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Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++17 Submitter: United States Opened: 2016-11-09 Last modified: 2017-07-30
Priority: 2
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Discussion:
Addresses US 122
unique_ptr should not satisfy is_constructible_v<unique_ptr<T, D>> unless D is DefaultConstructible and not a pointer type. This is important for interactions with pair, tuple, and variant constructors that rely on the is_default_constructible trait.Suggested resolution:
Add a Remarks: clause to constrain the default constructor to not exist unless the Requires clause is satisfied.[Issues Telecon 16-Dec-2016]
Priority 2; Howard and Ville to provide wording.
[2016-12-24: Howard and Ville provided wording.]
[2017-03-02, Kona, STL comments and tweaks the wording]
LWG discussed this issue, and we liked it, but we wanted to tweak the PR. I ran this past Ville (who drafted the PR with Howard), and he was in favor of tweaking this.
[Kona 2017-03-02]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
Modify the synopsis in 20.3.1.3 [unique.ptr.single] as follows:
[…]
constexpr unique_ptr(nullptr_t) noexcept;
: unique_ptr() { }
[…]
Modify 20.3.1.3.2 [unique.ptr.single.ctor] as follows:
constexpr unique_ptr() noexcept; constexpr unique_ptr(nullptr_t) noexcept;-1- Requires: D shall satisfy the requirements of DefaultConstructible (Table 22), and that construction shall not throw an exception.
-2- Effects: Constructs a unique_ptr object that owns nothing, value-initializing the stored pointer and the stored deleter. -3- Postconditions: get() == nullptr. get_deleter() returns a reference to the stored deleter. -4- Remarks:If this constructor is instantiated with a pointer type or reference type for the template argument D, the program is ill-formed.If is_pointer_v<deleter_type> is true or is_default_constructible_v<deleter_type> is false, this constructor shall not participate in overload resolution.explicit unique_ptr(pointer p) noexcept;[…]
-8- Remarks:
If this constructor is instantiated with a pointer type or reference type for the template argument D, the program is ill-formed.If is_pointer_v<deleter_type> is true or is_default_constructible_v<deleter_type> is false, this constructor shall not participate in overload resolution.
Modify the synopsis in 20.3.1.4 [unique.ptr.runtime] as follows:
[…] constexpr unique_ptr(nullptr_t) noexcept;: unique_ptr() { }[…]
Modify 20.3.1.4.2 [unique.ptr.runtime.ctor] as follows:
template <class U> explicit unique_ptr(U p) noexcept;This constructor behaves the same as the constructor that takes a pointer parameter in the primary template except that the following constraints are added for it to participate in overload resolution
U is the same type as pointer, or
pointer is the same type as element_type*, U is a pointer type V*, and V(*)[] is convertible to element_type(*)[].
template <class U> unique_ptr(U p, see below d) noexcept; template <class U> unique_ptr(U p, see below d) noexcept;1 These constructors behave the same as the constructors that take a pointer parameter in the primary template except that they shall not participate in overload resolution unless either