Section: 22.7.5 [any.nonmembers] Status: Voting Submitter: John Shaw Opened: 2019-10-16 Last modified: 2023-11-07 21:41:54 UTC
Priority: 2
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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Previous resolution [SUPERSEDED]: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.
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
Proposed resolution:
This wording is relative to N4950.
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>isfalse.-9- Returns: If operand != nullptr && operand->type() == typeid(T) is
[…]true, a pointer to the object contained by operand; otherwise, nullptr.