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Boost.Threadsscoped_try_lock |
Introduction
Header
Synopsis
Members
Example
This class template defines a generic lock type which meets the
ScopedTryLock requirements. The
try_mutex, timed_mutex,
recursive_try_mutex and
recursive_timed_mutex classes use this template
to define their scoped_try_lock
types.
Like all the Boost.Threads lock models,
scoped_try_lock
objects are meant to be short-lived. Objects of the
class are not thread-safe, and
so should not be shared between threads.
Class scoped_try_lock
follows the "resource acquisition is
initialization" idiom [Stroustrup
00 14.4.1] and is a realization of the "Scoped Locking Pattern"
[Schmidt-00]. Thus the usage is to let the
constructor do the locking, and then let the destructor do the unlocking automatically at
the end of the enclosing scope. The lock() and unlock() members are usually not
explicitly called, but are provided to allow for complex overlapping locks of multiple
mutexes.
Although this class is an implementation detail, it is publicly documented here because of its importance.
The type used to instantiate the class must meet the TryMutex requirements.
#include <boost/thread/detail/lock.hpp> This header is usually not included directly by programmers because it is supplied by <boost/thread/mutex.hpp> or <boost/thread/recursive_mutex.hpp>
namespace boost { namespace detail { namespace thread { template <typename TryMutex> class scoped_try_lock : private boost::noncopyable // Exposition only. // Class scoped_try_lock meets the NonCopyable requirement. { public: typedef TryMutex mutex_type; explicit scoped_try_lock(TryMutex& mx); scoped_try_lock(TryMutex& mx, bool initially_locked); ~scoped_try_lock(); void lock(); bool try_lock(); void unlock(); operator const void*() const; }; } // namespace thread } // namespace detail } // namespace boost
explicit scoped_try_lock(TryMutex& mx);
Effects: Associates mutex mx
with *this
.
Calls try_lock()
.
scoped_try_lock(TryMutex& mx, bool initially_locked);
Effects: Associates mutex mx
with *this
.
If initially_locked
is true,
calls lock()
.
~scoped_try_lock();
Effects: If locked()
, calls unlock()
. Destroys
*this
.
void lock();
Effects: If the associated mutex is already locked by another lock in the current thread, the effects depend on the locking strategy of the associated mutex, as shown in the following table:
Locking Strategy of associated mutex |
Effect if associated mutex is already locked by the current thread |
Recursive | As if an additional lock were added to the mutex. |
Checked | Throws lock_error. |
Unchecked | Undefined behavior [ISO 1.3.12] (but typically, deadlock.) |
If the associated mutex is already locked by some other thread, places the current thread in the Blocked state until the associated mutex is unlocked, after which the current thread is placed in the Ready state, eventually to be returned to the Running state. Places the associated mutex in the locked state.
Throws: lock_error if locked()
or
as indicated in Effects.
bool try_lock();
Effects: If the associated mutex is already locked by another lock in the current thread, the effects depend on the locking strategy of the associated mutex, as shown in the following table:
Locking Strategy of associated mutex |
Effect if associated mutex is already locked by the current thread |
Recursive | As if an additional lock were added to the mutex. |
Checked | Throws lock_error. |
Unspecified | Undefined behavior [ISO 1.3.12] (but typically, deadlock.) |
If the associated mutex is not already locked by some other thread, locks the associated mutex and returns true, else returns false.
Returns: See effects.
Throws: lock_error if locked()
or
as indicated in Effects.
void unlock();
Effects: Unlocks the associated mutex.
Throws: lock_error if !locked()
.
operator const void*() const;
Returns: If the associated mutex is currently locked, a value convertible to
true
, else a value convertible to false
.
Rationale: A const void*
conversion is considered safer
than a conversion to bool
.
bool locked() const;
Returns: this->operator const void*() != 0
.
#include <boost/thread/mutex.hpp> #include <iostream> int main(int, char*[]) { boost::mutex mutex; boost::mutex::try_lock lock(mutex); if (lock) std::cout << "locked" << std::endl; else std::cout << "unlocked" << std::endl; return 0; }
The output is:
locked
Revised 01 October, 2001
© Copyright William E. Kempf 2001 all rights reserved.