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1517. default_delete's default constructor should be trivial

Section: 20.3.1.2.2 [unique.ptr.dltr.dflt] Status: C++11 Submitter: Daniel Krügler Opened: 2010-09-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The current working draft does specify the default c'tor of default_delete in a manner to guarantee static initialization for default-constructed objects of static storage duration as a consequence of the acceptance of the proposal n2976 but this paper overlooked the fact that the suggested declaration does not ensure that the type will be a trivial type. The type default_delete was always considered as a simple wrapper for calling delete or delete[], respectivly and should be a trivial type.

In agreement with the new settled core language rules this easy to realize by just changing the declaration to

constexpr default_delete() = default;

This proposal also automatically solves the problem, that the semantics of the default constructor of the partial specialization default_delete<T[]> is not specified at all. By defaulting its default constructor as well, the semantics are well-defined.

[ Post-Rapperswil ]

Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The following wording changes are against N3126.

1. Change the synopsis of the primary template definition of default_delete in [unique.ptr.dltr.dflt] as indicated:

   namespace std {
     template <class T> struct default_delete {
       constexpr default_delete() = default;
       template <class U> default_delete(const default_delete<U>&);
       void operator()(T*) const;
     };
   }
   

2. Remove the prototype specification of the default_delete default constructor in [unique.ptr.dltr.dflt]/1. This brings it in harmony with the style used in the partial specialization default_delete<T[]>. Since there are neither implied nor explicit members, there is no possibility to misinterpret what the constructor does:

   constexpr default_delete();
   

   1 Effects: Default constructs a default_delete object.

3. Change the synopsis of the partial specialization of default_delete in [unique.ptr.dltr.dflt1] as indicated:

   namespace std {
     template <class T> struct default_delete<T[]> {
       constexpr default_delete() = default;
       void operator()(T*) const;
       template <class U> void operator()(U*) const = delete;
     };
   }