable1& l1,Lockable2& l2,Lockable3& l3,Lockable4& l4,Lockable5& l5);

Effects:

  Locks the `Lockable` objects supplied as arguments in an unspecified and indeterminate order in a way that avoids deadlock. It is safe to call this function concurrently from multiple threads with the same mutexes (or other lockable objects) in different orders without risk of deadlock. If any of the `lock()` or `try_lock()` operations on the supplied `Lockable` objects throws an exception any locks acquired by the function will be released before the function exits.

Throws:

  Any exceptions thrown by calling `lock()` or `try_lock()` on the supplied `Lockable` objects.

Postcondition:

  All the supplied `Lockable` objects are locked by the calling thread.

#### Non-member function `lock(begin,end)`

    template<typename ForwardIterator>
    void lock(ForwardIterator begin,ForwardIterator end);

Preconditions:

  The `value_type` of `ForwardIterator` must implement the `Lockable` concept

Effects:

  Locks all the `Lockable` objects in the supplied range in an unspecified and indeterminate order in a way that avoids deadlock. It is safe to call this function concurrently from multiple threads with the same mutexes (or other lockable objects) in different orders without risk of deadlock. If any of the `lock()` or `try_lock()` operations on the `Lockable` objects in the supplied range throws an exception any locks acquired by the function will be released before the function exits.

Throws:

  Any exceptions thrown by calling `lock()` or `try_lock()` on the supplied `Lockable` objects.

Postcondition:

  All the `Lockable` objects in the supplied range are locked by the calling thread.

#### Non-member function `try_lock(Lockable1,Lockable2,...)`

    template<typename Lockable1,typename Lockable2>
    int try_lock(Lockable1& l1,Lockable2& l2);

    template<typename Lockable1,typename Lockable2,typename Lockable3>
    int try_lock(Lockable1& l1,Lockable2& l2,Lockable3& l3);

    template<typename Lockable1,typename Lockable2,typename Lockable3,typename Lockable4>
    int try_lock(Lockable1& l1,Lockable2& l2,Lockable3& l3,Lockable4& l4);

    template<typename Lockable1,typename Lockable2,typename Lockable3,typename Lockable4,typename Lockable5>
    int try_lock(Lockable1& l1,Lockable2& l2,Lockable3& l3,Lockable4& l4,Lockable5& l5);

Effects:

  Calls `try_lock()` on each of the `Lockable` objects supplied as arguments. If any of the calls to `try_lock()` returns `false` then all locks acquired are released and the zero-based index of the failed lock is returned.

  If any of the `try_lock()` operations on the supplied `Lockable` objects throws an exception any locks acquired by the function will be released before the function exits.

Returns:

  `-1` if all the supplied `Lockable` objects are now locked by the calling thread, the zero-based index of the object which could not be locked otherwise.

Throws:

  Any exceptions thrown by calling `try_lock()` on the supplied `Lockable` objects.

Postcondition:

  If the function returns `-1`, all the supplied `Lockable` objects are locked by the calling thread. Otherwise any locks acquired by this function will have been released.

#### Non-member function `try_lock(begin,end)`

    template<typename ForwardIterator>
    ForwardIterator try_lock(ForwardIterator begin,ForwardIterator end);

Preconditions:

  The `value_type` of `ForwardIterator` must implement the `Lockable` concept

Effects:

  Calls `try_lock()` on each of the `Lockable` objects in the supplied range. If any of the calls to `try_lock()` returns `false` then all locks acquired are released and an iterator referencing the failed lock is returned.

  If any of the `try_lock()` operations on the supplied `Lockable` objects throws an exception any locks acquired by the function will be released before the function exits.

Returns:

  `end` if all the supplied `Lockable` objects are now locked by the calling thread, an iterator referencing the object which could not be locked otherwise.

Throws:

  Any exceptions thrown by calling `try_lock()` on the supplied `Lockable` objects.

Postcondition:

  If the function returns `end` then all the `Lockable` objects in the supplied range are locked by the calling thread, otherwise all locks acquired by the function have been released.

### Mutex Types

#### Class `mutex`

    #include <boost/thread/mutex.hpp>

    class mutex:
        boost::noncopyable
    {
    public:
        mutex();
        ~mutex();

        void lock();
        bool try_lock();
        void unlock();

        typedef platform-specific-type native_handle_type;
        native_handle_type native_handle();

        typedef unique_lock<mutex> scoped_lock;
        typedef unspecified-type scoped_try_lock;
    };

`boost::mutex` implements the `Lockable` concept to provide an exclusive-ownership mutex. At most one thread can own the lock on a given instance of `boost::mutex` at any time. Multiple concurrent calls to `lock()`, `try_lock()` and `unlock()` shall be permitted.

##### Member function `native_handle()`

    typedef platform-specific-type native_handle_type;
    native_handle_type native_handle();

Effects:

  Returns an instance of `native_handle_type` that can be used with platform-specific APIs to manipulate the underlying implementation. If no such instance exists, `native_handle()` and `native_handle_type` are not present.

Throws:

  Nothing.

#### Typedef `try_mutex`

    #include <boost/thread/mutex.hpp>

    typedef mutex try_mutex;

`boost::try_mutex` is a `typedef` to `boost::mutex`, provided for backwards compatibility with previous releases of boost.

#### Class `timed_mutex`

    #include <boost/thread/mutex.hpp>

    class timed_mutex:
        boost::noncopyable
    {
    public:
        timed_mutex();
        ~timed_mutex();

        void lock();
        void unlock();
        bool try_lock();
        bool timed_lock(system_time const & abs_time);

        template<typename TimeDuration>
        bool timed_lock(TimeDuration const & relative_time);

        typedef platform-specific-type native_handle_type;
        native_handle_type native_handle();

        typedef unique_lock<timed_mutex> scoped_timed_lock;
        typedef unspecified-type scoped_try_lock;
        typedef scoped_timed_lock scoped_lock;
    };

`boost::timed_mutex` implements the `TimedLockable` concept to provide an exclusive-ownership mutex. At most one thread can own the lock on a given instance of `boost::timed_mutex` at any time. Multiple concurrent calls to `lock()`, `try_lock()`, `timed_lock()`, `timed_lock()` and `unlock()` shall be permitted.

##### Member function `native_handle()`

    typedef platform-specific-type native_handle_type;
    native_handle_type native_handle();

Effects:

  Returns an instance of `native_handle_type` that can be used with platform-specific APIs to manipulate the underlying implementation. If no such instance exists, `native_handle()` and `native_handle_type` are not present.

Throws:

  Nothing.

#### Class `recursive_mutex`

    #include <boost/thread/recursive_mutex.hpp>

    class recursive_mutex:
        boost::noncopyable
    {
    public:
        recursive_mutex();
        ~recursive_mutex();

        void lock();
        bool try_lock();
        void unlock();

        typedef platform-specific-type native_handle_type;
        native_handle_type native_handle();

        typedef unique_lock<recursive_mutex> scoped_lock;
        typedef unspecified-type scoped_try_lock;
    };

`boost::recursive_mutex` implements the `Lockable` concept to provide an exclusive-ownership recursive mutex. At most one thread can own the lock on a given instance of `boost::recursive_mutex` at any time. Multiple concurrent calls to `lock()`, `try_lock()` and `unlock()` shall be permitted. A thread that already has exclusive ownership of a given `boost::recursive_mutex` instance can call `lock()` or `try_lock()` to acquire an additional level of ownership of the mutex. `unlock()` must be called once for each level of ownership acquired by a single thread before ownership can be acquired by another thread.

##### Member function `native_handle()`

    typedef platform-specific-type native_handle_type;
    native_handle_type native_handle();

Effects:

  Returns an instance of `native_handle_type` that can be used with platform-specific APIs to manipulate the underlying implementation. If no such instance exists, `native_handle()` and `native_handle_type` are not present.

Throws:

  Nothing.

#### Typedef `recursive_try_mutex`

    #include <boost/thread/recursive_mutex.hpp>

    typedef recursive_mutex recursive_try_mutex;

`boost::recursive_try_mutex` is a `typedef` to `boost::recursive_mutex`, provided for backwards compatibility with previous releases of boost.

#### Class `recursive_timed_mutex`

    #include <boost/thread/recursive_mutex.hpp>

    class recursive_timed_mutex:
        boost::noncopyable
    {
    public:
        recursive_timed_mutex();
        ~recursive_timed_mutex();

        void lock();
        bool try_lock();
        void unlock();

        bool timed_lock(system_time const & abs_time);

        template<typename TimeDuration>
        bool timed_lock(TimeDuration const & relative_time);

        typedef platform-specific-type native_handle_type;
        native_handle_type native_handle();

        typedef unique_lock<recursive_timed_mutex> scoped_lock;
        typedef unspecified-type scoped_try_lock;
        typedef scoped_lock scoped_timed_lock;
    };

`boost::recursive_timed_mutex` implements the `TimedLockable` concept to provide an exclusive-ownership recursive mutex. At most one thread can own the lock on a given instance of `boost::recursive_timed_mutex` at any time. Multiple concurrent calls to `lock()`, `try_lock()`, `timed_lock()`, `timed_lock()` and `unlock()` shall be permitted. A thread that already has exclusive ownership of a given `boost::recursive_timed_mutex` instance can call `lock()`, `timed_lock()`, `timed_lock()` or `try_lock()` to acquire an additional level of ownership of the mutex. `unlock()` must be called once for each level of ownership acquired by a single thread before ownership can be acquired by another thread.

##### Member function `native_handle()`

    typedef platform-specific-type native_handle_type;
    native_handle_type native_handle();

Effects:

  Returns an instance of `native_handle_type` that can be used with platform-specific APIs to manipulate the underlying implementation. If no such instance exists, `native_handle()` and `native_handle_type` are not present.

Throws:

  Nothing.

#### Class `shared_mutex`

    #include <boost/thread/shared_mutex.hpp>

    class shared_mutex
    {
    public:
        shared_mutex();
        ~shared_mutex();

        void lock_shared();
        bool try_lock_shared();
        bool timed_lock_shared(system_time const& timeout);
        void unlock_shared();

        void lock();
        bool try_lock();
        bool timed_lock(system_time const& timeout);
        void unlock();

        void lock_upgrade();
        void unlock_upgrade();

        void unlock_upgrade_and_lock();
        void unlock_and_lock_upgrade();
        void unlock_and_lock_shared();
        void unlock_upgrade_and_lock_shared();
    };

The class `boost::shared_mutex` provides an implementation of a multiple-reader / single-writer mutex. It implements the `UpgradeLockable` concept.

Multiple concurrent calls to `lock()`, `try_lock()`, `timed_lock()`, `lock_shared()`, `try_lock_shared()` and `timed_lock_shared()` shall be permitted.

### Condition Variables

##### Synopsis

The classes `condition_variable` and `condition_variable_any` provide a mechanism for one thread to wait for notification from another thread that a particular condition has become true. The general usage pattern is that one thread locks a mutex and then calls `wait` on an instance of `condition_variable` or `condition_variable_any`. When the thread is woken from the wait, then it checks to see if the appropriate condition is now true, and continues if so. If the condition is not true, then the thread then calls `wait` again to resume waiting. In the simplest case, this condition is just a boolean variable:

    boost::condition_variable cond;
    boost::mutex mut;
    bool data_ready;

    void process_data();

    void wait_for_data_to_process()
    {
        boost::unique_lock<boost::mutex> lock(mut);
        while(!data_ready)
        {
            cond.wait(lock);
        }
        process_data();
    }

Notice that the `lock` is passed to `wait`: `wait` will atomically add the thread to the set of threads waiting on the condition variable, and unlock the mutex. When the thread is woken, the mutex will be locked again before the call to `wait` returns. This allows other threads to acquire the mutex in order to update the shared data, and ensures that the data associated with the condition is correctly synchronized.

In the mean time, another thread sets the condition to `true`, and then calls either `notify_one` or `notify_all` on the condition variable to wake one waiting thread or all the waiting threads respectively.

    void retrieve_data();
    void prepare_data();

    void prepare_data_for_processing()
    {
        retrieve_data();
        prepare_data();
        {
            boost::lock_guard<boost::mutex> lock(mut);
            data_ready=true;
        }
        cond.notify_one();
    }

Note that the same mutex is locked before the shared data is updated, but that the mutex does not have to be locked across the call to `notify_one`.

This example uses an object of type `condition_variable`, but would work just as well with an object of type `condition_variable_any`: `condition_variable_any` is more general, and will work with any kind of lock or mutex, whereas `condition_variable` requires that the lock passed to `wait` is an instance of `boost::unique_lock<boost::mutex>`. This enables `condition_variable` to make optimizations in some cases, based on the knowledge of the mutex type; `condition_variable_any` typically has a more complex implementation than `condition_variable`.

#### Class `condition_variable`

    #include <boost/thread/condition_variable.hpp>

    namespace boost
    {
        class condition_variable
        {
        public:
            condition_variable();
            ~condition_variable();

            void notify_one();
            void notify_all();

            void wait(boost::unique_lock<boost::mutex>& lock);

            template<typename predicate_type>
            void wait(boost::unique_lock<boost::mutex>& lock,predicate_type predicate);

            bool timed_wait(boost::unique_lock<boost::mutex>& lock,boost::system_time const& abs_time);

            template<typename duration_type>
            bool timed_wait(boost::unique_lock<boost::mutex>& lock,duration_type const& rel_time);

            template<typename predicate_type>
            bool timed_wait(boost::unique_lock<boost::mutex>& lock,boost::system_time const& abs_time,predicate_type predicate);

            template<typename duration_type,typename predicate_type>
            bool timed_wait(boost::unique_lock<boost::mutex>& lock,duration_type const& rel_time,predicate_type predicate);

        // backwards compatibility

            bool timed_wait(boost::unique_lock<boost::mutex>& lock,boost::xtime const& abs_time);

            template<typename predicate_type>
            bool timed_wait(boost::unique_lock<boost::mutex>& lock,boost::xtime const& abs_time,predicate_type predicate);
        };
    }

##### `condition_variable()`

Effects:

  Constructs an object of class `condition_variable`.

Throws:

  `boost::thread_resource_error` if an error occurs.

##### `~condition_variable()`

Precondition:

  All threads waiting on `*this` have been notified by a call to `notify_one` or `notify_all` (though the respective calls to `wait` or `timed_wait` need not have returned).

Effects:

  Destroys the object.

Throws:

  Nothing.

##### `void notify_one()`

Effects:

  If any threads are currently blocked waiting on `*this` in a call to `wait` or `timed_wait`, unblocks one of those threads.

Throws:

  Nothing.

##### `void notify_all()`

Effects:

  If any threads are currently blocked waiting on `*this` in a call to `wait` or `timed_wait`, unblocks all of those threads.

Throws:

  Nothing.

##### `void wait(boost::unique_lock<boost::mutex>& lock)`

Precondition:

  `lock` is locked by the current thread, and either no other thread is currently waiting on `*this`, or the execution of the `mutex()` member function on the `lock` objects supplied in the calls to `wait` or `timed_wait` in all the threads currently waiting on `*this` would return the same value as `lock->mutex()` for this call to `wait`.

Effects:

  Atomically call `lock.unlock()` and blocks the current thread. The thread will unblock when notified by a call to `this->notify_one()` or `this->notify_all()`, or spuriously. When the thread is unblocked (for whatever reason), the lock is reacquired by invoking `lock.lock()` before the call to `wait` returns. The lock is also reacquired by invoking `lock.lock()` if the function exits with an exception.

Postcondition:

  `lock` is locked by the current thread.

Throws:

  `boost::thread_resource_error` if an error occurs. `boost::thread_interrupted` if the wait was interrupted by a call to `interrupt()` on the `boost::thread` object associated with the current thread of execution.

##### `template<typename predicate_type> void wait(boost::unique_lock<boost::mutex>& lock, predicate_type pred)`

Effects:

  As-if

      while(!pred())
      {
          wait(lock);
      }

##### `bool timed_wait(boost::unique_lock<boost::mutex>& lock,boost::system_time const& abs_time)`

Precondition:

  `lock` is locked by the current thread, and either no other thread is currently waiting on `*this`, or the execution of the `mutex()` member function on the `lock` objects supplied in the calls to `wait` or `timed_wait` in all the threads currently waiting on `*this` would return the same value as `lock->mutex()` for this call to `wait`.

Effects:

  Atomically call `lock.unlock()` and blocks the current thread. The thread will unblock when notified by a call to `this->notify_one()` or `this->notify_all()`, when the time as reported by `boost::get_system_time()` would be equal to or later than the specified `abs_time`, or spuriously. When the thread is unblocked (for whatever reason), the lock is reacquired by invoking `lock.lock()` before the call to `wait` returns. The lock is also reacquired by invoking `lock.lock()` if the function exits with an exception.

Returns:

  `false` if the call is returning because the time specified by `abs_time` was reached, `true` otherwise.

Postcondition:

  `lock` is locked by the current thread.

Throws:

  `boost::thread_resource_error` if an error occurs. `boost::thread_interrupted` if the wait was interrupted by a call to `interrupt()` on the `boost::thread` object associated with the current thread of execution.

##### `template<typename duration_type> bool timed_wait(boost::unique_lock<boost::mutex>& lock,duration_type const& rel_time)`

Precondition:

  `lock` is locked by the current thread, and either no other thread is currently waiting on `*this`, or the execution of the `mutex()` member function on the `lock` objects supplied in the calls to `wait` or `timed_wait` in all the threads currently waiting on `*this` would return the same value as `lock->mutex()` for this call to `wait`.

Effects:

  Atomically call `lock.unlock()` and blocks the current thread. The thread will unblock when notified by a call to `this->notify_one()` or `this->notify_all()`, after the period of time indicated by the `rel_time` argument has elapsed, or spuriously. When the thread is unblocked (for whatever reason), the lock is reacquired by invoking `lock.lock()` before the call to `wait` returns. The lock is also reacquired by invoking `lock.lock()` if the function exits with an exception.

Returns:

  `false` if the call is returning because the time period specified by `rel_time` has elapsed, `true` otherwise.

Postcondition:

  `lock` is locked by the current thread.

Throws:

  `boost::thread_resource_error` if an error occurs. `boost::thread_interrupted` if the wait was interrupted by a call to `interrupt()` on the `boost::thread` object associated with the current thread of execution.

| Note
The duration overload of timed_wait is difficult to use correctly. The overload taking a predicate should be preferred in most cases.

##### `template<typename predicate_type> bool timed_wait(boost::unique_lock<boost::mutex>& lock, boost::system_time const& abs_time, predicate_type pred)`

Effects:

  As-if

      while(!pred())
      {
          if(!timed_wait(lock,abs_time))
          {
              return pred();
          }
      }
      return true;

#### Class `condition_variable_any`

    #include <boost/thread/condition_variable.hpp>

    namespace boost
    {
        class condition_variable_any
        {
        public:
            condition_variable_any();
            ~condition_variable_any();

            void notify_one();
            void notify_all();

            template<typename lock_type>
            void wait(lock_type& lock);

            template<typename lock_type,typename predicate_type>
            void wait(lock_type& lock,predicate_type predicate);

            template<typename lock_type>
            bool timed_wait(lock_type& lock,boost::system_time const& abs_time);

            template<typename lock_type,typename duration_type>
            bool timed_wait(lock_type& lock,duration_type const& rel_time);

            template<typename lock_type,typename predicate_type>
            bool timed_wait(lock_type& lock,boost::system_time const& abs_time,predicate_type predicate);

            template<typename lock_type,typename duration_type,typename predicate_type>
            bool timed_wait(lock_type& lock,duration_type const& rel_time,predicate_type predicate);

        // backwards compatibility

            template<typename lock_type>
            bool timed_wait(lock_type>& lock,boost::xtime const& abs_time);

            template<typename lock_type,typename predicate_type>
            bool timed_wait(lock_type& lock,boost::xtime const& abs_time,predicate_type predicate);
        };
    }

##### `condition_variable_any()`

Effects:

  Constructs an object of class `condition_variable_any`.

Throws:

  `boost::thread_resource_error` if an error occurs.

##### `~condition_variable_any()`

Precondition:

  All threads waiting on `*this` have been notified by a call to `notify_one` or `notify_all` (though the respective calls to `wait` or `timed_wait` need not have returned).

Effects:

  Destroys the object.

Throws:

  Nothing.

##### `void notify_one()`

Effects:

  If any threads are currently blocked waiting on `*this` in a call to `wait` or `timed_wait`, unblocks one of those threads.

Throws:

  Nothing.

##### `void notify_all()`

Effects:

  If any threads are currently blocked waiting on `*this` in a call to `wait` or `timed_wait`, unblocks all of those threads.

Throws:

  Nothing.

##### `template<typename lock_type> void wait(lock_type& lock)`

Effects:

  Atomically call `lock.unlock()` and blocks the current thread. The thread will unblock when notified by a call to `this->notify_one()` or `this->notify_all()`, or spuriously. When the thread is unblocked (for whatever reason), the lock is reacquired by invoking `lock.lock()` before the call to `wait` returns. The lock is also reacquired by invoking `lock.lock()` if the function exits with an exception.

Postcondition:

  `lock` is locked by the current thread.

Throws:

  `boost::thread_resource_error` if an error occurs. `boost::thread_interrupted` if the wait was interrupted by a call to `interrupt()` on the `boost::thread` object associated with the current thread of execution.

##### `template<typename lock_type,typename predicate_type> void wait(lock_type& lock, predicate_type pred)`

Effects:

  As-if

      while(!pred())
      {
          wait(lock);
      }

##### `template<typename lock_type> bool timed_wait(lock_type& lock,boost::system_time const& abs_time)`

Effects:

  Atomically call `lock.unlock()` and blocks the current thread. The thread will unblock when notified by a call to `this->notify_one()` or `this->notify_all()`, when the time as reported by `boost::get_system_time()` would be equal to or later than the specified `abs_time`, or spuriously. When the thread is unblocked (for whatever reason), the lock is reacquired by invoking `lock.lock()` before the call to `wait` returns. The lock is also reacquired by invoking `lock.lock()` if the function exits with an exception.

Returns:

  `false` if the call is returning because the time specified by `abs_time` was reached, `true` otherwise.

Postcondition:

  `lock` is locked by the current thread.

Throws:

  `boost::thread_resource_error` if an error occurs. `boost::thread_interrupted` if the wait was interrupted by a call to `interrupt()` on the `boost::thread` object associated with the current thread of execution.

##### `template<typename lock_type,typename duration_type> bool timed_wait(lock_type& lock,duration_type const& rel_time)`

Effects:

  Atomically call `lock.unlock()` and blocks the current thread. The thread will unblock when notified by a call to `this->notify_one()` or `this->notify_all()`, after the period of time indicated by the `rel_time` argument has elapsed, or spuriously. When the thread is unblocked (for whatever reason), the lock is reacquired by invoking `lock.lock()` before the call to `wait` returns. The lock is also reacquired by invoking `lock.lock()` if the function exits with an exception.

Returns:

  `false` if the call is returning because the time period specified by `rel_time` has elapsed, `true` otherwise.

Postcondition:

  `lock` is locked by the current thread.

Throws:

  `boost::thread_resource_error` if an error occurs. `boost::thread_interrupted` if the wait was interrupted by a call to `interrupt()` on the `boost::thread` object associated with the current thread of execution.

| Note
The duration overload of timed_wait is difficult to use correctly. The overload taking a predicate should be preferred in most cases.

##### `template<typename lock_type,typename predicate_type> bool timed_wait(lock_type& lock, boost::system_time const& abs_time, predicate_type pred)`

Effects:

  As-if

      while(!pred())
      {
          if(!timed_wait(lock,abs_time))
          {
              return pred();
          }
      }
      return true;

#### Typedef `condition`

    #include <boost/thread/condition.hpp>

    typedef condition_variable_any condition;

The typedef `condition` is provided for backwards compatibility with previous boost releases.

### One-time Initialization

`boost::call_once` provides a mechanism for ensuring that an initialization routine is run exactly once without data races or deadlocks.

#### Typedef `once_flag`

    #include <boost/thread/once.hpp>

    typedef platform-specific-type once_flag;
    #define BOOST_ONCE_INIT platform-specific-initializer

Objects of type `boost::once_flag` shall be initialized with `BOOST_ONCE_INIT`:

    boost::once_flag f=BOOST_ONCE_INIT;

#### Non-member function `call_once`

    #include <boost/thread/once.hpp>

    template<typename Callable>
    void call_once(once_flag& flag,Callable func);

Requires:

  `Callable` is `CopyConstructible`. Copying `func` shall have no side effects, and the effect of calling the copy shall be equivalent to calling the original.

Effects:

  Calls to `call_once` on the same `once_flag` object are serialized. If there has been no prior effective `call_once` on the same `once_flag` object, the argument `func` (or a copy thereof) is called as-if by invoking `func()`, and the invocation of `call_once` is effective if and only if `func()` returns without exception. If an exception is thrown, the exception is propagated to the caller. If there has been a prior effective `call_once` on the same `once_flag` object, the `call_once` returns without invoking `func`.

Synchronization:

  The completion of an effective `call_once` invocation on a `once_flag` object, synchronizes with all subsequent `call_once` invocations on the same `once_flag` object.

Throws:

  `thread_resource_error` when the effects cannot be achieved. or any exception propagated from `func`.

Note:

  The function passed to `call_once` must not also call `call_once` passing the same `once_flag` object. This may cause deadlock, or invoking the passed function a second time. The alternative is to allow the second call to return immediately, but that assumes the code knows it has been called recursively, and can proceed even though the call to `call_once` didn't actually call the function, in which case it could also avoid calling `call_once` recursively.

    void call_once(void (*func)(),once_flag& flag);

This second overload is provided for backwards compatibility. The effects of `call_once(func,flag)` shall be the same as those of `call_once(flag,func)`.

### Barriers

A barrier is a simple concept. Also known as a rendezvous, it is a synchronization point between multiple threads. The barrier is configured for a particular number of threads (`n`), and as threads reach the barrier they must wait until all `n` threads have arrived. Once the `n`-th thread has reached the barrier, all the waiting threads can proceed, and the barrier is reset.

#### Class `barrier`

    #include <boost/thread/barrier.hpp>

    class barrier
    {
    public:
        barrier(unsigned int count);
        ~barrier();

        bool wait();
    };

Instances of `boost::barrier` are not copyable or movable.

###### Constructor

    barrier(unsigned int count);

Effects:

  Construct a barrier for `count` threads.

Throws:

  `boost::thread_resource_error` if an error occurs.

###### Destructor

    ~barrier();

Precondition:

  No threads are waiting on `*this`.

Effects:

  Destroys `*this`.

Throws:

  Nothing.

###### Member function `wait`

    bool wait();

Effects:

  Block until `count` threads have called `wait` on `*this`. When the `count`-th thread calls `wait`, all waiting threads are unblocked, and the barrier is reset.

Returns:

  `true` for exactly one thread from each batch of waiting threads, `false` otherwise.

Throws:

  `boost::thread_resource_error` if an error occurs.

### Futures

#### Overview

The futures library provides a means of handling synchronous future values, whether those values are generated by another thread, or on a single thread in response to external stimuli, or on-demand.

This is done through the provision of four class templates: `boost::unique_future` and `boost::shared_future` which are used to retrieve the asynchronous results, and `boost::promise` and `boost::packaged_task` which are used to generate the asynchronous results.

An instance of `boost::unique_future` holds the one and only reference to a result. Ownership can be transferred between instances using the move constructor or move-assignment operator, but at most one instance holds a reference to a given asynchronous result. When the result is ready, it is returned from `boost::unique_future<R>::get()` by rvalue-reference to allow the result to be moved or copied as appropriate for the type.

On the other hand, many instances of `boost::shared_future` may reference the same result. Instances can be freely copied and assigned, and `boost::shared_future<R>::get()` returns a `const` reference so that multiple calls to `boost::shared_future<R>::get()` are safe. You can move an instance of `boost::unique_future` into an instance of `boost::shared_future`, thus transferring ownership of the associated asynchronous result, but not vice-versa.

You can wait for futures either individually or with one of the `boost::wait_for_any()` and `boost::wait_for_all()` functions.

#### Creating asynchronous values

You can set the value in a future with either a `boost::promise` or a `boost::packaged_task`. A `boost::packaged_task` is a callable object that wraps a function or callable object. When the packaged task is invoked, it invokes the contained function in turn, and populates a future with the return value. This is an answer to the perennial question: "how do I return a value from a thread?": package the function you wish to run as a `boost::packaged_task` and pass the packaged task to the thread constructor. The future retrieved from the packaged task can then be used to obtain the return value. If the function throws an exception, that is stored in the future in place of the return value.

    int calculate_the_answer_to_life_the_universe_and_everything()
    {
        return 42;
    }

    boost::packaged_task<int> pt(calculate_the_answer_to_life_the_universe_and_everything);
    boost::unique_future<int> fi=pt.get_future();

    boost::thread task(boost::move(pt)); // launch task on a thread

    fi.wait(); // wait for it to finish

    assert(fi.is_ready());
    assert(fi.has_value());
    assert(!fi.has_exception());
    assert(fi.get_state()==boost::future_state::ready);
    assert(fi.get()==42);

A `boost::promise` is a bit more low level: it just provides explicit functions to store a value or an exception in the associated future. A promise can therefore be used where the value may come from more than one possible source, or where a single operation may produce multiple values.

    boost::promise<int> pi;
    boost::unique_future<int> fi;
    fi=pi.get_future();

    pi.set_value(42);

    assert(fi.is_ready());
    assert(fi.has_value());
    assert(!fi.has_exception());
    assert(fi.get_state()==boost::future_state::ready);
    assert(fi.get()==42);

#### Wait Callbacks and Lazy Futures

Both `boost::promise` and `boost::packaged_task` support wait callbacks that are invoked when a thread blocks in a call to `wait()` or `timed_wait()` on a future that is waiting for the result from the `boost::promise` or `boost::packaged_task`, in the thread that is doing the waiting. These can be set using the `set_wait_callback()` member function on the `boost::promise` or `boost::packaged_task` in question.

This allows lazy futures where the result is not actually computed until it is needed by some thread. In the example below, the call to `f.get()` invokes the callback `invoke_lazy_task`, which runs the task to set the value. If you remove the call to `f.get()`, the task is not ever run.

    int calculate_the_answer_to_life_the_universe_and_everything()
    {
        return 42;
    }

    void invoke_lazy_task(boost::packaged_task<int>& task)
    {
        try
        {
            task();
        }
        catch(boost::task_already_started&)
        {}
    }

    int main()
    {
        boost::packaged_task<int> task(calculate_the_answer_to_life_the_universe_and_everything);
        task.set_wait_callback(invoke_lazy_task);
        boost::unique_future<int> f(task.get_future());

        assert(f.get()==42);
    }

#### Futures Reference

##### `state` enum

    namespace future_state
    {
        enum state {uninitialized, waiting, ready};
    }

##### `unique_future` class template

    template <typename R>
    class unique_future
    {
        unique_future(unique_future & rhs);// = delete;
        unique_future& operator=(unique_future& rhs);// = delete;

    public:
        typedef future_state::state state;

        unique_future();
        ~unique_future();

        // move support
        unique_future(unique_future && other);
        unique_future& operator=(unique_future && other);

        void swap(unique_future& other);

        // retrieving the value
        R&& get();

        // functions to check state
        state get_state() const;
        bool is_ready() const;
        bool has_exception() const;
        bool has_value() const;

        // waiting for the result to be ready
        void wait() const;
        template<typename Duration>
        bool timed_wait(Duration const& rel_time) const;
        bool timed_wait_until(boost::system_time const& abs_time) const;
    };

###### Default Constructor

    unique_future();

Effects:

  Constructs an uninitialized future.

Postconditions:

  `this->is_ready` returns `false`. `this->get_state()` returns `boost::future_state::uninitialized`.

Throws:

  Nothing.

###### Destructor

    ~unique_future();

Effects:

  Destroys `*this`.

Throws:

  Nothing.

###### Move Constructor

    unique_future(unique_future && other);

Effects:

  Constructs a new future, and transfers ownership of the asynchronous result associated with `other` to `*this`.

Postconditions:

  `this->get_state()` returns the value of `other->get_state()` prior to the call. `other->get_state()` returns `boost::future_state::uninitialized`. If `other` was associated with an asynchronous result, that result is now associated with `*this`. `other` is not associated with any asynchronous result.

Throws:

  Nothing.

Notes:

  If the compiler does not support rvalue-references, this is implemented using the boost.thread move emulation.

###### Move Assignment Operator

    unique_future& operator=(unique_future && other);

Effects:

  Transfers ownership of the asynchronous result associated with `other` to `*this`.

Postconditions:

  `this->get_state()` returns the value of `other->get_state()` prior to the call. `other->get_state()` returns `boost::future_state::uninitialized`. If `other` was associated with an asynchronous result, that result is now associated with `*this`. `other` is not associated with any asynchronous result. If `*this` was associated with an asynchronous result prior to the call, that result no longer has an associated `boost::unique_future` instance.

Throws:

  Nothing.

Notes:

  If the compiler does not support rvalue-references, this is implemented using the boost.thread move emulation.

###### Member function `swap()`

    void swap(unique_future & other);

Effects:

  Swaps ownership of the asynchronous results associated with `other` and `*this`.

Postconditions:

  `this->get_state()` returns the value of `other->get_state()` prior to the call. `other->get_state()` returns the value of `this->get_state()` prior to the call. If `other` was associated with an asynchronous result, that result is now associated with `*this`, otherwise `*this` has no associated result. If `*this` was associated with an asynchronous result, that result is now associated with `other`, otherwise `other` has no associated result.

Throws:

  Nothing.

###### Member function `get()`

    R&& get();
    R& unique_future<R&>::get();
    void unique_future<void>::get();

Effects:

  If `*this` is associated with an asynchronous result, waits until the result is ready as-if by a call to `boost::unique_future<R>::wait()`, and retrieves the result (whether that is a value or an exception).

Returns:

  If the result type `R` is a reference, returns the stored reference. If `R` is `void`, there is no return value. Otherwise, returns an rvalue-reference to the value stored in the asynchronous result.

Postconditions:

  `this->is_ready()` returns `true`. `this->get_state()` returns `boost::future_state::ready`.

Throws:

  `boost::future_uninitialized` if `*this` is not associated with an asynchronous result. `boost::thread_interrupted` if the result associated with `*this` is not ready at the point of the call, and the current thread is interrupted. Any exception stored in the asynchronous result in place of a value.

Notes:

  `get()` is an interruption point.

###### Member function `wait()`

    void wait();

Effects:

  If `*this` is associated with an asynchronous result, waits until the result is ready. If the result is not ready on entry, and the result has a wait callback set, that callback is invoked prior to waiting.

Throws:

  `boost::future_uninitialized` if `*this` is not associated with an asynchronous result. `boost::thread_interrupted` if the result associated with `*this` is not ready at the point of the call, and the current thread is interrupted. Any exception thrown by the wait callback if such a callback is called.

Postconditions:

  `this->is_ready()` returns `true`. `this->get_state()` returns `boost::future_state::ready`.

Notes:

  `wait()` is an interruption point.

###### Member function `timed_wait()`

    template<typename Duration>
    bool timed_wait(Duration const& wait_duration);

Effects:

  If `*this` is associated with an asynchronous result, waits until the result is ready, or the time specified by `wait_duration` has elapsed. If the result is not ready on entry, and the result has a wait callback set, that callback is invoked prior to waiting.

Returns:

  `true` if `*this` is associated with an asynchronous result, and that result is ready before the specified time has elapsed, `false` otherwise.

Throws:

  `boost::future_uninitialized` if `*this` is not associated with an asynchronous result. `boost::thread_interrupted` if the result associated with `*this` is not ready at the point of the call, and the current thread is interrupted. Any exception thrown by the wait callback if such a callback is called.

Postconditions:

  If this call returned `true`, then `this->is_ready()` returns `true` and `this->get_state()` returns `boost::future_state::ready`.

Notes:

  `timed_wait()` is an interruption point. `Duration` must be a type that meets the Boost.DateTime time duration requirements.

###### Member function `timed_wait()`

    bool timed_wait(boost::system_time const& wait_timeout);

Effects:

  If `*this` is associated with an asynchronous result, waits until the result is ready, or the time point specified by `wait_timeout` has passed. If the result is not ready on entry, and the result has a wait callback set, that callback is invoked prior to waiting.

Returns:

  `true` if `*this` is associated with an asynchronous result, and that result is ready before the specified time has passed, `false` otherwise.

Throws:

  `boost::future_uninitialized` if `*this` is not associated with an asynchronous result. `boost::thread_interrupted` if the result associated with `*this` is not ready at the point of the call, and the current thread is interrupted. Any exception thrown by the wait callback if such a callback is called.

Postconditions:

  If this call returned `true`, then `this->is_ready()` returns `true` and `this->get_state()` returns `boost::future_state::ready`.

Notes:

  `timed_wait()` is an interruption point.

###### Member function `is_ready()`

    bool is_ready();

Effects:

  Checks to see if the asynchronous result associated with `*this` is set.

Returns:

  `true` if `*this` is associated with an asynchronous result, and that result is ready for retrieval, `false` otherwise.

Throws:

  Nothing.

###### Member function `has_value()`

    bool has_value();

Effects:

  Checks to see if the asynchronous result associated with `*this` is set with a value rather than an exception.

Returns:

  `true` if `*this` is associated with an asynchronous result, that result is ready for retrieval, and the result is a stored value, `false` otherwise.

Throws:

  Nothing.

###### Member function `has_exception()`

    bool has_exception();

Effects:

  Checks to see if the asynchronous result associated with `*this` is set with an exception rather than a value.

Returns:

  `true` if `*this` is associated with an asynchronous result, that result is ready for retrieval, and the result is a stored exception, `false` otherwise.

Throws:

  Nothing.

###### Member function `get_state()`

    future_state::state get_state();

Effects:

  Determine the state of the asynchronous result associated with `*this`, if any.

Returns:

  `boost::future_state::uninitialized` if `*this` is not associated with an asynchronous result. `boost::future_state::ready` if the asynchronous result associated with `*this` is ready for retrieval, `boost::future_state::waiting` otherwise.

Throws:

  Nothing.

##### `shared_future` class template

    template <typename R>
    class shared_future
    {
    public:
        typedef future_state::state state;

        shared_future();
        ~shared_future();

        // copy support
        shared_future(shared_future const& other);
        shared_future& operator=(shared_future const& other);

        // move support
        shared_future(shared_future && other);
        shared_future(unique_future<R> && other);
        shared_future& operator=(shared_future && other);
        shared_future& operator=(unique_future<R> && other);

        void swap(shared_future& other);

        // retrieving the value
        R get();

        // functions to check state, and wait for ready
        state get_state() const;
        bool is_ready() const;
        bool has_exception() const;
        bool has_value() const;

        // waiting for the result to be ready
        void wait() const;
        template<typename Duration>
        bool timed_wait(Duration const& rel_time) const;
        bool timed_wait_until(boost::system_time const& abs_time) const;
    };

###### Default Constructor

    shared_future();

Effects:

  Constructs an uninitialized future.

Postconditions:

  `this->is_ready` returns `false`. `this->get_state()` returns `boost::future_state::uninitialized`.

Throws:

  Nothing.

###### Member function `get()`

    const R& get();

Effects:

  If `*this` is associated with an asynchronous result, waits until the result is ready as-if by a call to `boost::shared_future<R>::wait()`, and returns a `const` reference to the result.

Returns:

  If the result type `R` is a reference, returns the stored reference. If `R` is `void`, there is no return value. Otherwise, returns a `const` reference to the value stored in the asynchronous result.

Throws:

  `boost::future_uninitialized` if `*this` is not associated with an asynchronous result. `boost::thread_interrupted` if the result associated with `*this` is not ready at the point of the call, and the current thread is interrupted.

Notes:

  `get()` is an interruption point.

###### Member function `wait()`

    void wait();

Effects:

  If `*this` is associated with an asynchronous result, waits until the result is ready. If the result is not ready on entry, and the result has a wait callback set, that callback is invoked prior to waiting.

Throws:

  `boost::future_uninitialized` if `*this` is not associated with an asynchronous result. `boost::thread_interrupted` if the result associated with `*this` is not ready at the point of the call, and the current thread is interrupted. Any exception thrown by the wait callback if such a callback is called.

Postconditions:

  `this->is_ready()` returns `true`. `this->get_state()` returns `boost::future_state::ready`.

Notes:

  `wait()` is an interruption point.

###### Member function `timed_wait()`

    template<typename Duration>
    bool timed_wait(Duration const& wait_duration);

Effects:

  If `*this` is associated with an asynchronous result, waits until the result is ready, or the time specified by `wait_duration` has elapsed. If the result is not ready on entry, and the result has a wait callback set, that callback is invoked prior to waiting.

Returns:

  `true` if `*this` is associated with an asynchronous result, and that result is ready before the specified time has elapsed, `false` otherwise.

Throws:

  `boost::future_uninitialized` if `*this` is not associated with an asynchronous result. `boost::thread_interrupted` if the result associated with `*this` is not ready at the point of the call, and the current thread is interrupted. Any exception thrown by the wait callback if such a callback is called.

Postconditions:

  If this call returned `true`, then `this->is_ready()` returns `true` and `this->get_state()` returns `boost::future_state::ready`.

Notes:

  `timed_wait()` is an interruption point. `Duration` must be a type that meets the Boost.DateTime time duration requirements.

###### Member function `timed_wait()`

    bool timed_wait(boost::system_time const& wait_timeout);

Effects:

  If `*this` is associated with an asynchronous result, waits until the result is ready, or the time point specified by `wait_timeout` has passed. If the result is not ready on entry, and the result has a wait callback set, that callback is invoked prior to waiting.

Returns:

  `true` if `*this` is associated with an asynchronous result, and that result is ready before the specified time has passed, `false` otherwise.

Throws:

  `boost::future_uninitialized` if `*this` is not associated with an asynchronous result. `boost::thread_interrupted` if the result associated with `*this` is not ready at the point of the call, and the current thread is interrupted. Any exception thrown by the wait callback if such a callback is called.

Postconditions:

  If this call returned `true`, then `this->is_ready()` returns `true` and `this->get_state()` returns `boost::future_state::ready`.

Notes:

  `timed_wait()` is an interruption point.

###### Member function `is_ready()`

    bool is_ready();

Effects:

  Checks to see if the asynchronous result associated with `*this` is set.

Returns:

  `true` if `*this` is associated with an asynchronous result, and that result is ready for retrieval, `false` otherwise.

Throws:

  Nothing.

###### Member function `has_value()`

    bool has_value();

Effects:

  Checks to see if the asynchronous result associated with `*this` is set with a value rather than an exception.

Returns:

  `true` if `*this` is associated with an asynchronous result, that result is ready for retrieval, and the result is a stored value, `false` otherwise.

Throws:

  Nothing.

###### Member function `has_exception()`

    bool has_exception();

Effects:

  Checks to see if the asynchronous result associated with `*this` is set with an exception rather than a value.

Returns:

  `true` if `*this` is associated with an asynchronous result, that result is ready for retrieval, and the result is a stored exception, `false` otherwise.

Throws:

  Nothing.

###### Member function `get_state()`

    future_state::state get_state();

Effects:

  Determine the state of the asynchronous result associated with `*this`, if any.

Returns:

  `boost::future_state::uninitialized` if `*this` is not associated with an asynchronous result. `boost::future_state::ready` if the asynchronous result associated with `*this` is ready for retrieval, `boost::future_state::waiting` otherwise.

Throws:

  Nothing.

##### `promise` class template

    template <typename R>
    class promise
    {
        promise(promise & rhs);// = delete;
        promise & operator=(promise & rhs);// = delete;
    public:
        // template <class Allocator> explicit promise(Allocator a);

        promise();
        ~promise();

        // Move support
        promise(promise && rhs);
        promise & operator=(promise&& rhs);

        void swap(promise& other);
        // Result retrieval
        unique_future<R> get_future();

        // Set the value
        void set_value(R& r);
        void set_value(R&& r);
        void set_exception(boost::exception_ptr e);

        template<typename F>
        void set_wait_callback(F f);
    };

###### Default Constructor

    promise();

Effects:

  Constructs a new `boost::promise` with no associated result.

Throws:

  Nothing.

###### Move Constructor

    promise(promise && other);

Effects:

  Constructs a new `boost::promise`, and transfers ownership of the result associated with `other` to `*this`, leaving `other` with no associated result.

Throws:

  Nothing.

Notes:

  If the compiler does not support rvalue-references, this is implemented using the boost.thread move emulation.

###### Move Assignment Operator

    promise& operator=(promise && other);

Effects:

  Transfers ownership of the result associated with `other` to `*this`, leaving `other` with no associated result. If there was already a result associated with `*this`, and that result was not ready, sets any futures associated with that result to ready with a `boost::broken_promise` exception as the result.

Throws:

  Nothing.

Notes:

  If the compiler does not support rvalue-references, this is implemented using the boost.thread move emulation.

###### Destructor

    ~promise();

Effects:

  Destroys `*this`. If there was a result associated with `*this`, and that result is not ready, sets any futures associated with that task to ready with a `boost::broken_promise` exception as the result.

Throws:

  Nothing.

###### Member Function `get_future()`

    unique_future<R> get_future();

Effects:

  If `*this` was not associated with a result, allocate storage for a new asynchronous result and associate it with `*this`. Returns a `boost::unique_future` associated with the result associated with `*this`.

Throws:

  `boost::future_already_retrieved` if the future associated with the task has already been retrieved. `std::bad_alloc` if any memory necessary could not be allocated.

###### Member Function `set_value()`

    void set_value(R&& r);
    void set_value(const R& r);
    void promise<R&>::set_value(R& r);
    void promise<void>::set_value();

Effects:

  If `*this` was not associated with a result, allocate storage for a new asynchronous result and associate it with `*this`. Store the value `r` in the asynchronous result associated with `*this`. Any threads blocked waiting for the asynchronous result are woken.

Postconditions:

  All futures waiting on the asynchronous result are ready and `boost::unique_future<R>::has_value()` or `boost::shared_future<R>::has_value()` for those futures shall return `true`.

Throws:

  `boost::promise_already_satisfied` if the result associated with `*this` is already ready. `std::bad_alloc` if the memory required for storage of the result cannot be allocated. Any exception thrown by the copy or move-constructor of `R`.

###### Member Function `set_exception()`

    void set_exception(boost::exception_ptr e);

Effects:

  If `*this` was not associated with a result, allocate storage for a new asynchronous result and associate it with `*this`. Store the exception `e` in the asynchronous result associated with `*this`. Any threads blocked waiting for the asynchronous result are woken.

Postconditions:

  All futures waiting on the asynchronous result are ready and `boost::unique_future<R>::has_exception()` or `boost::shared_future<R>::has_exception()` for those futures shall return `true`.

Throws:

  `boost::promise_already_satisfied` if the result associated with `*this` is already ready. `std::bad_alloc` if the memory required for storage of the result cannot be allocated.

###### Member Function `set_wait_callback()`

    template<typename F>
    void set_wait_callback(F f);

Preconditions:

  The expression `f(t)` where `t` is a lvalue of type `boost::promise` shall be well-formed. Invoking a copy of `f` shall have the same effect as invoking `f`

Effects:

  Store a copy of `f` with the asynchronous result associated with `*this` as a wait callback. This will replace any existing wait callback store alongside that result. If a thread subsequently calls one of the wait functions on a `boost::unique_future` or `boost::shared_future` associated with this result, and the result is not ready, `f(*this)` shall be invoked.

Throws:

  `std::bad_alloc` if memory cannot be allocated for the required storage.

##### `packaged_task` class template

    template<typename R>
    class packaged_task
    {
        packaged_task(packaged_task&);// = delete;
        packaged_task& operator=(packaged_task&);// = delete;

    public:
        // construction and destruction
        template <class F>
        explicit packaged_task(F const& f);

        explicit packaged_task(R(*f)());

        template <class F>
        explicit packaged_task(F&& f);

        // template <class F, class Allocator>
        // explicit packaged_task(F const& f, Allocator a);
        // template <class F, class Allocator>
        // explicit packaged_task(F&& f, Allocator a);

        ~packaged_task()
        {}

        // move support
        packaged_task(packaged_task&& other);
        packaged_task& operator=(packaged_task&& other);

        void swap(packaged_task& other);
        // result retrieval
        unique_future<R> get_future();

        // execution
        void operator()();

        template<typename F>
        void set_wait_callback(F f);
    };

###### Task Constructor

    template<typename F>
    packaged_task(F const &f);

    packaged_task(R(*f)());

    template<typename F>
    packaged_task(F&&f);

Preconditions:

  `f()` is a valid expression with a return type convertible to `R`. Invoking a copy of `f` shall behave the same as invoking `f`.

Effects:

  Constructs a new `boost::packaged_task` with a copy of `f` stored as the associated task.

Throws:

  Any exceptions thrown by the copy (or move) constructor of `f`. `std::bad_alloc` if memory for the internal data structures could not be allocated.

###### Move Constructor

    packaged_task(packaged_task && other);

Effects:

  Constructs a new `boost::packaged_task`, and transfers ownership of the task associated with `other` to `*this`, leaving `other` with no associated task.

Throws:

  Nothing.

Notes:

  If the compiler does not support rvalue-references, this is implemented using the boost.thread move emulation.

###### Move Assignment Operator

    packaged_task& operator=(packaged_task && other);

Effects:

  Transfers ownership of the task associated with `other` to `*this`, leaving `other` with no associated task. If there was already a task associated with `*this`, and that task has not been invoked, sets any futures associated with that task to ready with a `boost::broken_promise` exception as the result.

Throws:

  Nothing.

Notes:

  If the compiler does not support rvalue-references, this is implemented using the boost.thread move emulation.

###### Destructor

    ~packaged_task();

Effects:

  Destroys `*this`. If there was a task associated with `*this`, and that task has not been invoked, sets any futures associated with that task to ready with a `boost::broken_promise` exception as the result.

Throws:

  Nothing.

###### Member Function `get_future()`

    unique_future<R> get_future();

Effects:

  Returns a `boost::unique_future` associated with the result of the task associated with `*this`.

Throws:

  `boost::task_moved` if ownership of the task associated with `*this` has been moved to another instance of `boost::packaged_task`. `boost::future_already_retrieved` if the future associated with the task has already been retrieved.

###### Member Function `operator()()`

    void operator()();

Effects:

  Invoke the task associated with `*this` and store the result in the corresponding future. If the task returns normally, the return value is stored as the asynchronous result, otherwise the exception thrown is stored. Any threads blocked waiting for the asynchronous result associated with this task are woken.

Postconditions:

  All futures waiting on the asynchronous result are ready

Throws:

  `boost::task_moved` if ownership of the task associated with `*this` has been moved to another instance of `boost::packaged_task`. `boost::task_already_started` if the task has already been invoked.

###### Member Function `set_wait_callback()`

    template<typename F>
    void set_wait_callback(F f);

Preconditions:

  The expression `f(t)` where `t` is a lvalue of type `boost::packaged_task` shall be well-formed. Invoking a copy of `f` shall have the same effect as invoking `f`

Effects:

  Store a copy of `f` with the task associated with `*this` as a wait callback. This will replace any existing wait callback store alongside that task. If a thread subsequently calls one of the wait functions on a `boost::unique_future` or `boost::shared_future` associated with this task, and the result of the task is not ready, `f(*this)` shall be invoked.

Throws:

  `boost::task_moved` if ownership of the task associated with `*this` has been moved to another instance of `boost::packaged_task`.

##### Non-member function `wait_for_any()`

    template<typename Iterator>
    Iterator wait_for_any(Iterator begin,Iterator end);

    template<typename F1,typename F2>
    unsigned wait_for_any(F1& f1,F2& f2);

    template<typename F1,typename F2,typename F3>
    unsigned wait_for_any(F1& f1,F2& f2,F3& f3);

    template<typename F1,typename F2,typename F3,typename F4>
    unsigned wait_for_any(F1& f1,F2& f2,F3& f3,F4& f4);

    template<typename F1,typename F2,typename F3,typename F4,typename F5>
    unsigned wait_for_any(F1& f1,F2& f2,F3& f3,F4& f4,F5& f5);

Preconditions:

  The types `Fn` shall be specializations of `boost::unique_future` or `boost::shared_future`, and `Iterator` shall be a forward iterator with a `value_type` which is a specialization of `boost::unique_future` or `boost::shared_future`.

Effects:

  Waits until at least one of the specified futures is ready.

Returns:

  The range-based overload returns an `Iterator` identifying the first future in the range that was detected as ready. The remaining overloads return the zero-based index of the first future that was detected as ready (first parameter => 0, second parameter => 1, etc.).

Throws:

  `boost::thread_interrupted` if the current thread is interrupted. Any exception thrown by the wait callback associated with any of the futures being waited for. `std::bad_alloc` if memory could not be allocated for the internal wait structures.

Notes:

  `wait_for_any()` is an interruption point.

##### Non-member function `wait_for_all()`

    template<typename Iterator>
    void wait_for_all(Iterator begin,Iterator end);

    template<typename F1,typename F2>
    void wait_for_all(F1& f1,F2& f2);

    template<typename F1,typename F2,typename F3>
    void wait_for_all(F1& f1,F2& f2,F3& f3);

    template<typename F1,typename F2,typename F3,typename F4>
    void wait_for_all(F1& f1,F2& f2,F3& f3,F4& f4);

    template<typename F1,typename F2,typename F3,typename F4,typename F5>
    void wait_for_all(F1& f1,F2& f2,F3& f3,F4& f4,F5& f5);

Preconditions:

  The types `Fn` shall be specializations of `boost::unique_future` or `boost::shared_future`, and `Iterator` shall be a forward iterator with a `value_type` which is a specialization of `boost::unique_future` or `boost::shared_future`.

Effects:

  Waits until all of the specified futures are ready.

Throws:

  Any exceptions thrown by a call to `wait()` on the specified futures.

Notes:

  `wait_for_all()` is an interruption point.