Re: Possible bug in shared_ptr(unique_ptr) constructor?

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 7 October 2013 22:34, Brad Spencer wrote:
>
> Sorry for the delay.  Bug 58659 filed and you CCed.

Got it - thanks!

>> And here's a suggested patch to fix it:
>
> I hacked in the patch to my installed toolchain headers and both my
> minimal test case and real program worked as expected.  I'll do a full
> toolchain rebuild now with the patch.  Thanks!

Great, thanks for testing it.

> BTW, I noticed that when it's created from unique_ptr, shared_ptr uses
> _Sp_counted_deleter even in the cases where the deleter is the
> std::default_delete<T>.  Since _M_del is just a normal member of
> _Sp_counted_deleter (no empty base optimization, etc.), does this mean
> that a shared_ptr made from a unique_ptr uses a slightly larger than
> necessary count object?  Is it worth trying to use _Sp_counted_ptr in
> that case?

It would be a nice space optimization and I was about to start coding
it up, but I remembered two points:

1) Users are allowed to specialize std::default_delete<T> if T depends
on a user-defined type (e.g.
default_delete<std::vector<SomeNonStdType>>) in which case it could do
something sneaky like log to std::cout every time it is invoked, so we
can't unconditionally remove any use of std::default_delete.

2) on the trunk (what will be GCC 4.9) both the allocator and the
deleter use the empty base optimization, so constructing a
shared_ptr<T> from a unique_ptr<T, default_delete<T>> should already
be no larger than necessary.

Valgrind shows me that this allocates 32 bytes with GCC 4.8 and 24
bytes with trunk:
  std::shared_ptr<int>(std::unique_ptr<int>());

> (I feel greedy for even asking.)

Not at all, it was a good idea.

Thanks again for the report and the suggestion.