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2762. unique_ptr operator*() should be noexcept

Section: 20.11.1.3.5 [unique.ptr.single.observers] Status: Tentatively Ready Submitter: Ville Voutilainen Opened: 2016-08-04 Last modified: 2021-06-23

Priority: 3

View all other issues in [unique.ptr.single.observers].

View all issues with Tentatively Ready status.

Discussion:

See LWG 2337. 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Previous resolution [SUPERSEDED]: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 20.11.1.3 [unique.ptr.single] synopsis, edit as follows:

    add_lvalue_reference_t<T> operator*() const noexcept;
    
  2. Before 20.11.1.3.5 [unique.ptr.single.observers]/1, edit as follows:

    add_lvalue_reference_t<T> operator*() const noexcept;
    
  3. In 20.6.3 [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.

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 20.11.1.3.1 [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:
        […]
    
        // 20.11.1.3.5 [unique.ptr.single.observers], observers
        add_lvalue_reference_t<T> operator*() const noexcept(see below);
        […]
      };
    }
    
  2. Edit 20.11.1.3.5 [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 20.6.3.1 [optional.optional.general], class template optional synopsis, as indicated:

    namespace std {
      template<class T>
      class optional {
      public:
        […]
    
        // 20.6.3.6 [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 20.6.3.6 [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);