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Boost.Flyweight Tracking policies reference


Preliminary concepts

A tracking policy helper provides access to some of the functionality of a factory so as to be used in the implementation of an associated Tracking Policy. In the following table, TrackingHelper is a tracking policy helper associated to a factory type Factory of elements of type Entry, h is a value of Factory::handle_type associated to a Factory f and check is a value of a Predicate type Checker with argument of type Factory::handle_type.

Tracking Policy Helper requirements.
expression return type assertion/note
TrackingHelper::entry(h); const Entry& Returns f.entry(h).
TrackingHelper::erase(h,check); void If check(h), invokes f.erase(h).

The execution of TrackingHelper::erase (including the invocation of check(h)) is done in a synchronized manner so as to prevent any other thread of execution from simultaneously accessing the factory's insertion or deletion facilities.

Tracking policies

A tracking policy defines the strategy to be followed by a flyweight instantiation when all the flyweight objects associated to a given value are destroyed. The tracking policy contributes some type information necessary for the definition of the flyweight internal factory.

A type Tracking is a tracking policy if:

Tracking::handle_type is parameterized by a helper that provides access to some of the functionality of the factory associated to the tracking policy. This factory's associated entry and handle types are the types Entry and Handle defined above, respectively.

Header "boost/flyweight/tracking_tag.hpp" synopsis

namespace boost{

namespace flyweights{

struct tracking_marker;

template<typename T>
struct is_tracking

template<typename T>
struct tracking;

} // namespace boost::flyweights

} // namespace boost

Class template is_tracking

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

Class template tracking

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

Header "boost/flyweight/refcounted_fwd.hpp" synopsis

namespace boost{

namespace flyweights{

struct refcounted;

} // namespace boost::flyweights

} // namespace boost

refcounted_fwd.hpp forward declares the class refcounted.

Header "boost/flyweight/refcounted.hpp" synopsis

Class refcounted

Tracking Policy providing flyweight instantiations with reference counting semantics: when all the flyweight objects associated to a given value are destroyed, the corresponding entry is erased from flyweight's internal factory.

Header "boost/flyweight/no_tracking_fwd.hpp" synopsis

namespace boost{

namespace flyweights{

struct no_tracking;

} // namespace boost::flyweights

} // namespace boost

no_tracking_fwd.hpp forward declares the class no_tracking.

Header "boost/flyweight/no_tracking.hpp" synopsis

Class no_tracking

Null Tracking Policy: elements inserted into flyweight's internal factory are not erased until program termination.

Revised April 24th 2019

© Copyright 2006-2019 Joaquín M López Muñoz. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at