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.
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   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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  24 
  25 #ifndef SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP
  26 #define SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP
  27 
  28 // This trait is used for injecting optional classes in inheritance hierarchies
  29 // if they have been defined. This trait may not be used in such a way that
  30 // in some instantiations in the same compilation unit,  the optional class is
  31 // defined and in others it is not as that can cause inconsistent behaviour.
  32 // The ::type of this trait will be the Optional type if it was defined
  33 // and is a subtype of Required, otherwise it will be Required.
  34 template <class Optional, class Required>
  35 class SelectBaseClass {
  36   typedef char yes[1];
  37   typedef char no[2];
  38 
  39   template<typename T>
  40   static yes &check(Optional*, T);
  41   static no &check(Required*, int);
  42 
  43   struct OptionalCheckHost {
  44     operator Required*() const;
  45     operator Optional*();
  46   };
  47 
  48   template<bool optional_valid, typename R, typename O>
  49   struct OptionalCheck { typedef O type; };
  50 
  51   template<typename R, typename O>
  52   struct OptionalCheck<false, R, O> { typedef R type; };
  53 public:
  54   typedef typename OptionalCheck<sizeof(check(OptionalCheckHost(), int())) == sizeof(yes), Required, Optional>::type type;
  55 };
  56 
  57 #endif // SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP
  58