text: //
  // Copyright 2016 Pixar
  //
  // Licensed under the Apache License, Version 2.0 (the "Apache License")
  // with the following modification; you may not use this file except in
  // compliance with the Apache License and the following modification to it:
  // Section 6. Trademarks. is deleted and replaced with:
  //
  // 6. Trademarks. This License does not grant permission to use the trade
  //    names, trademarks, service marks, or product names of the Licensor
  //    and its affiliates, except as required to comply with Section 4(c) of
  //    the License and to reproduce the content of the NOTICE file.
  //
  // You may obtain a copy of the Apache License at
  //
  //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the Apache License with the above modification is
  // distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  // KIND, either express or implied. See the Apache License for the specific
  // language governing permissions and limitations under the Apache License.
  //
  #ifndef USD_SHARED_H
  #define USD_SHARED_H
  
  #include <boost/functional/hash.hpp>
  #include <boost/smart_ptr/intrusive_ptr.hpp>
  #include <atomic>
  
  // Implementation storage + refcount for Usd_Shared.
  template <class T>
  struct Usd_Counted {
      constexpr Usd_Counted() : count(0) {}
      explicit Usd_Counted(T const &data) : data(data), count(0) {}
      explicit Usd_Counted(T &&data) : data(std::move(data)), count(0) {}
      
      friend inline void
      intrusive_ptr_add_ref(Usd_Counted const *c) { ++c->count; }
      friend inline void
      intrusive_ptr_release(Usd_Counted const *c) {
          if (--c->count == 0)
              delete c;
      }
  
      T data;
      mutable std::atomic_int count;
  };
  
  struct Usd_EmptySharedTagType {};
  constexpr Usd_EmptySharedTagType Usd_EmptySharedTag{};
  
  // This class provides a simple way to share a data object between clients.  It
  // can be used to do simple copy-on-write, etc.
  template <class T>
  struct Usd_Shared
  {
      // Construct a Usd_Shared with a value-initialized T instance.
      Usd_Shared() : _held(new Usd_Counted<T>()) {}
      // Create a copy of \p obj.
      explicit Usd_Shared(T const &obj) : _held(new Usd_Counted<T>(obj)) {}
      // Move from \p obj.
      explicit Usd_Shared(T &&obj) : _held(new Usd_Counted<T>(std::move(obj))) {}
  
      // Create an empty shared, which may not be accessed via Get(),
      // GetMutable(), IsUnique(), Clone(), or MakeUnique().  This is useful when
      // using the insert() or emplace() methods on associative containers, to
      // avoid allocating a temporary in case the object is already present in the
      // container.
      Usd_Shared(Usd_EmptySharedTagType) {}
  
      // Return a const reference to the shared data.
      T const &Get() const { return _held->data; }
      // Return a mutable reference to the shared data.
      T &GetMutable() const { return _held->data; }
      // Return true if no other Usd_Shared instance shares this instance's data.
      bool IsUnique() const { return _held->count == 1; }
      // Make a new copy of the held data and refer to it.
      void Clone() { _held.reset(new Usd_Counted<T>(Get())); }
      // Ensure this Usd_Shared instance has unique data.  Equivalent to:
      // \code
      // if (not shared.IsUnique()) { shared.Clone(); }
      // \endcode
      void MakeUnique() { if (not IsUnique()) Clone(); }
  
      // Equality and inequality.
      bool operator==(Usd_Shared const &other) const {
          return _held == other._held or _held->data == other._held->data;
      }
      bool operator!=(Usd_Shared const &other) const { return *this != other; }
  
      // Swap.
      void swap(Usd_Shared &other) { _held.swap(other._held); }
      friend inline void swap(Usd_Shared &l, Usd_Shared &r) { l.swap(r); }
  
      // hash_value.
      friend inline size_t hash_value(Usd_Shared const &sh) {
          using boost::hash_value;
          return hash_value(sh._held->data);
      }
  private:
      boost::intrusive_ptr<Usd_Counted<T>> _held;
  };
  
  #endif // USD_SHARED_H
  
  