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 ///////////////////////////////////////////////////////////////////////////////
 //
 // Copyright (c) 2015 Microsoft Corporation. All rights reserved.
 //
 // This code is licensed under the MIT License (MIT).
 //
 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 //
 ///////////////////////////////////////////////////////////////////////////////
 
 #ifndef GSL_POINTERS_H
 #define GSL_POINTERS_H
 
 #include <gsl/assert> // for Ensures, Expects
 
 #include <algorithm>    // for forward
 #include <cstddef>      // for ptrdiff_t, nullptr_t, size_t
 #include <memory>       // for shared_ptr, unique_ptr
 #include <system_error> // for hash
 #include <type_traits>  // for enable_if_t, is_convertible, is_assignable
 
 #if !defined(GSL_NO_IOSTREAMS)
 #include <iosfwd> // for ostream
 #endif            // !defined(GSL_NO_IOSTREAMS)
 
 namespace gsl
 {
 
 namespace details
 {
     template <typename T, typename = void>
     struct is_comparable_to_nullptr : std::false_type
     {
     };
 
     template <typename T>
     struct is_comparable_to_nullptr<
         T,
         std::enable_if_t<std::is_convertible<decltype(std::declval<T>() != nullptr), bool>::value>>
         : std::true_type
     {
     };
 } // namespace details
 
 //
 // GSL.owner: ownership pointers
 //
 using std::shared_ptr;
 using std::unique_ptr;
 
 //
 // owner
 //
 // owner<T> is designed as a bridge for code that must deal directly with owning pointers for some
 // reason
 //
 // T must be a pointer type
 // - disallow construction from any type other than pointer type
 //
 template <class T, class = std::enable_if_t<std::is_pointer<T>::value>>
 using owner = T;
 
 //
 // not_null
 //
 // Restricts a pointer or smart pointer to only hold non-null values.
 //
 // Has zero size overhead over T.
 //
 // If T is a pointer (i.e. T == U*) then
 // - allow construction from U*
 // - disallow construction from nullptr_t
 // - disallow default construction
 // - ensure construction from null U* fails
 // - allow implicit conversion to U*
 //
 template <class T>
 class not_null
 {
 public:
     static_assert(details::is_comparable_to_nullptr<T>::value, "T cannot be compared to nullptr.");
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
     constexpr not_null(U&& u) : ptr_(std::forward<U>(u))
     {
         Expects(ptr_ != nullptr);
     }
 
     template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
     constexpr not_null(T u) : ptr_(std::move(u))
     {
         Expects(ptr_ != nullptr);
     }
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
     constexpr not_null(const not_null<U>& other) : not_null(other.get())
     {}
 
     not_null(const not_null& other) = default;
     not_null& operator=(const not_null& other) = default;
     constexpr std::conditional_t<std::is_copy_constructible<T>::value, T, const T&> get() const
     {
         Ensures(ptr_ != nullptr);
         return ptr_;
     }
 
     constexpr operator T() const { return get(); }
     constexpr decltype(auto) operator->() const { return get(); }
     constexpr decltype(auto) operator*() const { return *get(); }
 
     // prevents compilation when someone attempts to assign a null pointer constant
     not_null(std::nullptr_t) = delete;
     not_null& operator=(std::nullptr_t) = delete;
 
     // unwanted operators...pointers only point to single objects!
     not_null& operator++() = delete;
     not_null& operator--() = delete;
     not_null operator++(int) = delete;
     not_null operator--(int) = delete;
     not_null& operator+=(std::ptrdiff_t) = delete;
     not_null& operator-=(std::ptrdiff_t) = delete;
     void operator[](std::ptrdiff_t) const = delete;
 
 private:
     T ptr_;
 };
 
 template <class T>
 auto make_not_null(T&& t) noexcept
 {
     return not_null<std::remove_cv_t<std::remove_reference_t<T>>>{std::forward<T>(t)};
 }
 
 #if !defined(GSL_NO_IOSTREAMS)
 template <class T>
 std::ostream& operator<<(std::ostream& os, const not_null<T>& val)
 {
     os << val.get();
     return os;
 }
 #endif // !defined(GSL_NO_IOSTREAMS)
 
 template <class T, class U>
 auto operator==(const not_null<T>& lhs,
                 const not_null<U>& rhs) noexcept(noexcept(lhs.get() == rhs.get()))
     -> decltype(lhs.get() == rhs.get())
 {
     return lhs.get() == rhs.get();
 }
 
 template <class T, class U>
 auto operator!=(const not_null<T>& lhs,
                 const not_null<U>& rhs) noexcept(noexcept(lhs.get() != rhs.get()))
     -> decltype(lhs.get() != rhs.get())
 {
     return lhs.get() != rhs.get();
 }
 
 template <class T, class U>
 auto operator<(const not_null<T>& lhs,
                const not_null<U>& rhs) noexcept(noexcept(lhs.get() < rhs.get()))
     -> decltype(lhs.get() < rhs.get())
 {
     return lhs.get() < rhs.get();
 }
 
 template <class T, class U>
 auto operator<=(const not_null<T>& lhs,
                 const not_null<U>& rhs) noexcept(noexcept(lhs.get() <= rhs.get()))
     -> decltype(lhs.get() <= rhs.get())
 {
     return lhs.get() <= rhs.get();
 }
 
 template <class T, class U>
 auto operator>(const not_null<T>& lhs,
                const not_null<U>& rhs) noexcept(noexcept(lhs.get() > rhs.get()))
     -> decltype(lhs.get() > rhs.get())
 {
     return lhs.get() > rhs.get();
 }
 
 template <class T, class U>
 auto operator>=(const not_null<T>& lhs,
                 const not_null<U>& rhs) noexcept(noexcept(lhs.get() >= rhs.get()))
     -> decltype(lhs.get() >= rhs.get())
 {
     return lhs.get() >= rhs.get();
 }
 
 // more unwanted operators
 template <class T, class U>
 std::ptrdiff_t operator-(const not_null<T>&, const not_null<U>&) = delete;
 template <class T>
 not_null<T> operator-(const not_null<T>&, std::ptrdiff_t) = delete;
 template <class T>
 not_null<T> operator+(const not_null<T>&, std::ptrdiff_t) = delete;
 template <class T>
 not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;
 
 } // namespace gsl
 
 namespace std
 {
 template <class T>
 struct hash<gsl::not_null<T>>
 {
     std::size_t operator()(const gsl::not_null<T>& value) const { return hash<T>{}(value.get()); }
 };
 
 } // namespace std
 
 namespace gsl
 {
 
 //
 // strict_not_null
 //
 // Restricts a pointer or smart pointer to only hold non-null values,
 //
 // - provides a strict (i.e. explicit constructor from T) wrapper of not_null
 // - to be used for new code that wishes the design to be cleaner and make not_null
 //   checks intentional, or in old code that would like to make the transition.
 //
 //   To make the transition from not_null, incrementally replace not_null
 //   by strict_not_null and fix compilation errors
 //
 //   Expect to
 //   - remove all unneeded conversions from raw pointer to not_null and back
 //   - make API clear by specifying not_null in parameters where needed
 //   - remove unnecessary asserts
 //
 template <class T>
 class strict_not_null : public not_null<T>
 {
 public:
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
     constexpr explicit strict_not_null(U&& u) : not_null<T>(std::forward<U>(u))
     {}
 
     template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
     constexpr explicit strict_not_null(T u) : not_null<T>(u)
     {}
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
     constexpr strict_not_null(const not_null<U>& other) : not_null<T>(other)
     {}
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
     constexpr strict_not_null(const strict_not_null<U>& other) : not_null<T>(other)
     {}
 
     strict_not_null(strict_not_null&& other) = default;
     strict_not_null(const strict_not_null& other) = default;
     strict_not_null& operator=(const strict_not_null& other) = default;
     strict_not_null& operator=(const not_null<T>& other)
     {
         not_null<T>::operator=(other);
         return *this;
     }
 
     // prevents compilation when someone attempts to assign a null pointer constant
     strict_not_null(std::nullptr_t) = delete;
     strict_not_null& operator=(std::nullptr_t) = delete;
 
     // unwanted operators...pointers only point to single objects!
     strict_not_null& operator++() = delete;
     strict_not_null& operator--() = delete;
     strict_not_null operator++(int) = delete;
     strict_not_null operator--(int) = delete;
     strict_not_null& operator+=(std::ptrdiff_t) = delete;
     strict_not_null& operator-=(std::ptrdiff_t) = delete;
     void operator[](std::ptrdiff_t) const = delete;
 };
 
 // more unwanted operators
 template <class T, class U>
 std::ptrdiff_t operator-(const strict_not_null<T>&, const strict_not_null<U>&) = delete;
 template <class T>
 strict_not_null<T> operator-(const strict_not_null<T>&, std::ptrdiff_t) = delete;
 template <class T>
 strict_not_null<T> operator+(const strict_not_null<T>&, std::ptrdiff_t) = delete;
 template <class T>
 strict_not_null<T> operator+(std::ptrdiff_t, const strict_not_null<T>&) = delete;
 
 template <class T>
 auto make_strict_not_null(T&& t) noexcept
 {
     return strict_not_null<std::remove_cv_t<std::remove_reference_t<T>>>{std::forward<T>(t)};
 }
 
 #if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
 
 // deduction guides to prevent the ctad-maybe-unsupported warning
 template <class T>
 not_null(T) -> not_null<T>;
 template <class T>
 strict_not_null(T) -> strict_not_null<T>;
 
 #endif // ( defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L) )
 
 } // namespace gsl
 
 namespace std
 {
 template <class T>
 struct hash<gsl::strict_not_null<T>>
 {
     std::size_t operator()(const gsl::strict_not_null<T>& value) const
     {
         return hash<T>{}(value.get());
     }
 };
 
 } // namespace std
 
 #endif // GSL_POINTERS_H

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