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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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#ifndef SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP
#define SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP

#include "utilities/traits/enableIf.hpp"
#include "utilities/traits/isBaseOf.hpp"
#include "utilities/traits/isClassDefined.hpp"

// This trait is used for injecting optional classes in inheritance hierarchies
// if they have been defined. This trait may not be used in such a way that
// in some instantiations in the same compilation unit,  the optional class is
// defined and in others it is not as that can cause inconsistent behaviour.
// The ::type of this trait will be the Optional type if it was defined
// and is a subtype of Required, otherwise it will be Required.
template <class Optional, class Required>
class SelectBaseClass {
  template<bool, class T, class U>
  struct InheritanceCheck { typedef T type; };

  template<class T, class U>
  struct InheritanceCheck<true, T, U> { typedef U type; };

  template<bool, class T, class U>
  struct ExistenceCheck { typedef T type; };

  template<class T, class U>
  struct ExistenceCheck<true, T, U> {
    typedef typename InheritanceCheck<IsBaseOf<Required, Optional>::value, T, U>::type type;
  };
public:
  typedef typename ExistenceCheck<IsClassDefined<Optional>::value, Required, Optional>::type type;
};

#endif // SHARE_VM_UTILITIES_TRAITS_SELECT_BASE_CLASS_HPP