84 lines
3 KiB
C++
84 lines
3 KiB
C++
/*
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NULL-VALUE.hpp - maintain per-type NIL values in static memory
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Copyright (C) Lumiera.org
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2010, 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 null-value.hpp
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** Singleton-style holder for \em NIL or \em default values.
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** This template allows to maintain a "Null Object" value, with type-based access.
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** It is implemented as a cluster of Meyer's Singletons, thus the creation happens
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** on demand, and the destruction happens "sometime" during application shutdown.
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** Contrary to the generic lumiera Singleton holder, this implementation is
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** lightweight and without any further prerequisites, contains no logging,
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** no assertions and no locking.
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** @warning we can't make any assumptions regarding the exact time when the dtor
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** is called, and it is even impossible to detect if this happened already.
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** Any access after that point will use a defunct object, thus the user
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** needs to assure this facility is <b>never used during application shutdown</b>
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**
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** \par purpose of NIL objects
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** Employing the NIL object pattern instead of NULL pointers typically leads to
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** greatly simplified and more robust code. Usually the only problem is these NIL
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** marker objects need to exist somewhere. In case no factory is used for object
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** creation, this NullValue holder might satisfy this need.
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**
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** NIL objects are assumed to be default constructible for now. Initialisation
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** may introduce a race, which is considered acceptable here, as these objects
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** are assumed to be simple, constant and value-like.
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**
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** @todo initialisation could be extended to use a free function to fabricate
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** the NIL value object, so to support the special case of an NIL object
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** not being default constructible
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**
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** @see null-value-test.cpp
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** @see lib::advice::Request usage example
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**
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*/
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#ifndef LIB_NULL_VALUE_H
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#define LIB_NULL_VALUE_H
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namespace lib {
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/**
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* Singleton holder for NIL or default value objects.
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* Implemented as a cluster of Meyer's singletons, maintaining
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* a single value per type.
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*/
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template<class TY>
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struct NullValue
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{
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static TY const&
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get()
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{
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static TY nilValue;
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return nilValue;
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}
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};
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} // namespace lib
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#endif
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