Title
any_cast<void>
Status
wp
Section
[any.nonmembers]
Submitter
John Shaw

Created on 2019-10-16.00:00:00 last changed 4 months ago

Messages

Date: 2023-11-13.14:08:10

Proposed resolution:

This wording is relative to N4950.

  1. Modify [any.nonmembers] as indicated:

    template<class T>
      const T* any_cast(const any* operand) noexcept;
    template<class T>
      T* any_cast(any* operand) noexcept;
    

    -8- Mandates: is_void_v<T> is false.

    -9- Returns: If operand != nullptr && operand->type() == typeid(T) is true, a pointer to the object contained by operand; otherwise, nullptr.

    […]

Date: 2023-11-11.00:00:00

[ 2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP. ]

Date: 2023-06-14.00:00:00

[ 2023-06-14 Varna; Move to Ready ]

Poll: 7-0-1

Date: 2023-06-14.00:00:00

[ 2023-06-14 Varna; Jonathan provides improved wording ]

Date: 2023-06-14.07:34:48

[ 2022-02 Currently ill-formed in MSVC ("error C2338: std::any cannot contain void") and returns null pointer in libstdc++ and libc++. ]

This wording is relative to N4835.

  1. Modify [any.nonmembers] as indicated:

    template<class T>
      const T* any_cast(const any* operand) noexcept;
    template<class T>
      T* any_cast(any* operand) noexcept;
    

    -9- Returns: If operand != nullptr && operand->type() == typeid(T) && is_object_v<T>, a pointer to the object contained by operand; otherwise, nullptr.

    […]

Date: 2022-02-03.10:15:29

[ 2020-02 LWG discussion in Prague did not reach consensus. Status to Open. ]

There was discussion about whether or not any_cast<void>(a) should be ill-formed, or return nullptr.

Poll "should it return nullptr" was 0-4-5-5-1.

Date: 2019-11-04.13:23:43

[ 2019-11 Priority to 2 during Monday issue prioritization in Belfast. There is implementation divergence here. ]

Date: 2019-10-16.00:00:00
any foo;
void* p = any_cast<void>(&foo);

Per [any.nonmembers]/9, since the operand isn't nullptr and operand->type() == typeid(T) (because T = void in this case), we should return a pointer to the object contained by operand. But there is no such object.

We need to handle the T = void case, probably by just explicitly returning nullptr.

History
Date User Action Args
2023-11-13 14:08:10adminsetmessages: + msg13836
2023-11-13 14:08:10adminsetstatus: voting -> wp
2023-11-07 21:41:54adminsetstatus: ready -> voting
2023-06-14 07:38:05adminsetmessages: + msg13631
2023-06-14 07:38:05adminsetstatus: open -> ready
2023-06-14 07:34:48adminsetmessages: + msg13630
2022-02-03 00:31:03adminsetmessages: + msg12336
2020-02-14 06:55:06adminsetmessages: + msg11093
2020-02-14 06:55:06adminsetstatus: new -> open
2019-11-04 13:23:43adminsetmessages: + msg10725
2019-10-19 20:23:42adminsetmessages: + msg10707
2019-10-16 00:00:00admincreate