293 lines
9.3 KiB
C++
293 lines
9.3 KiB
C++
/*
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VARIANT.hpp - simple variant wrapper (typesafe union)
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Copyright (C) Lumiera.org
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2008, Hermann Vosseler <Ichthyostega@web.de>
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This program is free software; you can redistribute it and/or
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modify it under the terms of the GNU General Public License as
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published by the Free Software Foundation; either version 2 of
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the License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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/** @file variant.hpp
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** This file defines a simple alternative to boost::variant.
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** It pulls in fewer headers and has a shorter code path, but also
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** doesn't deal with alignment issues and is <b>not threadsafe</b>.
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**
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** Values can be stored using \c operator= . In order to access the value
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** stored in lib::Variant, you additionally need to define a "functor"
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** - with a typedef "Ret" for the return type
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** - providing a \code static Ret access(ELM&) \endcode function
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** for each of the types used in the Variant</li>
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**
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** @todo the instance handling for the accessor seems somewhat
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** misaligned: why do we create another accessor as static var
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** within the access() function?? Why not putting that into the
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** Holder::Storage instance created by the client code?
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** @todo write an unit test ///////////////////////////////////////TICKET #141
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** @see wrapperptr.hpp usage example
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*/
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#ifndef LIB_VARIANT_H
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#define LIB_VARIANT_H
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#include "lib/meta/typelist-manip.hpp"
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#include "lib/meta/generator.hpp"
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#include <boost/noncopyable.hpp>
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namespace lib {
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namespace variant {
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using lumiera::typelist::count;
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using lumiera::typelist::maxSize;
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using lumiera::typelist::InstantiateWithIndex;
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/**
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* internal helper used to build a variant storage wrapper.
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* Parametrised with a collection of types, it provides functionality
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* to copy a value of one of these types into an internal buffer, while
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* remembering which of these types was used to place this copy.
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* The value can be later on extracted using a visitation like mechanism,
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* which takes a functor class and invokes a function \c access(T&) with
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* the type matching the current value in storage
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*/
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template<typename TYPES>
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struct Holder
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{
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enum { TYPECNT = count<TYPES>::value
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, SIZE = maxSize<TYPES>::value
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};
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/** Storage to hold the actual value */
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struct Buffer
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{
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char buffer_[SIZE];
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uint which_;
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Buffer() : which_(TYPECNT) {}
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void*
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put (void)
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{
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deleteCurrent();
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return 0;
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}
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void
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deleteCurrent (); // depends on the Deleter, see below
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};
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template<typename T, class BASE, uint idx>
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struct PlacementAdapter : BASE
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{
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T&
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put (T const& toStore)
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{
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BASE::deleteCurrent(); // remove old content, if any
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//
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T& storedObj = *new(BASE::buffer_) T (toStore);
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BASE::which_ = idx; // remember the actual type selected
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return storedObj;
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}
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using BASE::put; // inherited alternate put() for other types T
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};
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typedef InstantiateWithIndex< TYPES
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, PlacementAdapter
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, Buffer
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>
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Storage;
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/** provide a dispatcher table based visitation mechanism */
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template<class FUNCTOR>
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struct CaseSelect
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{
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typedef typename FUNCTOR::Ret Ret;
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typedef Ret (Func)(Buffer&);
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Func* table_[TYPECNT];
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CaseSelect ()
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{
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for (uint i=0; i<TYPECNT; ++i)
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table_[i] = 0;
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}
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template<typename T>
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static Ret
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trampoline (Buffer& storage)
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{
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T& content = reinterpret_cast<T&> (storage.buffer_);
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return FUNCTOR::access (content);
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}
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Ret
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invoke (Buffer& storage)
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{
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if (TYPECNT <= storage.which_)
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return FUNCTOR::ifEmpty ();
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else
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{
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Func& access = *table_[storage.which_];
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return access (storage);
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}
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}
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};
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/** initialise the dispatcher (trampoline)
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* for the case of accessing type T */
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template< class T, class BASE, uint i >
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struct CasePrepare
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: BASE
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{
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CasePrepare () : BASE()
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{
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BASE::table_[i] = &BASE::template trampoline<T>;
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}
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};
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/** access the variant's inline buffer,
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* using the configured access functor.
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* @note the actual accessor instance
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* is created on demand (static)
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* @todo shouldn't this rather be located within
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* the Holder::Storage created by clients??
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*/
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template<class FUNCTOR>
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static typename FUNCTOR::Ret
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access (Buffer& buf)
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{
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typedef InstantiateWithIndex< TYPES
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, CasePrepare
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, CaseSelect<FUNCTOR>
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>
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Accessor;
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static Accessor select_case;
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return select_case.invoke(buf);
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}
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struct Deleter
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{
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typedef void Ret;
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template<typename T>
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static void access (T& elem) { elem.~T(); }
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static void ifEmpty () { }
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};
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};
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template<typename TYPES>
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inline void
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Holder<TYPES>::Buffer::deleteCurrent ()
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{
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access<Deleter>(*this); // remove old content, if any
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which_ = TYPECNT; // mark as empty
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}
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} // namespace variant
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/**
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* A variant wrapper (typesafe union) capable of holding a value of any
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* of a bounded collection of types. The value is stored in a local buffer
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* directly within the object and may be accessed by a typesafe visitation.
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*
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* \par
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* This utility class is similar to boost::variant and indeed was implemented
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* (5/08) in an effort to replace the latter in a draft solution for the problem
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* of typesafe access to the correct wrapper class from within some builder tool.
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* Well -- after finishing this "exercise" I must admit that it is not really
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* much more simple than what boost::variant does internally. At least we are
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* pulling in fewer headers and the actual code path is shorter compared with
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* boost::variant, at the price of being not so generic, not caring for
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* alignment issues within the buffer and being <b>not threadsafe</b>
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*
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* @param TYPES collection of possible types to be stored in this variant object
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* @param Access policy how to access the stored value
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*/
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template< typename TYPES
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, template<typename> class Access
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>
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class Variant
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: boost::noncopyable
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{
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typedef variant::Holder<TYPES> Holder;
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typedef typename Holder::Deleter Deleter;
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/** storage: buffer holding either an "empty" marker,
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* or an instance of one of the configured payload types */
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typename Holder::Storage holder_;
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public:
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void reset () { holder_.deleteCurrent();}
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/** store a copy of the given argument within the
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* variant holder buffer, thereby typically casting
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* or converting the given source type to the best
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* suited (base) type (out of the collection of possible
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* types for this Variant instance)
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*/
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template<typename SRC>
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Variant&
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operator= (SRC src)
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{
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if (src) holder_.put (src); // see Holder::PlacementAdaptor::put
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else reset();
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return *this;
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}
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/** retrieve current content of the variant,
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* trying to cast or convert it to the given type.
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* Actually, the function \c access(T&) on the
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* Access-policy (template param) is invoked with the
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* type currently stored in the holder buffer.
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* May return NULL if conversion fails.
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*/
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template<typename TAR>
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TAR
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get ()
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{
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typedef Access<TAR> Extractor;
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return Holder::template access<Extractor> (this->holder_);
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}
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};
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} // namespace lib
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#endif
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