2874. Constructor shared_ptr::shared_ptr(Y*) should be constrained

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 125

Paragraph 4: This constructor should not participate in overload resolution unless the Requires clause is satisfied. Note that this would therefore apply to some assignment operator and reset overloads, via Effects: equivalent to some code wording.

Proposed change:

Add a Remarks: clause to constrain this constructor not to participate in overload resolution unless the Requires clause is satisfied.

[2017-02-23, Jonathan provides wording]

Previous resolution [SUPERSEDED]:

This wording is relative to N4640.

1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   [Drafting note: This also adds a hyphen to "well defined"]

   template<class Y> explicit shared_ptr(Y* p);
   

   -4- Requires: Y shall be a complete type. The expression delete[] p, when T is an array type, or delete p, when T is not an array type, shall be well formed, shall have well -defined behavior, and shall not throw exceptions. When T is U[N], Y(*)[N] shall be convertible to T*; when T is U[], Y(*)[] shall be convertible to T*; otherwise, Y* shall be convertible to T*.

   -5- Effects: […]

   -6- Postconditions: […]

   -7- Throws: […]

   -?- Remarks: When T is an array type, this constructor shall not participate in overload resolution unless the expression delete[] p is well-formed and either T is U[N] and Y(*)[N] is convertible to T*, or Y(*)[] is convertible to T*. When T is not an array type, this constructor shall not participate in overload resolution unless the expression delete p is well-formed and Y* is convertible to T*.

[Kona 2017-02-27: Jonathan updates wording after LWG review]

[Kona 2017-02-27]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

This wording is relative to N4640.

1. Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:

   [Drafting note: This also adds a hyphen to "well defined"]

   template<class Y> explicit shared_ptr(Y* p);
   

   -4- Requires: Y shall be a complete type. The expression delete[] p, when T is an array type, or delete p, when T is not an array type, shall be well formed, shall have well -defined behavior, and shall not throw exceptions. When T is U[N], Y(*)[N] shall be convertible to T*; when T is U[], Y(*)[] shall be convertible to T*; otherwise, Y* shall be convertible to T*.

   -5- Effects: […]

   -6- Postconditions: […]

   -7- Throws: […]

   -?- Remarks: When T is an array type, this constructor shall not participate in overload resolution unless the expression delete[] p is well-formed and either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*. When T is not an array type, this constructor shall not participate in overload resolution unless the expression delete p is well-formed and Y* is convertible to T*.