20 General utilities library [utilities]

20.11 Smart pointers [smartptr]

20.11.1 Class template unique_­ptr [unique.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; }; }
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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.
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If the deleter's type D is not a reference type, D shall meet the Cpp17Destructible requirements (Table 32).
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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 33).
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[Example 1:
Given an allocator type X (Table 36) 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]

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constexpr unique_ptr() noexcept; constexpr unique_ptr(nullptr_t) noexcept;
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Constraints: is_­pointer_­v<deleter_­type> is false and is_­default_­constructible_­v<deleter_­type> is true.
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Preconditions: D meets the Cpp17DefaultConstructible requirements (Table 27), and that construction does not throw an exception.
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Effects: Constructs a unique_­ptr object that owns nothing, value-initializing the stored pointer and the stored deleter.
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Postconditions: get() == nullptr.
get_­deleter() returns a reference to the stored deleter.
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explicit unique_ptr(pointer p) noexcept;
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Constraints: is_­pointer_­v<deleter_­type> is false and is_­default_­constructible_­v<deleter_­type> is true.
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Mandates: This constructor is not selected by class template argument deduction ([over.match.class.deduct]).
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Preconditions: D meets the Cpp17DefaultConstructible requirements (Table 27), and that construction does not throw an exception.
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Effects: Constructs a unique_­ptr which owns p, initializing the stored pointer with p and value-initializing the stored deleter.
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Postconditions: get() == p.
get_­deleter() returns a reference to the stored deleter.
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unique_ptr(pointer p, const D& d) noexcept; unique_ptr(pointer p, remove_reference_t<D>&& d) noexcept;
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Constraints: is_­constructible_­v<D, decltype(d)> is true.
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Mandates: These constructors are not selected by class template argument deduction ([over.match.class.deduct]).
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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.
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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).
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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.
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Remarks: If D is a reference type, the second constructor is defined as deleted.
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[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]
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unique_ptr(unique_ptr&& u) noexcept;
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Constraints: is_­move_­constructible_­v<D> is true.
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Preconditions: If D is not a reference type, D meets the Cpp17MoveConstructible requirements (Table 28).
Construction of the deleter from an rvalue of type D does not throw an exception.
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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]
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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.
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template<class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;
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Constraints:
  • (21.1)
    unique_­ptr<U, E>​::​pointer is implicitly convertible to pointer,
  • (21.2)
    U is not an array type, and
  • (21.3)
    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.
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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.
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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]
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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]

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~unique_ptr();
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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]
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Effects: If get() == nullptr there are no effects.
Otherwise get_­deleter()(get()).

20.11.1.3.4 Assignment [unique.ptr.single.asgn]

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unique_ptr& operator=(unique_ptr&& u) noexcept;
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Constraints: is_­move_­assignable_­v<D> is true.
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Preconditions: If D is not a reference type, D meets the Cpp17MoveAssignable requirements (Table 30) 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.
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Effects: Calls reset(u.release()) followed by get_­deleter() = std​::​forward<D>(u.get_­deleter()).
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Postconditions: u.get() == nullptr.
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Returns: *this.
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template<class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
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Constraints:
  • (6.1)
    unique_­ptr<U, E>​::​pointer is implicitly convertible to pointer, and
  • (6.2)
    U is not an array type, and
  • (6.3)
    is_­assignable_­v<D&, E&&> is true.
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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.
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Effects: Calls reset(u.release()) followed by get_­deleter() = std​::​forward<E>(u.get_­deleter()).
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Postconditions: u.get() == nullptr.
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Returns: *this.
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unique_ptr& operator=(nullptr_t) noexcept;
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Effects: As if by reset().
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Postconditions: get() == nullptr.
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Returns: *this.

20.11.1.3.5 Observers [unique.ptr.single.observers]

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

20.11.1.3.6 Modifiers [unique.ptr.single.modifiers]

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pointer release() noexcept;
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Postconditions: get() == nullptr.
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Returns: The value get() had at the start of the call to release.
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void reset(pointer p = pointer()) noexcept;
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Preconditions: The expression get_­deleter()(get()) is well-formed, has well-defined behavior, and does not throw exceptions.
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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]
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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]
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void swap(unique_ptr& u) noexcept;
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Preconditions: get_­deleter() is swappable ([swappable.requirements]) and does not throw an exception under swap.
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Effects: Invokes swap on the stored pointers and on the stored deleters of *this and u.