meta/select.h

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/** @file
 *  @author Bram de Greve (bramz@users.sourceforge.net)
 *  @author Tom De Muer (tomdemuer@users.sourceforge.net)
 *
 *  *** BEGIN LICENSE INFORMATION ***
 *  
 *  The contents of this file are subject to the Common Public Attribution License 
 *  Version 1.0 (the "License"); you may not use this file except in compliance with 
 *  the License. You may obtain a copy of the License at 
 *  http://lass.sourceforge.net/cpal-license. The License is based on the 
 *  Mozilla Public License Version 1.1 but Sections 14 and 15 have been added to cover 
 *  use of software over a computer network and provide for limited attribution for 
 *  the Original Developer. In addition, Exhibit A has been modified to be consistent 
 *  with Exhibit B.
 *  
 *  Software distributed under the License is distributed on an "AS IS" basis, WITHOUT 
 *  WARRANTY OF ANY KIND, either express or implied. See the License for the specific 
 *  language governing rights and limitations under the License.
 *  
 *  The Original Code is LASS - Library of Assembled Shared Sources.
 *  
 *  The Initial Developer of the Original Code is Bram de Greve and Tom De Muer.
 *  The Original Developer is the Initial Developer.
 *  
 *  All portions of the code written by the Initial Developer are:
 *  Copyright (C) 2004-2007 the Initial Developer.
 *  All Rights Reserved.
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 *  GNU General Public License Version 2 or later (the GPL), in which case the 
 *  provisions of GPL are applicable instead of those above.  If you wish to allow use
 *  of your version of this file only under the terms of the GPL and not to allow 
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 *  *** END LICENSE INFORMATION ***
 */

#ifndef LASS_GUARDIAN_OF_INCLUSION_META_SELECT_H
#define LASS_GUARDIAN_OF_INCLUSION_META_SELECT_H

#include "meta_common.h"
#include "bool.h"

namespace lass
{
namespace meta
{

/** Select a type based on a meta condition that evaluates to meta::True or meta::False.
 *
 *  It differs from meta::If by the fact that TrueType and FalseType do not have to be
 *  meta expressions, as a typedef is used rather than inheritance to select the type.
 *  meta::Select is useful if TrueType and FalseType are regular types like for example
 *  int or std::string.
 *
 *  Using meta::Wrap, you can express the functionality of meta::Select in terms
 *  of meta::If as following:
 *
 *  @code
 *  Select<Cond, A, B>::Type <-> If<Cond, Wrap<A>, Wrap<B> >::Type;
 *  @endcode
 */
template <typename Condition, typename TrueType, typename FalseType> 
struct Select: public Select<typename Condition::Type, TrueType, FalseType>
{
};

template <typename TrueType, typename FalseType>
struct Select<True, TrueType, FalseType>
{
    typedef TrueType Type;
};

template <typename TrueType, typename FalseType>
struct Select<False, TrueType, FalseType>
{
    typedef FalseType Type;
};

}
}

#endif

// EOF