1 /*
   2  * Copyright (c) 2011, 2013, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
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  23  */
  24 
  25 #ifndef SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP
  26 #define SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP
  27 
  28 #include "utilities/traits/enableIf.hpp"
  29 #include "utilities/traits/isBaseOf.hpp"
  30 #include "utilities/traits/isClassDefined.hpp"
  31 
  32 // This trait is used for injecting optional classes in inheritance hierarchies
  33 // if they have been defined. This trait may not be used in such a way that
  34 // in some instantiations in the same compilation unit,  the optional class is
  35 // defined and in others it is not as that can cause inconsistent behaviour.
  36 // The ::type of this trait will be the Optional type if it was defined
  37 // and is a subtype of Required, otherwise it will be Required.
  38 template <class Optional, class Required>
  39 class SelectBaseClass {
  40   template<bool, class T, class U>
  41   struct InheritanceCheck { typedef T type; };
  42 
  43   template<class T, class U>
  44   struct InheritanceCheck<true, T, U> { typedef U type; };
  45 
  46   template<bool, class T, class U>
  47   struct ExistenceCheck { typedef T type; };
  48 
  49   template<class T, class U>
  50   struct ExistenceCheck<true, T, U> {
  51     typedef typename InheritanceCheck<IsBaseOf<Required, Optional>::value, T, U>::type type;
  52   };
  53 public:
  54   typedef typename ExistenceCheck<IsClassDefined<Optional>::value, Required, Optional>::type type;
  55 };
  56 
  57 #endif // SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP
  58