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2786. Annex C should mention shared_ptr changes for array support

Section: C.3.9 [diff.cpp14.utilities] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-10-18 Last modified: 2017-07-30

Priority: 0

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Discussion:

This is valid in C++11 and C++14:

shared_ptr<int> s{unique_ptr<int[]>{new int[1]}};

The shared_ptr copies the default_delete<int[]> deleter from the unique_ptr, which does the right thing on destruction.

In C++17 it won't compile, because !is_convertible_v<int(*)[], int*>.

The solution is to use shared_ptr<int[]>, which doesn't work well in C++14, so there's no transition path. This should be called out in Annex C.

[2016-11-12, Issaquah]

Sat AM: Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4606.

1. Add to C.3.9 [diff.cpp14.utilities]:

   20.3.2.2 [util.smartptr.shared]

   Change: Different constraint on conversions from unique_ptr.

   Rationale: Adding array support to shared_ptr, via the syntax shared_ptr<T[]> and shared_ptr<T[N]>.

   Effect on original feature: Valid code may fail to compile or change meaning in this International Standard. For example:

   #include <memory>
   std::unique_ptr<int[]> arr(new int[1]);
   std::shared_ptr<int> ptr(std::move(arr)); // error: int(*)[] is not compatible with int*