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Boost.Flyweight Locking policies reference



Contents

Preliminary concepts

A mutex is a type whose objects can be in either of two states, called locked and unlocked, with the property that when a thread A has locked a mutex m and a different thread B tries to lock m, B is blocked until A unlocks m. Additionally, a mutex is said to support recursive locking if a thread can succesfully invoke the locking operation for a mutex already locked by this same thread; in this case, it is required that the thread execute as many unlock operations as lock operations it has performed for the mutex to become effectively unlocked. A scoped lock is a type associated to some mutex type whose objects do the locking/unlocking of a mutex on construction/destruction time.

In the following table, Mutex is a mutex type, m is an object of type Mutex, Lock is a scoped lock associated to Mutex and lk is a value of Lock.

Mutex and Scoped Lock requirements.
expression return type assertion/note
pre/post-condition
Mutex m;   Post: m is unlocked.
(&m)->~Mutex(); void Pre: m is unlocked.
Lock lk(m);   Associates m to lk and locks m.
(&lk)->~Lock(); void Unlocks the mutex associated to lk.

These concepts are very similar, but not entirely equivalent, to the homonym ones described in the Boost Thread Library.

Locking policies

Locking policies describe a mutex type and an associated scoped lock type. flyweight uses a given locking policy to synchronize the access to its internal factory.

A type Locking is a locking policy if:

Header "boost/flyweight/locking_tag.hpp" synopsis

namespace boost{

namespace flyweights{

struct locking_marker;

template<typename T>
struct is_locking

template<typename T>
struct locking;

} // namespace boost::flyweights

} // namespace boost

Class template is_locking

Unless specialized by the user, is_locking<T>::type is boost::mpl::true_ if T is derived from locking_marker, and it is boost::mpl::false_ otherwise.

Class template locking

locking<T> is a syntactic construct meant to indicate that T is a locking policy without resorting to the mechanisms provided by the is_locking class template.

Header "boost/flyweight/simple_locking_fwd.hpp" synopsis

namespace boost{

namespace flyweights{

struct simple_locking;

} // namespace boost::flyweights

} // namespace boost

simple_locking_fwd.hpp forward declares the class simple_locking.

Header "boost/flyweight/simple_locking.hpp" synopsis

Class simple_locking

Locking Policy that specifies a basic mutex type based on the simplest synchronization mechanisms provided by the environment; When no threading capabilities are available, simple_locking specifies a dummy type without actual synchronization capabilities.

Header "boost/flyweight/no_locking_fwd.hpp" synopsis

namespace boost{

namespace flyweights{

struct no_locking;

} // namespace boost::flyweights

} // namespace boost

no_locking_fwd.hpp forward declares the class no_locking.

Header "boost/flyweight/no_locking.hpp" synopsis

Class no_locking

Null Locking Policy: it specifies a dummy type that satisfies the formal requirements for the Mutex concept but does not perform thread blocking. no_locking should only be used in single-threaded environments.




Revised January 27th 2009

© Copyright 2006-2009 Joaquín M López Muñoz. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)