[PATCH] Make std::enable_shared_from_this cope with ambiguity

[Jonathan Wakely]

On 19/10/16 21:13 +0100, Jonathan Wakely wrote:
>> This patch does three things:
>> 
>> 1. Refactor std::enable_shared_from_this support code.
>> 
>> Instead of several overloads of __enable_shared_from_this_helper
>> that contain the same code but operating on slightly different types
>> I've split it into two parts. Upcasting to an accessible+unambiguous
>> enable_shared_from_this base class is done by new functions found by
>> ADL, __enable_shared_from_this_base. That is called by a new
>> template function __shared_ptr::_M_enable_shared_from_this_with()
>> which actually does the enabling by calling _M_weak_assign.
>> 
>> Calls to _M_enable_shared_from_this_with(p) closely match the
>> wording from my https://wg21.link/p0033r1 paper ("enables
>> shared_from_this with p") and are constrained so they don't give an
>> error if the __enable_shared_from_this_base() call is ambiguous. We
>> already gracefully handled ambiguous std::enable_shared_from_this
>> bases (PR56383) but failed noisily if a type had a combination of
>> std::enable_shared_from_this, std::__enable_shared_from_this and
>> std::experimental::enable_shared_from_this bases. Now we treat those
>> combinations gracefully.
>> 
>> 2. Change std::experimental::enable_shared_from_this bases to be
>> initialized independently of std::enable_shared_from_this bases.
>> It's now possible to have both, and for both to be enabled. See
>> enable_shared_from_this.cc which tests this.
>> 
>> The standard says we have to enable shared_from_this for types with
>> an accessible and unambiguous std::enable_shared_from_this base
>> class, and we should be able to do that even if the class also has
>> an experimental::enable_shared_from_this base class. Now we can.
>> 
>> Potentially we could do the same for std::__enable_shared_from_this,
>> but I'm happy for those bases to be considered ambiguous with
>> std::enable_shared_from_this.
>> 
>> 3. Don't enable shared_from_this for arrays stored in
>> experimental::shared_ptr.  This matches the boost::shared_ptr
>> semantics, and I intend to propose this as a fix for C++17 too. It
>> doesn't make sense to treat the first element of an array specially
>> and enable shared_from_this for the first element only.
>
>I forgot to say that 2. and 3. are appropriate for gcc-6-branch too,
>as they only touch the experimental TS code.

This adds only that part to gcc-6-branch, so that we don't get
ambiguities between std::enable_shared_from_this base-classes and
experimental::enable_shared_from_this base-class

Tested x86_64-linux, committed to gcc-6-branch.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: patch.txt
Type: text/x-patch
Size: 8988 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20161212/09d3ecf8/attachment.bin>