20 General utilities library [utilities]

20.11 Smart pointers [smartptr]

20.11.1 Class template unique_­ptr [unique.ptr]

20.11.1.1 General [unique.ptr.general]

A unique pointer is an object that owns another object and manages that other object through a pointer.
More precisely, a unique pointer is an object u that stores a pointer to a second object p and will dispose of p when u is itself destroyed (e.g., when leaving block scope ([stmt.dcl])).
In this context, u is said to own p.
The mechanism by which u disposes of p is known as p's associated deleter, a function object whose correct invocation results in p's appropriate disposition (typically its deletion).
Let the notation u.p denote the pointer stored by u, and let u.d denote the associated deleter.
Upon request, u can reset (replace) u.p and u.d with another pointer and deleter, but properly disposes of its owned object via the associated deleter before such replacement is considered completed.
Each object of a type U instantiated from the unique_­ptr template specified in [unique.ptr] has the strict ownership semantics, specified above, of a unique pointer.
In partial satisfaction of these semantics, each such U is Cpp17MoveConstructible and Cpp17MoveAssignable, but is not Cpp17CopyConstructible nor Cpp17CopyAssignable.
The template parameter T of unique_­ptr may be an incomplete type.
[Note 1:
The uses of unique_­ptr include providing exception safety for dynamically allocated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function.
— end note]

20.11.1.2 Default deleters [unique.ptr.dltr]

20.11.1.2.1 In general [unique.ptr.dltr.general]

The class template default_­delete serves as the default deleter (destruction policy) for the class template unique_­ptr.
The template parameter T of default_­delete may be an incomplete type.

20.11.1.2.2 default_­delete [unique.ptr.dltr.dflt]

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 U> default_delete(const default_delete<U>& other) noexcept;
Constraints: U* is implicitly convertible to T*.
Effects: Constructs a default_­delete object from another default_­delete<U> object.
void operator()(T* ptr) const;
Mandates: T is a complete type.
Effects: Calls delete on ptr.

20.11.1.2.3 default_­delete<T[]> [unique.ptr.dltr.dflt1]

namespace std { 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; }; }
template<class U> default_delete(const default_delete<U[]>& other) noexcept;
Constraints: U(*)[] is convertible to T(*)[].
Effects: Constructs a default_­delete object from another default_­delete<U[]> object.
template<class U> void operator()(U* ptr) const;
Constraints: U(*)[] is convertible to T(*)[].
Mandates: U is a complete type.
Effects: Calls delete[] on ptr.

20.11.1.3 unique_­ptr for single objects [unique.ptr.single]

20.11.1.3.1 General [unique.ptr.single.general]

namespace std { template<class T, class D = default_delete<T>> class unique_ptr { public: using pointer = see below; using element_type = T; using deleter_type = D; // [unique.ptr.single.ctor], constructors constexpr unique_ptr() noexcept; explicit unique_ptr(pointer p) noexcept; unique_ptr(pointer p, see below d1) noexcept; unique_ptr(pointer p, see below d2) noexcept; unique_ptr(unique_ptr&& u) noexcept; constexpr unique_ptr(nullptr_t) noexcept; template<class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept; // [unique.ptr.single.dtor], destructor ~unique_ptr(); // [unique.ptr.single.asgn], 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; // [unique.ptr.single.observers], 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; // [unique.ptr.single.modifiers], 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; }; }
The default type for the template parameter D is default_­delete.
A client-supplied template argument D shall be a function object type, lvalue reference to function, or lvalue reference to function object type for which, given a value d of type D and a value ptr of type unique_­ptr<T, D>​::​pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter.
If the deleter's type D is not a reference type, D shall meet the Cpp17Destructible requirements (Table 34).
If the qualified-id remove_­reference_­t<D>​::​pointer is valid and denotes a type ([temp.deduct]), then unique_­ptr<T, D>​::​pointer shall be a synonym for remove_­reference_­t<D>​::​pointer.
Otherwise unique_­ptr<T, D>​::​pointer shall be a synonym for element_­type*.
The type unique_­ptr<T, D>​::​pointer shall meet the Cpp17NullablePointer requirements (Table 35).
[Example 1:
Given an allocator type X (Table 38) and letting A be a synonym for allocator_­traits<X>, the types A​::​pointer, A​::​const_­pointer, A​::​void_­pointer, and A​::​const_­void_­pointer may be used as unique_­ptr<T, D>​::​pointer.
— end example]

20.11.1.3.2 Constructors [unique.ptr.single.ctor]

constexpr unique_ptr() noexcept; constexpr unique_ptr(nullptr_t) noexcept;
Constraints: is_­pointer_­v<deleter_­type> is false and is_­default_­constructible_­v<deleter_­type> is true.
Preconditions: D meets the Cpp17DefaultConstructible requirements (Table 29), and that construction does not throw an exception.
Effects: Constructs a unique_­ptr object that owns nothing, value-initializing the stored pointer and the stored deleter.
Postconditions: get() == nullptr.
get_­deleter() returns a reference to the stored deleter.
explicit unique_ptr(pointer p) noexcept;
Constraints: is_­pointer_­v<deleter_­type> is false and is_­default_­constructible_­v<deleter_­type> is true.
Mandates: This constructor is not selected by class template argument deduction ([over.match.class.deduct]).
Preconditions: D meets the Cpp17DefaultConstructible requirements (Table 29), and that construction does not throw an exception.
Effects: Constructs a unique_­ptr which owns p, initializing the stored pointer with p and value-initializing the stored deleter.
Postconditions: get() == p.
get_­deleter() returns a reference to the stored deleter.
unique_ptr(pointer p, const D& d) noexcept; unique_ptr(pointer p, remove_reference_t<D>&& d) noexcept;
Constraints: is_­constructible_­v<D, decltype(d)> is true.
Mandates: These constructors are not selected by class template argument deduction ([over.match.class.deduct]).
Preconditions: For the first constructor, if D is not a reference type, D meets the Cpp17CopyConstructible requirements and such construction does not exit via an exception.
For the second constructor, if D is not a reference type, D meets the Cpp17MoveConstructible requirements and such construction does not exit via an exception.
Effects: Constructs a unique_­ptr object which owns p, initializing the stored pointer with p and initializing the deleter from std​::​forward<decltype(d)>(d).
Postconditions: get() == p.
get_­deleter() returns a reference to the stored deleter.
If D is a reference type then get_­deleter() returns a reference to the lvalue d.
Remarks: If D is a reference type, the second constructor is defined as deleted.
[Example 1: D d; unique_ptr<int, D> p1(new int, D()); // D must be Cpp17MoveConstructible unique_ptr<int, D> p2(new int, d); // D must be Cpp17CopyConstructible unique_ptr<int, D&> p3(new int, d); // p3 holds a reference to d unique_ptr<int, const D&> p4(new int, D()); // error: rvalue deleter object combined // with reference deleter type — end example]
unique_ptr(unique_ptr&& u) noexcept;
Constraints: is_­move_­constructible_­v<D> is true.
Preconditions: If D is not a reference type, D meets the Cpp17MoveConstructible requirements (Table 30).
Construction of the deleter from an rvalue of type D does not throw an exception.
Effects: Constructs a unique_­ptr from u.
If D is a reference type, this deleter is copy constructed from u's deleter; otherwise, this deleter is move constructed from u's deleter.
[Note 1:
The construction of the deleter can be implemented with std​::​forward<D>.
— end note]
Postconditions: get() yields the value u.get() yielded before the construction.
u.get() == nullptr.
get_­deleter() returns a reference to the stored deleter that was constructed from u.get_­deleter().
If D is a reference type then get_­deleter() and u.get_­deleter() both reference the same lvalue deleter.
template<class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;
Constraints:
  • unique_­ptr<U, E>​::​pointer is implicitly convertible to pointer,
  • U is not an array type, and
  • either D is a reference type and E is the same type as D, or D is not a reference type and E is implicitly convertible to D.
Preconditions: If E is not a reference type, construction of the deleter from an rvalue of type E is well-formed and does not throw an exception.
Otherwise, E is a reference type and construction of the deleter from an lvalue of type E is well-formed and does not throw an exception.
Effects: Constructs a unique_­ptr from u.
If E is a reference type, this deleter is copy constructed from u's deleter; otherwise, this deleter is move constructed from u's deleter.
[Note 2:
The deleter constructor can be implemented with std​::​forward<E>.
— end note]
Postconditions: get() yields the value u.get() yielded before the construction.
u.get() == nullptr.
get_­deleter() returns a reference to the stored deleter that was constructed from u.get_­deleter().

20.11.1.3.3 Destructor [unique.ptr.single.dtor]

~unique_ptr();
Preconditions: The expression get_­deleter()(get()) is well-formed, has well-defined behavior, and does not throw exceptions.
[Note 1:
The use of default_­delete requires T to be a complete type.
— end note]
Effects: If get() == nullptr there are no effects.
Otherwise get_­deleter()(get()).

20.11.1.3.4 Assignment [unique.ptr.single.asgn]

unique_ptr& operator=(unique_ptr&& u) noexcept;
Constraints: is_­move_­assignable_­v<D> is true.
Preconditions: If D is not a reference type, D meets the Cpp17MoveAssignable requirements (Table 32) and assignment of the deleter from an rvalue of type D does not throw an exception.
Otherwise, D is a reference type; remove_­reference_­t<D> meets the Cpp17CopyAssignable requirements and assignment of the deleter from an lvalue of type D does not throw an exception.
Effects: Calls reset(u.release()) followed by get_­deleter() = std​::​forward<D>(u.get_­deleter()).
Postconditions: If this != addressof(u), u.get() == nullptr, otherwise u.get() is unchanged.
Returns: *this.
template<class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
Constraints:
  • unique_­ptr<U, E>​::​pointer is implicitly convertible to pointer, and
  • U is not an array type, and
  • is_­assignable_­v<D&, E&&> is true.
Preconditions: If E is not a reference type, assignment of the deleter from an rvalue of type E is well-formed and does not throw an exception.
Otherwise, E is a reference type and assignment of the deleter from an lvalue of type E is well-formed and does not throw an exception.
Effects: Calls reset(u.release()) followed by get_­deleter() = std​::​forward<E>(u.get_­deleter()).
Postconditions: u.get() == nullptr.
Returns: *this.
unique_ptr& operator=(nullptr_t) noexcept;
Effects: As if by reset().
Postconditions: get() == nullptr.
Returns: *this.

20.11.1.3.5 Observers [unique.ptr.single.observers]

add_lvalue_reference_t<T> operator*() const;
Preconditions: get() != nullptr.
Returns: *get().
pointer operator->() const noexcept;
Preconditions: get() != nullptr.
Returns: get().
[Note 1:
The use of this function typically requires that T be a complete type.
— end note]
pointer get() const noexcept;
Returns: The stored pointer.
deleter_type& get_deleter() noexcept; const deleter_type& get_deleter() const noexcept;
Returns: A reference to the stored deleter.
explicit operator bool() const noexcept;
Returns: get() != nullptr.

20.11.1.3.6 Modifiers [unique.ptr.single.modifiers]

pointer release() noexcept;
Postconditions: get() == nullptr.
Returns: The value get() had at the start of the call to release.
void reset(pointer p = pointer()) noexcept;
Preconditions: The expression get_­deleter()(get()) is well-formed, has well-defined behavior, and does not throw exceptions.
Effects: Assigns p to the stored pointer, and then if and only if the old value of the stored pointer, old_­p, was not equal to nullptr, calls get_­deleter()(old_­p).
[Note 1:
The order of these operations is significant because the call to get_­deleter() might destroy *this.
— end note]
Postconditions: get() == p.
[Note 2:
The postcondition does not hold if the call to get_­deleter() destroys *this since this->get() is no longer a valid expression.
— end note]
void swap(unique_ptr& u) noexcept;
Preconditions: get_­deleter() is swappable ([swappable.requirements]) and does not throw an exception under swap.
Effects: Invokes swap on the stored pointers and on the stored deleters of *this and u.

20.11.1.4 unique_­ptr for array objects with a runtime length [unique.ptr.runtime]

20.11.1.4.1 General [unique.ptr.runtime.general]

namespace std { template<class T, class D> class unique_ptr<T[], D> { public: using pointer = see below; using element_type = T; using deleter_type = D; // [unique.ptr.runtime.ctor], constructors constexpr unique_ptr() noexcept; template<class U> explicit unique_ptr(U p) noexcept; template<class U> unique_ptr(U p, see below d) noexcept; template<class U> unique_ptr(U p, see below 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; // 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; // [unique.ptr.runtime.observers], 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; // [unique.ptr.runtime.modifiers], 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; }; }
A specialization for array types is provided with a slightly altered interface.
  • Conversions between different types of unique_­ptr<T[], D> that would be disallowed for the corresponding pointer-to-array types, and conversions to or from the non-array forms of unique_­ptr, produce an ill-formed program.
  • Pointers to types derived from T are rejected by the constructors, and by reset.
  • The observers operator* and operator-> are not provided.
  • The indexing observer operator[] is provided.
  • The default deleter will call delete[].
Descriptions are provided below only for members that differ from the primary template.
The template argument T shall be a complete type.

20.11.1.4.2 Constructors [unique.ptr.runtime.ctor]

template<class U> explicit unique_ptr(U p) noexcept;
This constructor behaves the same as the constructor in the primary template that takes a single parameter of type pointer.
Constraints:
  • U is the same type as pointer, or
  • pointer is the same type as element_­type*, U is a pointer type V*, and V(*)[] is convertible to element_­type(*)[].
template<class U> unique_ptr(U p, see below d) noexcept; template<class U> unique_ptr(U p, see below d) noexcept;
These constructors behave the same as the constructors in the primary template that take a parameter of type pointer and a second parameter.
Constraints:
  • U is the same type as pointer,
  • U is nullptr_­t, or
  • pointer is the same type as element_­type*, U is a pointer type V*, and V(*)[] is convertible to element_­type(*)[].
template<class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;
This constructor behaves the same as in the primary template.
Constraints: Where UP is unique_­ptr<U, E>:
  • U is an array type, and
  • pointer is the same type as element_­type*, and
  • UP​::​pointer is the same type as UP​::​element_­type*, and
  • UP​::​element_­type(*)[] is convertible to element_­type(*)[], and
  • either D is a reference type and E is the same type as D, or D is not a reference type and E is implicitly convertible to D.
[Note 1:
This replaces the Constraints: specification of the primary template.
— end note]

20.11.1.4.3 Assignment [unique.ptr.runtime.asgn]

template<class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
This operator behaves the same as in the primary template.
Constraints: Where UP is unique_­ptr<U, E>:
  • U is an array type, and
  • pointer is the same type as element_­type*, and
  • UP​::​pointer is the same type as UP​::​element_­type*, and
  • UP​::​element_­type(*)[] is convertible to element_­type(*)[], and
  • is_­assignable_­v<D&, E&&> is true.
[Note 1:
This replaces the Constraints: specification of the primary template.
— end note]

20.11.1.4.4 Observers [unique.ptr.runtime.observers]

T& operator[](size_t i) const;
Preconditions: the number of elements in the array to which the stored pointer points.
Returns: get()[i].

20.11.1.4.5 Modifiers [unique.ptr.runtime.modifiers]

void reset(nullptr_t p = nullptr) noexcept;
Effects: Equivalent to reset(pointer()).
template<class U> void reset(U p) noexcept;
This function behaves the same as the reset member of the primary template.
Constraints:
  • U is the same type as pointer, or
  • pointer is the same type as element_­type*, U is a pointer type V*, and V(*)[] is convertible to element_­type(*)[].

20.11.1.5 Creation [unique.ptr.create]

template<class T, class... Args> unique_ptr<T> make_unique(Args&&... args);
Constraints: T is not an array type.
Returns: unique_­ptr<T>(new T(std​::​forward<Args>(args)...)).
template<class T> unique_ptr<T> make_unique(size_t n);
Constraints: T is an array of unknown bound.
Returns: unique_­ptr<T>(new remove_­extent_­t<T>[n]()).
template<class T, class... Args> unspecified make_unique(Args&&...) = delete;
Constraints: T is an array of known bound.
template<class T> unique_ptr<T> make_unique_for_overwrite();
Constraints: T is not an array type.
Returns: unique_­ptr<T>(new T).
template<class T> unique_ptr<T> make_unique_for_overwrite(size_t n);
Constraints: T is an array of unknown bound.
Returns: unique_­ptr<T>(new remove_­extent_­t<T>[n]).
template<class T, class... Args> unspecified make_unique_for_overwrite(Args&&...) = delete;
Constraints: T is an array of known bound.

20.11.1.6 Specialized algorithms [unique.ptr.special]

template<class T, class D> void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;
Constraints: is_­swappable_­v<D> is true.
Effects: Calls x.swap(y).
template<class T1, class D1, class T2, class D2> bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Returns: x.get() == y.get().
template<class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Let CT denote common_type_t<typename unique_ptr<T1, D1>::pointer, typename unique_ptr<T2, D2>::pointer>
Mandates:
  • unique_­ptr<T1, D1>​::​pointer is implicitly convertible to CT and
  • unique_­ptr<T2, D2>​::​pointer is implicitly convertible to CT.
Preconditions: The specialization less<CT> is a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.
Returns: less<CT>()(x.get(), y.get()).
template<class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Returns: y < x.
template<class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Returns: !(y < x).
template<class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Returns: !(x < y).
template<class T1, class D1, class T2, class D2> requires three_­way_­comparable_­with<typename unique_ptr<T1, D1>::pointer, typename unique_ptr<T2, D2>::pointer> compare_three_way_result_t<typename unique_ptr<T1, D1>::pointer, typename unique_ptr<T2, D2>::pointer> operator<=>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
Returns: compare_­three_­way()(x.get(), y.get()).
template<class T, class D> bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
Returns: !x.
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>& x);
Preconditions: The specialization less<unique_­ptr<T, D>​::​pointer> is a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.
Returns: The first function template returns less<unique_ptr<T, D>::pointer>()(x.get(), nullptr)
The second function template returns less<unique_ptr<T, D>::pointer>()(nullptr, x.get())
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>& x);
Returns: The first function template returns nullptr < x.
The second function template returns x < nullptr.
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>& x);
Returns: The first function template returns !(nullptr < x).
The second function template returns !(x < nullptr).
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>& x);
Returns: The first function template returns !(x < nullptr).
The second function template returns !(nullptr < x).
template<class T, class D> requires three_­way_­comparable<typename unique_ptr<T, D>::pointer> compare_three_way_result_t<typename unique_ptr<T, D>::pointer> operator<=>(const unique_ptr<T, D>& x, nullptr_t);
Returns: compare_three_way()(x.get(), static_cast<typename unique_ptr<T, D>::pointer>(nullptr)).

20.11.1.7 I/O [unique.ptr.io]

template<class E, class T, class Y, class D> basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const unique_ptr<Y, D>& p);
Constraints: os << p.get() is a valid expression.
Effects: Equivalent to: os << p.get();
Returns: os.