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### 2964. Apparently redundant requirement for `dynamic_pointer_cast`

Section: 20.3.2.2.10 [util.smartptr.shared.cast] Status: C++20 Submitter: Tim Song Opened: 2017-05-11 Last modified: 2021-02-25

Priority: 0

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Discussion:

Currently 20.3.2.2.10 [util.smartptr.shared.cast]/4 says:

> Requires: The expression `dynamic_cast<T*>((U*)nullptr)` shall be well formed and shall have well defined behavior.

A `dynamic_cast` of a null pointer, if well-formed, always has well-defined behavior: it returns a null pointer. The second part is therefore redundant as currently worded. The C++14 version, on the other hand, requires `dynamic_cast<T*>(r.get())` to have well-defined behavior, which actually adds something: it requires the user to not trigger the undefined case in [class.cdtor]/5, for instance.

[2017-07 Toronto Monday issue prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Edit 20.3.2.2.10 [util.smartptr.shared.cast] as indicated:

   >     shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;

   > > -4- Requires: The expression `dynamic_cast<T*>((U*)0)` shall be well formed. The expression `dynamic_cast<typename shared_ptr<T>::element_type*>(r.get())` shall be well formed and shall have well defined behavior.

   > > […]