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Alexandrescu, C++
Coding Standards

`flyweight`

reference`"boost/flyweight/flyweight_fwd.hpp"`

synopsis
namespace boost{ namespace flyweights{ template< typename T, typename Arg1=implementation defined, typename Arg2=implementation defined, typename Arg3=implementation defined, typename Arg4=implementation defined, typename Arg5=implementation defined> class flyweight; // comparison: //OPis any of ==,<,!=,>,>=,<= template< typename T1,typename Arg11,...,typename Arg15, typename T2,typename Arg21,...,typename Arg25 > bool operator( const flyweight<T1,Arg11,...,Arg15>& x, const flyweight<T2,Arg21,...,Arg25>& y); template< typename T1,typename Arg11,...,typename Arg15, typename T2 > bool operatorOP(const flyweight<T1,Arg11,...,Arg15>& x,const T2& y); template< typename T1, typename T2,typename Arg21,...,typename Arg25 > bool operatorOP(const T1& x,const flyweight<T2,Arg21,...,Arg25>& y); // specialized algorithms: template<typename T,typename Arg1,...,typename Arg5> inline void swap( flyweight<T,Arg1,...,Arg5>& x,flyweight<T,Arg1,...,Arg5>& y); template< typename ElemType,typename Traits, typename T,typename Arg1,...,typename Arg5 > inline std::basic_ostream<ElemType,Traits>& operator<<( std::basic_ostream<ElemType,Traits>& out, const flyweight<T,Arg1,...,Arg5>& x); template< typename ElemType,typename Traits, typename T,typename Arg1,...,typename Arg5 > inline std::basic_ostream<ElemType,Traits>& operator>>( std::basic_istream<ElemType,Traits>& in, flyweight<T,Arg1,...,Arg5>& x); } // namespace boost::flyweights using flyweights::flyweight; } // namespace boostOP

`flyweight_fwd.hpp`

forward declares the class template
`flyweight`

and its associated global functions.

`"boost/flyweight/flyweight.hpp"`

synopsis
`flyweight`

Objects of type `flyweight<...>`

provide access to immutable
values of type `flyweight<...>::value_type`

, with the following advantages over using
plain `value_type`

objects:

- Flyweight objects with equivalent value share the same representation
(the associated
`value_type`

object). - The size of flyweights is typically that of a pointer, which is in general
smaller than
`sizeof(value_type)`

.

`flyweight`

for `value_type`

results in a reduction in memory usage.
`flyweight`

is parameterized according to some aspects:

- Types
`key_value`

and`value_type`

(possibly equal), where`key_type`

serves as a key type to lookup and construct internal shared instances of objects of`value_type`

. - An optional tag type meant to syntactically differentiate between otherwise identical instantiations.
- The factory class used to store and retrieve the shared value objects.
- The type of holder used to
instantiate the flyweight factory and a mutex object, both of
which are unique to each specialization of the
`flyweight`

class template. - A locking policy determining the synchronization mechanisms for internal access to shared resources.
- A tracking policy which controls how values are treated when all their associated flyweight objects are destroyed.

`flyweight`

instantiation in the following manner:
- Each instantation of
`flyweight`

internally owns a unique factory object and a unique synchronization mutex object, both of which are created through the use of an associated holder type. - The flyweight factory stores elements of an undisclosed type
`Entry`

that is implicitly convertible to`const key_type&`

and also stores a subobject of`value_type`

. Every flyweight object is associated to a`value_type`

subobject of some`Entry`

stored in the factory. - The associated mutex object is used to protect all invocations to the insertion and deletion functions of the internal flyweight factory.
- Each flyweight object internally stores a value of some
undisclosed type
`Handle`

.`Handle`

and the`Entry`

type referred to above are obtained from invocations to the associated tracking policy, in the manner described for this concept.

`key_type`

equivalence
refers to the equivalence relationship induced by the factory class used.
Also, two values of `value_type`

are considered equivalent
if they are constructed from equivalent keys, or are copies of
objects constructed from equivalent keys.
template< typename T, typename Arg1,typename Arg2,typename Arg3,typename Arg4,typename Arg5 > class flyweight { public: typedefdependent on Tkey_type; typedefdependent on Tvalue_type; // static data initialization: static bool init(); class initializer{public:initializer();}; // construct/copy/destroy: flyweight(); flyweight(const flyweight& x); flyweight(flyweight& x); // forwarding constructors: // n is implementation defined. All combinations of constant // and non constant reference arguments are provided. template<typename T0> explicit flyweight([const] T0& t0); template<typename T0,typename T1> flyweight([const] T0& t0,[const] T1& t1); ... template<typename T0,...,typename Tn-1> flyweight([const] T0& t0,...,[const] Tn-1& tn-1); flyweight& operator=(const flyweight& x); flyweight& operator=(const value_type& x); // convertibility to underlying type: const key_type& get_key()const; const value_type& get()const; operator const value_type&()const; // modifiers: void swap(flyweight& x); };

`T`

can be either:

- An arbitrary type,
- a type of the form
`key_value<Key,Value[,KeyFromValue]>`

.

`key_type`

and `value_type`

are both equal to `T`

. In the second case, `key_type`

=`Key`

,
`value_type`

=`Value`

; we say then that the instantiation
of `flyweight`

is a `value_type`

is the type of the values flyweight objects give access to,
while value lookup is based on associated `key_type`

values.
`key_value`

must be
`Assignable`

and `value_type`

must be constructible from `key_type`

;
additionally, `key_value`

must
conform to any extra requirements imposed by the type of factory used.
For key-value flyweights, it is guaranteed that the creation or assignment of a flyweight
object results in at most one construction (or copy construction in some
particular cases) of an object
of `value_type`

, and this construction only occurs in the case that no
equivalent value existed previously in the flyweight factory.
The types `Arg1`

, ... , `Arg5`

, if provided, must be any
of the following, in no particular order:

- A tag,
- a factory specifier,
- a holder specifier,
- a locking policy,
- a tracking policy.

`flyweight`

instantiation is obtained through use of the
corresponding specifier; for instance, the factory results from a
certain (MPL) invocation of the given factory specifier, the internal
mutex from the given locking policy, etc.
The default configuration arguments are:
- No tagging,
`hashed_factory<>`

,`static_holder`

,`simple_locking`

,`refcounted`

tracking policy.

The static data internal to a given `flyweight`

instantiation
(factory instance, etc.) is constructed during the dynamic initialization
phase of the program and always before the first program-wide use of the
instantiated class. The following utilities can be
used when more control about the moment of construction is required.

`static bool init();`

Effects:After execution of this function the static data associated to the instantiation of`flyweight`

is guaranteed to be constructed.

Note:Concurrent execution of this function is not thread safe.

`initializer::initializer();`

Effects:Executes`init()`

.

`flyweight();`

Requires:`key_type`

is`Default Constructible`

.

Effects:Constructs a`flyweight`

object associated with value`value_type(key_type())`

.

`flyweight(const flyweight& x);`

flyweight(flyweight& x);

Effects:Constructs a`flyweight`

object with value`x.get()`

.

Exception safety:`nothrow`

.

`template<typename T0>`

explicit flyweight([const] T0& t0);

template<typename T0,typename T1>

flyweight([const] T0& t0,[const] T1& t1);

...

template<typename T0,...,typename Tn-1>

flyweight([const] T0& t0,...,[const] Tn-1& tn-1);

Effects:Constructs a`flyweight`

object with value`value_type(key_type(t0,...,ti))`

, up to an implementation defined number of arguments.

Note:In this implementation, the maximum number of arguments can be globally configured by the user.

`flyweight& operator=(const flyweight& x);`

Effects:Associates the`flyweight`

object with the same value as`x`

.

Returns:`*this`

.

Exception safety:`nothrow`

.

`flyweight& operator=(const value_type& x);`

Requires:If`flyweight`

is key-value,`value_type`

is`Assignable`

and the`Key Extractor`

`KeyFromValue`

must have been supplied as part of the`key_value<>`

construct.

Effects:Associates the`flyweight`

object with a copy of`x`

or with a`value_type`

constructed from a key equivalent to that associated to`x`

. For non-key-value flyweights,`x`

is its own key; for key-value flyweights, the key is extracted through use of an object of type`KeyFromValue`

.

Returns:`*this`

.

`const key_type& get_key()const;`

Return:A copy of the key used to construct the`value_type`

associated to the`flyweight`

object.

Exception safety:If`flyweight`

is not key-value or if`KeyFromValue`

was not provided,`nothrow`

.

`const value_type& get()const;`

operator const value_type&()const;

Return:The value associated to the`flyweight`

object.

Exception safety:`nothrow`

.

`void swap(flyweight& x);`

Effects:Swaps the associations to`value_type`

s each flyweight object has. No swapping of`key_type`

or`value_type`

objects is done.

Exception safety:`nothrow`

.

`template<`

typename T1,typename Arg11,...,typename Arg15,

typename T2,typename Arg21,...,typename Arg25

>

bool operator ==(

const flyweight<T1,Arg11,...,Arg15>& x,

const flyweight<T2,Arg21,...,Arg25>& y);

Returns:If`x`

and`y`

are of the same type, returns`true`

if and only if they are associated to the same value; if`x`

and`y`

have different types, returns`x.get()==y.get()`

.

Exception safety:If`x`

and`y`

are of the same type,`nothrow`

.

`template<`

typename T1,typename Arg11,...,typename Arg15,

typename T2

>

bool operator ==(const flyweight<T1,Arg11,...,Arg15>& x,const T2& y);

Returns:`x.get()==y`

.

`template<`

typename T1,

typename T2,typename Arg21,...,typename Arg25

>

bool operator ==(const T1& x,const flyweight<T2,Arg21,...,Arg25>& y);

Returns:`x()==y.get()`

.

`template<`

typename T1,typename Arg11,...,typename Arg15,

typename T2,typename Arg21,...,typename Arg25

>

bool operator <(

const flyweight<T1,Arg11,...,Arg15>& x,

const flyweight<T2,Arg21,...,Arg25>& y);

Returns:`x.get()<y.get()`

.

`template<`

typename T1,typename Arg11,...,typename Arg15,

typename T2

>

bool operator <(const flyweight<T1,Arg11,...,Arg15>& x,const T2& y);

Returns:`x.get()<y`

.

`template<`

typename T1,

typename T2,typename Arg21,...,typename Arg25

>

bool operator <(const T1& x,const flyweight<T2,Arg21,...,Arg25>& y);

Returns:`x()<y.get()`

.

`template<`

typename T1,typename Arg11,...,typename Arg15,

typename T2,typename Arg21,...,typename Arg25

>

bool operator *OP*(

const flyweight<T1,Arg11,...,Arg15>& x,

const flyweight<T2,Arg21,...,Arg25>& y);

template<

typename T1,typename Arg11,...,typename Arg15,

typename T2

>

bool operator *OP*(const flyweight<T1,Arg11,...,Arg15>& x,const T2& y);

template<

typename T1,

typename T2,typename Arg21,...,typename Arg25

>

bool operator *OP*(const T1& x,const flyweight<T2,Arg21,...,Arg25>& y);

(

is any of *OP*`!=`

, `>`

,
`>=`

, `<=`

.)

Returns:`true`

if and only if`!(x==y)`

(`is`

OP`!=`

),

`y< x`

(`is`

OP`>`

),

`!(x< y)`

(`is`

OP`>=`

),

`!(y< x)`

(`is`

OP`<=`

).

`template<typename T,typename Arg1,...,typename Arg5>`

inline void swap(

flyweight<T,Arg1,...,Arg5>& x,flyweight<T,Arg1,...,Arg5>& y);

Effects:`x.swap(y)`

.

`template<`

typename ElemType,typename Traits,

typename T,typename Arg1,...,typename Arg5

>

inline std::basic_ostream<ElemType,Traits>& operator<<(

std::basic_ostream<ElemType,Traits>& out,

const flyweight<T,Arg1,...,Arg5>& x);

Effects:`out<<x.get()`

.

Returns:`out`

.

`template<`

typename ElemType,typename Traits,

typename T,typename Arg1,...,typename Arg5

>

inline std::basic_ostream<ElemType,Traits>& operator>>(

std::basic_istream<ElemType,Traits>& in,

flyweight<T,Arg1,...,Arg5>& x);

Requires:If`flyweight`

is key-value,`value_type`

is`Assignable`

and the`Key Extractor`

`KeyFromValue`

must have been supplied as part of the`key_value<>`

construct.

Effects:Reads an object of type`value_type`

from`in`

and assigns it to`x`

.

Returns:`in`

.

`BOOST_FLYWEIGHT_LIMIT_PERFECT_FWD_ARGS`

Effects:Globally define this macro to set the maximum number of arguments accepted by`flyweight`

forwarding constructors, which by default is 5.

Revised December 2nd 2008

© Copyright 2006-2008 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)