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operator<=>(const shared_ptr<T>&, nullptr_t) definition ill-formedSection: 20.3.2.2.8 [util.smartptr.shared.cmp] Status: C++23 Submitter: Daniel Krügler Opened: 2020-04-11 Last modified: 2023-11-22
Priority: 0
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Discussion:
This is similar to LWG 3426(i): This time the definition of
operator<=>(const shared_ptr<T>&, nullptr_t) is ill-formed, whose effects are essentially
specified as calling:
compare_three_way()(a.get(), nullptr)
This call will be ill-formed by constraint-violation, because both nullptr <=> nullptr as well as
((T*) 0) <=> nullptr are ill-formed.
a.get() to (element_type*) nullptr instead of to nullptr directly.
As a long-term solution we should at least consider to deprecate the mixed relational operators as well as the
mixed three-way comparison operator of all our smart-pointers with std::nullptr_t since the core language
has eliminated relational comparisons of pointers with std::nullptr_t with
N3624 four years after they had been originally accepted by
CWG 879. Consequently, for C++20, the mixed three-way comparison between
pointers and nullptr is not supported either. For this long-term solution I'm suggesting to handle this
via a proposal.
[2020-05-16 Reflector discussions]
Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 20.3.2.2.8 [util.smartptr.shared.cmp] as indicated:
template<class T> strong_ordering operator<=>(const shared_ptr<T>& a, nullptr_t) noexcept;-5- Returns:
compare_three_way()(a.get(), static_cast<typename shared_ptr<T>::element_type*>(nullptr)).