if.h

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/** @file
 *  @author Bram de Greve (bram@cocamware.com)
 *  @author Tom De Muer (tom@cocamware.com)
 *
 *  *** 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) 2004-2011 the Initial Developer.
 *  All Rights Reserved.
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 */
 
#ifndef LASS_GUADIAN_OF_INCLUSION_META_IF_H
#define LASS_GUADIAN_OF_INCLUSION_META_IF_H
 
#include "meta_common.h"
#include "bool.h"
 
namespace lass
{
namespace meta
{
namespace impl
{
 
template <typename Bool, typename TrueType, typename FalseType> struct IfImpl;
 
}
 
/** Evaluates meta expression TrueType or FalseType based on a meta condition Condition.
 *  
 *  Depending on whether Condition is meta::True or meta::False, meta::If derives from
 *  TrueType or FalseType, thereby selecting the meta code path.
 *  
 *  meta::If can roughly be expressed as meta::Select if only the resulting Type is needed.
 *
 *  @code
 *  If<Cond, A, B>::Type <-> Select<Cond, A::Type, B::Type>::Type
 *  @endcode
 *
 *  Apart from being the shorter expression, using meta::If has the following benefits:
 *  @arg meta::It is faster. Using meta::Select requires the compiler to both evaluate A::Type and B::Type,
 *      regardless fo the outcome.  meta::If postpones the evaluation by deriving from the result.
 *  @arg meta::Select only has the Type field as result.  If other fields are needed, you should
 *      use meta::If as it derives directly from A or B.  Any field that is available in A or B
 *      is thus also available in the result.
 *
 *  meta::If however cannot be used with fundamental types as you need to be able to derive from the result.
 *  This is not really a short coming, as meta::If is designed to select meta code paths, and not to select
 *  a type.  Use meta::Select instead.
 */
template <typename Condition, typename TrueType, typename FalseType>
struct If: public If<typename Condition::Type, TrueType, FalseType> 
{
};
 
template <typename TrueType, typename FalseType> 
struct If<True, TrueType, FalseType>: public TrueType 
{
};
 
template <typename TrueType, typename FalseType> 
struct If<False, TrueType, FalseType>: public FalseType 
{
};
 
}
}
 
#endif
 
// EOF