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2762. unique_ptr operator*() should be noexcept

Section: 20.3.1.3.5 [unique.ptr.single.observers] Status: C++23 Submitter: Ville Voutilainen Opened: 2016-08-04 Last modified: 2023-11-22

Priority: 3

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Discussion:

See LWG 2337⁽ⁱ⁾. Since we aren't removing noexcept from shared_ptr's operator*, we should consider adding noexcept to unique_ptr's operator*.

[2016-08 — Chicago]

Thurs PM: P3, and status to 'LEWG'

[2016-08-05 Chicago]

Ville provides an initial proposed wording.

[LEWG Kona 2017]

->Open: Believe these should be noexcept for consistency. We like these. We agree with the proposed resolution. operator->() already has noexcept.

Also adds optional::operator*

Alisdair points out that fancy pointers might intentionally throw from operator*, and we don't want to prohibit that.

Go forward with conditional noexcept(noexcept(*decltype())).

Previous resolution [SUPERSEDED]:

This wording is relative to N4606.

[Drafting note: since this issue is all about consistency, optional's pointer-like operators are additionally included.]

1. In 20.3.1.3 [unique.ptr.single] synopsis, edit as follows:

   add_lvalue_reference_t<T> operator*() const noexcept;
   

2. Before 20.3.1.3.5 [unique.ptr.single.observers]/1, edit as follows:

   add_lvalue_reference_t<T> operator*() const noexcept;
   

3. In 22.5.3 [optional.optional] synopsis, edit as follows:

   constexpr T const *operator->() const noexcept;
   constexpr T *operator->() noexcept;
   constexpr T const &operator*() const & noexcept;
   constexpr T &operator*() & noexcept;
   constexpr T &&operator*() && noexcept;
   constexpr const T &&operator*() const && noexcept;
   

4. Before [optional.object.observe]/1, edit as follows:

   constexpr T const* operator->() const noexcept;
   constexpr T* operator->() noexcept;
   

5. Before [optional.object.observe]/5, edit as follows:

   constexpr T const& operator*() const & noexcept;
   constexpr T& operator*() & noexcept;
   

6. Before [optional.object.observe]/9, edit as follows:

   constexpr T&& operator*() && noexcept;
   constexpr const T&& operator*() const && noexcept;
   

[2021-06-19 Tim updates wording]

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4892.

[Drafting note: since this issue is all about consistency, optional's pointer-like operators are additionally included.]

1. Edit 20.3.1.3.1 [unique.ptr.single.general], class template unique_ptr synopsis, as indicated:

   namespace std {
     template<class T, class D = default_delete<T>> class unique_ptr {
     public:
       […]
   
       // 20.3.1.3.5 [unique.ptr.single.observers], observers
       add_lvalue_reference_t<T> operator*() const noexcept(see below);
       […]
     };
   }
   

2. Edit 20.3.1.3.5 [unique.ptr.single.observers] as indicated:

   add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));
   

   -1- Preconditions: get() != nullptr.

   -2- Returns: *get().

3. Edit 22.5.3.1 [optional.optional.general], class template optional synopsis, as indicated:

   namespace std {
     template<class T>
     class optional {
     public:
       […]
   
       // 22.5.3.7 [optional.observe], observers
       constexpr const T* operator->() const noexcept;
       constexpr T* operator->() noexcept;
       constexpr const T& operator*() const & noexcept;
       constexpr T& operator*() & noexcept;
       constexpr T&& operator*() && noexcept;
       constexpr const T&& operator*() const && noexcept;
   
       […]
     };
   }
   

4. Edit 22.5.3.7 [optional.observe] as indicated:

   constexpr const T* operator->() const noexcept;
   constexpr T* operator->() noexcept;
   

   -1- Preconditions: *this contains a value.

   -2- Returns: val.

   -3- Throws: Nothing.

   -4- Remarks: These functions are constexpr functions.

   constexpr const T& operator*() const & noexcept;
   constexpr T& operator*() & noexcept;
   

   -5- Preconditions: *this contains a value.

   -6- Returns: *val.

   -7- Throws: Nothing.

   -8- Remarks: These functions are constexpr functions.

   constexpr T&& operator*() && noexcept;
   constexpr const T&& operator*() const && noexcept;
   

   -9- Preconditions: *this contains a value.

   -10- Effects: Equivalent to: return std::move(*val);