# Standard library header <memory>

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< cpp‎ | header

This header is part of the dynamic memory management library.

##### Smart pointers categories

unique_ptr (C++11) | smart pointer with unique object ownership semantics (class template)
shared_ptr (C++11) | smart pointer with shared object ownership semantics (class template)
weak_ptr (C++11) | weak reference to an object managed by std::shared_ptr (class template)
auto_ptr (until C++17) | smart pointer with strict object ownership semantics (class template)

##### Helper classes

owner_less (C++11) | provides mixed-type owner-based ordering of shared and weak pointers (class template)
enable_shared_from_this (C++11) | allows an object to create a `shared_ptr` referring to itself (class template)
bad_weak_ptr (C++11) | exception thrown when accessing a `weak_ptr` which refers to already destroyed object (class)
default_delete (C++11) | default deleter for `unique_ptr` (class template)

##### Allocators

allocator | the default allocator (class template)
allocator_traits (C++11) | provides information about allocator types (class template)
allocator_arg_t (C++11) | tag type used to select allocator-aware constructor overloads (class)
allocator_arg (C++11) | an object of type std::allocator_arg_t used to select allocator-aware constructors (constant)
uses_allocator (C++11) | checks if the specified type supports uses-allocator construction (class template)

##### Uninitialized storage

uninitialized_copy | copies a range of objects to an uninitialized area of memory (function template)
uninitialized_copy_n (C++11) | copies a number of objects to an uninitialized area of memory (function template)
uninitialized_fill | copies an object to an uninitialized area of memory, defined by a range (function template)
uninitialized_fill_n | copies an object to an uninitialized area of memory, defined by a start and a count (function template)
uninitialized_move (C++17) | moves a range of objects to an uninitialized area of memory (function template)
uninitialized_move_n (C++17) | moves a number of objects to an uninitialized area of memory (function template)
uninitialized_default_construct (C++17) | constructs objects by default-initialization in an uninitialized area of memory, defined by a range (function template)
uninitialized_default_construct_n (C++17) | constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count (function template)
uninitialized_value_construct (C++17) | constructs objects by value-initialization in an uninitialized area of memory, defined by a range (function template)
uninitialized_value_construct_n (C++17) | constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (function template)
destroy_at (C++17) | destroys an object at a given address (function template)
destroy (C++17) | destroys a range of objects (function template)
destroy_n (C++17) | destroys a number of objects in a range (function template)
raw_storage_iterator (deprecated in C++17) | an iterator that allows standard algorithms to store results in uninitialized memory (class template)
get_temporary_buffer (deprecated in C++17) | obtains uninitialized storage (function template)
return_temporary_buffer (deprecated in C++17) | frees uninitialized storage (function template)

##### Garbage collector support

declare_reachable (C++11) | declares that an object can not be recycled (function)
undeclare_reachable (C++11) | declares that an object can be recycled (function template)
declare_no_pointers (C++11) | declares that a memory area does not contain traceable pointers (function)
undeclare_no_pointers (C++11) | cancels the effect of std::declare_no_pointers (function)
pointer_safety (C++11) | lists pointer safety models (class)
get_pointer_safety (C++11) | returns the current pointer safety model (function)

##### Miscellaneous

pointer_traits (C++11) | provides information about pointer-like types (class template)
addressof (C++11) | obtains actual address of an object, even if the & operator is overloaded (function template)
align (C++11) | aligns a pointer in a buffer (function)

##### Smart pointer non-member operations

make_unique (C++14) | creates a unique pointer that manages a new object (function template)
operator==operator!=operator<operator<=operator>operator>= | compares to another `unique_ptr` or with nullptr (function template)
make_shared | creates a shared pointer that manages a new object (function template)
allocate_shared | creates a shared pointer that manages a new object allocated using an allocator (function template)
static_pointer_cast dynamic_pointer_cast const_pointer_cast reinterpret_pointer_cast (C++17) | applies `static_cast`, `dynamic_cast`, `const_cast`, or `reinterpret_cast` to the stored pointer (function template)
get_deleter | returns the deleter of specified type, if owned (function template)
operator==operator!=operator<operator<=operator>operator>= | compares with another `shared_ptr` or with nullptr (function template)
operator<< | outputs the value of the stored pointer to an output stream (function template)

std::atomic_is_lock_free(std::shared_ptr)std::atomic_load(std::shared_ptr)std::atomic_load_explicit(std::shared_ptr)std::atomic_store(std::shared_ptr)std::atomic_store_explicit(std::shared_ptr)std::atomic_exchange(std::shared_ptr)std::atomic_exchange_explicit(std::shared_ptr)std::atomic_compare_exchange_weak(std::shared_ptr)std::atomic_compare_exchange_strong(std::shared_ptr)std::atomic_compare_exchange_weak_explicit(std::shared_ptr)std::atomic_compare_exchange_strong_explicit(std::shared_ptr)

specializes atomic operations for std::shared_ptr
(function template)

std::swap(std::unique_ptr) (C++11) | specializes the std::swap algorithm (function template)
std::swap(std::weak_ptr) (C++11) | specializes the std::swap algorithm (function template)
std::swap(std::shared_ptr) (C++11) | specializes the std::swap algorithm (function template)

##### Smart pointer helper classes

std::hash<std::unique_ptr> (C++11) | hash support for std::unique_ptr (class template specialization)
std::hash<std::shared_ptr> (C++11) | hash support for std::shared_ptr (class template specialization)

### Synopsis

    namespace std {
      // pointer traits
      template <class Ptr> struct pointer_traits;
      template <class T> struct pointer_traits<T*>;
      // pointer safety
      enum class pointer_safety { relaxed, preferred, strict };
      void declare_reachable(void* p);
      template <class T> T* undeclare_reachable(T* p);
      void declare_no_pointers(char* p, size_t n);
      void undeclare_no_pointers(char* p, size_t n);
      pointer_safety get_pointer_safety() noexcept;
      // pointer alignment function
      void* align(size_t alignment, size_t size, void*& ptr, size_t& space);
      // allocator argument tag
      struct allocator_arg_t { explicit allocator_arg_t() = default; };
      constexpr allocator_arg_t allocator_arg{};
      // uses_allocator
      template <class T, class Alloc> struct uses_allocator;
      // allocator traits
      template <class Alloc> struct allocator_traits;
      // the default allocator
      template <class T> class allocator;
      template <class T, class U>
      bool operator==(const allocator<T>&, const allocator<U>&) noexcept;
      template <class T, class U>
      bool operator!=(const allocator<T>&, const allocator<U>&) noexcept;
      // specialized algorithms
      template <class T> constexpr T* addressof(T& r) noexcept;
      template <class T> const T* addressof(const T&&) = delete;
      template <class ForwardIterator>
      void uninitialized_default_construct(ForwardIterator first, ForwardIterator last);
      template <class ExecutionPolicy, class ForwardIterator>
      void uninitialized_default_construct(ExecutionPolicy&& exec,
                                           ForwardIterator first, ForwardIterator last);
      template <class ForwardIterator, class Size>
      ForwardIterator uninitialized_default_construct_n(ForwardIterator first, Size n);
      template <class ExecutionPolicy, class ForwardIterator, class Size>
      ForwardIterator uninitialized_default_construct_n(ExecutionPolicy&& exec,
                                                        ForwardIterator first, Size n);
      template <class ForwardIterator>
      void uninitialized_value_construct(ForwardIterator first, ForwardIterator last);
      template <class ExecutionPolicy, class ForwardIterator>
      void uninitialized_value_construct(ExecutionPolicy&& exec,
                                         ForwardIterator first, ForwardIterator last);
      template <class ForwardIterator, class Size>
      ForwardIterator uninitialized_value_construct_n(ForwardIterator first, Size n);
      template <class ExecutionPolicy, class ForwardIterator, class Size>
      ForwardIterator uninitialized_value_construct_n(ExecutionPolicy&& exec,
                                                      ForwardIterator first, Size n);
      template <class InputIterator, class ForwardIterator>
      ForwardIterator uninitialized_copy(InputIterator first, InputIterator last,
                                         ForwardIterator result);
      template <class ExecutionPolicy, class InputIterator, class ForwardIterator>
      ForwardIterator uninitialized_copy(ExecutionPolicy&& exec,
                                         InputIterator first, InputIterator last,
                                         ForwardIterator result);
      template <class InputIterator, class Size, class ForwardIterator>
      ForwardIterator uninitialized_copy_n(InputIterator first, Size n,
                                           ForwardIterator result);
      template <class ExecutionPolicy, class InputIterator, class Size, class ForwardIterator>
      ForwardIterator uninitialized_copy_n(ExecutionPolicy&& exec,
                                           InputIterator first, Size n,
                                           ForwardIterator result);
      template <class InputIterator, class ForwardIterator>
      ForwardIterator uninitialized_move(InputIterator first, InputIterator last,
                                         ForwardIterator result);
      template <class ExecutionPolicy, class InputIterator, class ForwardIterator>
      ForwardIterator uninitialized_move(ExecutionPolicy&& exec,
                                         InputIterator first, InputIterator last,
                                         ForwardIterator result);
      template <class InputIterator, class Size, class ForwardIterator>
      pair<InputIterator, ForwardIterator>
        uninitialized_move_n(InputIterator first, Size n, ForwardIterator result);
      template <class ExecutionPolicy, class InputIterator, class Size, class ForwardIterator>
      pair<InputIterator, ForwardIterator>
        uninitialized_move_n(ExecutionPolicy&& exec,
                             InputIterator first, Size n, ForwardIterator result);
      template <class ForwardIterator, class T>
      void uninitialized_fill(ForwardIterator first, ForwardIterator last, const T& x);
      template <class ExecutionPolicy, class ForwardIterator, class T>
      void uninitialized_fill(ExecutionPolicy&& exec,
                              ForwardIterator first, ForwardIterator last, const T& x);
      template <class ForwardIterator, class Size, class T>
      ForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);
      template <class ExecutionPolicy, class ForwardIterator, class Size, class T>
      ForwardIterator uninitialized_fill_n(ExecutionPolicy&& exec,
                                           ForwardIterator first, Size n, const T& x);
      template <class T>
      void destroy_at(T* location);
      template <class ForwardIterator>
      void destroy(ForwardIterator first, ForwardIterator last);
      template <class ExecutionPolicy, class ForwardIterator>
      void destroy(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last);
      template <class ForwardIterator, class Size>
      ForwardIterator destroy_n(ForwardIterator first, Size n);
      template <class ExecutionPolicy, class ForwardIterator, class Size>
      ForwardIterator destroy_n(ExecutionPolicy&& exec, ForwardIterator first, Size n);
      // class template unique_ptr
      template <class T> struct default_delete;
      template <class T> struct default_delete<T[]>;
      template <class T, class D = default_delete<T>> class unique_ptr;
      template <class T, class D> class unique_ptr<T[], D>;
      template <class T, class... Args> unique_ptr<T> make_unique(Args&&... args);
      template <class T> unique_ptr<T> make_unique(size_t n);
      template <class T, class... Args> /*unspecified*/ make_unique(Args&&...) = delete;
      template <class T, class D> void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;
      template <class T1, class D1, class T2, class D2>
      bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template <class T1, class D1, class T2, class D2>
      bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template <class T1, class D1, class T2, class D2>
      bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template <class T1, class D1, class T2, class D2>
      bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template <class T1, class D1, class T2, class D2>
      bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template <class T1, class D1, class T2, class D2>
      bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
      template <class T, class D>
      bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
      template <class T, class D>
      bool operator==(nullptr_t, const unique_ptr<T, D>& y) noexcept;
      template <class T, class D>
      bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept;
      template <class T, class D>
      bool operator!=(nullptr_t, const unique_ptr<T, D>& y) noexcept;
      template <class T, class D>
      bool operator<(const unique_ptr<T, D>& x, nullptr_t);
      template <class T, class D>
      bool operator<(nullptr_t, const unique_ptr<T, D>& y);
      template <class T, class D>
      bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
      template <class T, class D>
      bool operator<=(nullptr_t, const unique_ptr<T, D>& y);
      template <class T, class D>
      bool operator>(const unique_ptr<T, D>& x, nullptr_t);
      template <class T, class D>
      bool operator>(nullptr_t, const unique_ptr<T, D>& y);
      template <class T, class D>
      bool operator>=(const unique_ptr<T, D>& x, nullptr_t);
      template <class T, class D>
      bool operator>=(nullptr_t, const unique_ptr<T, D>& y);
      // class bad_weak_ptr
      class bad_weak_ptr;
      // class template shared_ptr
      template<class T> class shared_ptr;
      // shared_ptr creation
      template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args);
      template<class T, class A, class... Args>
      shared_ptr<T> allocate_shared(const A& a, Args&&... args);
      // shared_ptr comparisons
      template<class T, class U>
      bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
      template<class T, class U>
      bool operator!=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
      template<class T, class U>
      bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
      template<class T, class U>
      bool operator>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
      template<class T, class U>
      bool operator<=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
      template<class T, class U>
      bool operator>=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
      template <class T>
      bool operator==(const shared_ptr<T>& x, nullptr_t) noexcept;
      template <class T>
      bool operator==(nullptr_t, const shared_ptr<T>& y) noexcept;
      template <class T>
      bool operator!=(const shared_ptr<T>& x, nullptr_t) noexcept;
      template <class T>
      bool operator!=(nullptr_t, const shared_ptr<T>& y) noexcept;
      template <class T>
      bool operator<(const shared_ptr<T>& x, nullptr_t) noexcept;
      template <class T>
      bool operator<(nullptr_t, const shared_ptr<T>& y) noexcept;
      template <class T>
      bool operator<=(const shared_ptr<T>& x, nullptr_t) noexcept;
      template <class T>
      bool operator<=(nullptr_t, const shared_ptr<T>& y) noexcept;
      template <class T>
      bool operator>(const shared_ptr<T>& x, nullptr_t) noexcept;
      template <class T>
      bool operator>(nullptr_t, const shared_ptr<T>& y) noexcept;
      template <class T>
      bool operator>=(const shared_ptr<T>& x, nullptr_t) noexcept;
      template <class T>
      bool operator>=(nullptr_t, const shared_ptr<T>& y) noexcept;
      // shared_ptr specialized algorithms
      template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;
      // shared_ptr casts
      template<class T, class U>
      shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept;
      template<class T, class U>
      shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
      template<class T, class U>
      shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;
      // shared_ptr get_deleter
      template<class D, class T> D* get_deleter(const shared_ptr<T>& p) noexcept;
      // shared_ptr I/O
      template<class E, class T, class Y>
      basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, const shared_ptr<Y>& p);
      // class template weak_ptr
      template<class T> class weak_ptr;
      // weak_ptr specialized algorithms
      template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
      // class template owner_less
      template<class T = void> struct owner_less;
      // class template enable_shared_from_this
      template<class T> class enable_shared_from_this;
      // shared_ptr atomic access
      template<class T>
      bool atomic_is_lock_free(const shared_ptr<T>* p);
      template<class T>
      shared_ptr<T> atomic_load(const shared_ptr<T>* p);
      template<class T>
      shared_ptr<T> atomic_load_explicit(const shared_ptr<T>* p, memory_order mo);
      template<class T>
      void atomic_store(shared_ptr<T>* p, shared_ptr<T> r);
      template<class T>
      void atomic_store_explicit(shared_ptr<T>* p, shared_ptr<T> r, memory_order mo);
      template<class T>
      shared_ptr<T> atomic_exchange(shared_ptr<T>* p, shared_ptr<T> r);
      template<class T>
      shared_ptr<T> atomic_exchange_explicit(shared_ptr<T>* p, shared_ptr<T> r,
                                             memory_order mo);
      template<class T>
      bool atomic_compare_exchange_weak(shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);
      template<class T>
      bool atomic_compare_exchange_strong(shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);
      template<class T>
      bool atomic_compare_exchange_weak_explicit(shared_ptr<T>* p, shared_ptr<T>* v,
                                                 shared_ptr<T> w, memory_order success,
                                                 memory_order failure);
      template<class T>
      bool atomic_compare_exchange_strong_explicit(shared_ptr<T>* p, shared_ptr<T>* v,
                                                   shared_ptr<T> w, memory_order success,
                                                   memory_order failure);
      // hash support
      template <class T> struct hash;
      template <class T, class D> struct hash<unique_ptr<T, D>>;
      template <class T> struct hash<shared_ptr<T>>;
      // uses_allocator
      template <class T, class Alloc>
      constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value;
    }

#### Class template std::pointer_traits

    namespace std {
      template <class Ptr> struct pointer_traits {
        using pointer = Ptr;
        using element_type = /*see definition*/ ;
        using difference_type = /*see definition*/ ;
        template <class U> using rebind = /*see definition*/ ;
        static pointer pointer_to(/*see definition*/ r);
      };
      template <class T> struct pointer_traits<T*> {
        using pointer = T*;
        using element_type = T;
        using difference_type = ptrdiff_t;
        template <class U> using rebind = U*;
        static pointer pointer_to(/*see definition*/ r) noexcept;
      };
    }

#### Class template std::allocator_traits

    namespace std {
      template <class Alloc> struct allocator_traits {
        using allocator_type = Alloc;
        using value_type = typename Alloc::value_type;
        using pointer = /*see definition*/ ;
        using const_pointer = /*see definition*/ ;
        using void_pointer = /*see definition*/ ;
        using const_void_pointer = /*see definition*/ ;
        using difference_type = /*see definition*/ ;
        using size_type = /*see definition*/ ;
        using propagate_on_container_copy_assignment = /*see definition*/ ;
        using propagate_on_container_move_assignment = /*see definition*/ ;
        using propagate_on_container_swap = /*see definition*/ ;
        using is_always_equal = /*see definition*/ ;
        template <class T> using rebind_alloc = /*see definition*/ ;
        template <class T> using rebind_traits = allocator_traits<rebind_alloc<T>>;
        static pointer allocate(Alloc& a, size_type n);
        static pointer allocate(Alloc& a, size_type n, const_void_pointer hint);
        static void deallocate(Alloc& a, pointer p, size_type n);
        template <class T, class... Args>
        static void construct(Alloc& a, T* p, Args&&... args);
        template <class T>
        static void destroy(Alloc& a, T* p);
        static size_type max_size(const Alloc& a) noexcept;
        static Alloc select_on_container_copy_construction(const Alloc& rhs);
      };
    }

#### Class template std::allocator

    namespace std {
      template <class T> class allocator {
      public:
        using value_type = T;
        using propagate_on_container_move_assignment = true_type;
        using is_always_equal = true_type;
        allocator() noexcept;
        allocator(const allocator&) noexcept;
        template <class U> allocator(const allocator<U>&) noexcept;
        ~allocator();
        T* allocate(size_t n);
        void deallocate(T* p, size_t n);
      };
    }

#### Class template std::default_delete

    namespace std {
      template <class T>
      struct default_delete {
        constexpr default_delete() noexcept = default;
        template <class U> default_delete(const default_delete<U>&) noexcept;
        void operator()(T*) const;
      };
      template <class T>
      struct default_delete<T[]> {
        constexpr default_delete() noexcept = default;
        template <class U> default_delete(const default_delete<U[]>&) noexcept;
        template <class U> void operator()(U* ptr) const;
      };
    }

#### Class template std::unique_ptr

    namespace std {
      template <class T, class D = default_delete<T>>
      class unique_ptr {
      public:
        using pointer = /*see definition*/ ;
        using element_type = T;
        using deleter_type = D;
        // constructors
        constexpr unique_ptr() noexcept;
        explicit unique_ptr(pointer p) noexcept;
        unique_ptr(pointer p, /*see definition*/ d1) noexcept;
        unique_ptr(pointer p, /*see definition*/ d2) noexcept;
        unique_ptr(unique_ptr&& u) noexcept;
        constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
        template <class U, class E>
        unique_ptr(unique_ptr<U, E>&& u) noexcept;
        // destructor
        ~unique_ptr();
        // assignment
        unique_ptr& operator=(unique_ptr&& u) noexcept;
        template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
        unique_ptr& operator=(nullptr_t) noexcept;
        // observers
        add_lvalue_reference_t<T> operator*() const;
        pointer operator->() const noexcept;
        pointer get() const noexcept;
        deleter_type& get_deleter() noexcept;
        const deleter_type& get_deleter() const noexcept;
        explicit operator bool() const noexcept;
        // modifiers
        pointer release() noexcept;
        void reset(pointer p = pointer()) noexcept;
        void swap(unique_ptr& u) noexcept;
        // disable copy from lvalue
        unique_ptr(const unique_ptr&) = delete;
        unique_ptr& operator=(const unique_ptr&) = delete;
      };

      template <class T, class D>
      class unique_ptr<T[], D> {
      public:
        using pointer = /*see definition*/ ;
        using element_type = T;
        using deleter_type = D;
        // constructors
        constexpr unique_ptr() noexcept;
        template <class U> explicit unique_ptr(U p) noexcept;
        template <class U> unique_ptr(U p, /*see definition*/d) noexcept;
        template <class U> unique_ptr(U p, /*see definition*/d) noexcept;
        unique_ptr(unique_ptr&& u) noexcept;
        template <class U, class E>
        unique_ptr(unique_ptr<U, E>&& u) noexcept;
        constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
        // destructor
        ~unique_ptr();
        // assignment
        unique_ptr& operator=(unique_ptr&& u) noexcept;
        template <class U, class E>
        unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
        unique_ptr& operator=(nullptr_t) noexcept;
        // observers
        T& operator[](size_t i) const;
        pointer get() const noexcept;
        deleter_type& get_deleter() noexcept;
        const deleter_type& get_deleter() const noexcept;
        explicit operator bool() const noexcept;
        // modifiers
        pointer release() noexcept;
        template <class U> void reset(U p) noexcept;
        void reset(nullptr_t = nullptr) noexcept;
        void swap(unique_ptr& u) noexcept;
        // disable copy from lvalue
        unique_ptr(const unique_ptr&) = delete;
        unique_ptr& operator=(const unique_ptr&) = delete;
      };
    }

#### Class std::bad_weak_ptr

    namespace std {
      class bad_weak_ptr : public exception {
      public:
        bad_weak_ptr() noexcept;
      };
    }

#### Class template std::shared_ptr

    namespace std {
      template<class T>
      class shared_ptr {
      public:
        using element_type = remove_extent_t<T>;
        using weak_type = weak_ptr<T>;
        // constructors
        constexpr shared_ptr() noexcept;
        template<class Y> explicit shared_ptr(Y* p);
        template<class Y, class D> shared_ptr(Y* p, D d);
        template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
        template <class D> shared_ptr(nullptr_t p, D d);
        template <class D, class A> shared_ptr(nullptr_t p, D d, A a);
        template<class Y> shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept;
        shared_ptr(const shared_ptr& r) noexcept;
        template<class Y> shared_ptr(const shared_ptr<Y>& r) noexcept;
        shared_ptr(shared_ptr&& r) noexcept;
        template<class Y> shared_ptr(shared_ptr<Y>&& r) noexcept;
        template<class Y> explicit shared_ptr(const weak_ptr<Y>& r);
        template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);
        constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }
        // destructor
        ~shared_ptr();
        // assignment
        shared_ptr& operator=(const shared_ptr& r) noexcept;
        template<class Y> shared_ptr& operator=(const shared_ptr<Y>& r) noexcept;
        shared_ptr& operator=(shared_ptr&& r) noexcept;
        template<class Y> shared_ptr& operator=(shared_ptr<Y>&& r) noexcept;
        template <class Y, class D> shared_ptr& operator=(unique_ptr<Y, D>&& r);
        // modifiers
        void swap(shared_ptr& r) noexcept;
        void reset() noexcept;
        template<class Y> void reset(Y* p);
        template<class Y, class D> void reset(Y* p, D d);
        template<class Y, class D, class A> void reset(Y* p, D d, A a);
        // observers
        element_type* get() const noexcept;
        T& operator*() const noexcept;
        T* operator->() const noexcept;
        element_type& operator[](ptrdiff_t i) const;
        long use_count() const noexcept;
        explicit operator bool() const noexcept;
        template<class U> bool owner_before(const shared_ptr<U>& b) const;
        template<class U> bool owner_before(const weak_ptr<U>& b) const;
      };
    }

#### Class template std::weak_ptr

    namespace std {
      template<class T>
      class weak_ptr {
      public:
        using element_type = T;
        // constructors
        constexpr weak_ptr() noexcept;
        template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;
        weak_ptr(const weak_ptr& r) noexcept;
        template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept;
        weak_ptr(weak_ptr&& r) noexcept;
        template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;
        // destructor
        ~weak_ptr();
        // assignment
        weak_ptr& operator=(const weak_ptr& r) noexcept;
        template<class Y> weak_ptr& operator=(const weak_ptr<Y>& r) noexcept;
        template<class Y> weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;
        weak_ptr& operator=(weak_ptr&& r) noexcept;
        template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;
        // modifiers
        void swap(weak_ptr& r) noexcept;
        void reset() noexcept;
        // observers
        long use_count() const noexcept;
        bool expired() const noexcept;
        shared_ptr<T> lock() const noexcept;
        template<class U> bool owner_before(const shared_ptr<U>& b) const;
        template<class U> bool owner_before(const weak_ptr<U>& b) const;
      };
    }

#### Class template std::owner_less

    namespace std {
      template<class T = void> struct owner_less;
      template<class T> struct owner_less<shared_ptr<T>> {
        bool operator()(const shared_ptr<T>&, const shared_ptr<T>&) const;
        bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const;
        bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const;
      };
      template<class T> struct owner_less<weak_ptr<T>> {
        bool operator()(const weak_ptr<T>&, const weak_ptr<T>&) const;
        bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const;
        bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const;
      };
      template<> struct owner_less<void> {
        template<class T, class U>
        bool operator()(const shared_ptr<T>&, const shared_ptr<U>&) const;
        template<class T, class U>
        bool operator()(const shared_ptr<T>&, const weak_ptr<U>&) const;
        template<class T, class U>
        bool operator()(const weak_ptr<T>&, const shared_ptr<U>&) const;
        template<class T, class U>
        bool operator()(const weak_ptr<T>&, const weak_ptr<U>&) const;
        using is_transparent = /*unspecified*/ ;
      };
    }

#### Class template std::enable_shared_from_this

    namespace std {
      template<class T> class enable_shared_from_this {
      protected:
        constexpr enable_shared_from_this() noexcept;
        enable_shared_from_this(const enable_shared_from_this&) noexcept;
        enable_shared_from_this& operator=(const enable_shared_from_this&) noexcept;
        ~enable_shared_from_this();
      public:
        shared_ptr<T> shared_from_this();
        shared_ptr<T const> shared_from_this() const;
        weak_ptr<T> weak_from_this() noexcept;
        weak_ptr<T const> weak_from_this() const noexcept;
      private:
        mutable weak_ptr<T> weak_this; // exposition only
      };
    }

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