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3305. any_cast<void>

Section: 22.7.5 [any.nonmembers] Status: WP Submitter: John Shaw Opened: 2019-10-16 Last modified: 2023-11-22

Priority: 2

View all other issues in [any.nonmembers].

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Discussion:

any foo;
void* p = any_cast<void>(&foo);

Per 22.7.5 [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.

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

[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.

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

Previous resolution [SUPERSEDED]:

This wording is relative to N4835.

1. Modify 22.7.5 [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.

   […]

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

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

Poll: 7-0-1

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

Proposed resolution:

This wording is relative to N4950.

1. Modify 22.7.5 [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.

   […]