Re: [GSoC] Patches for shared_ptr array and polymorphic_allocator

[Jonathan Wakely <jwakely at redhat dot com>]

On 18/07/15 23:29 -0700, Tim Shen wrote:
> On Sat, Jul 18, 2015 at 3:01 PM, Fan You <youfan.noey@gmail.com> wrote:
> > However [8.8.6] said, If r is non-null, sets the value of the default memory
> > resource pointer to r, otherwise sets the default memory resource pointer to
> > new_delete_resource().
>
> Ah right, so the only place we should care about is set_default_resource:
> void
> set_default_resource(memory_resource __r)
> {
>  std::atomic<memory_resource> __new_ptr = __r ? __r : new_delete_resource();
>  return __new_ptr.exchange(memory_resource::_S_default_resource);
> }

This won't compile, because std::atomic<T>::exchange takes T not a
std::atomic<T>.

We don't need __new_ptr to be atomic, the only thing that needs to be
atomic is the exchange operation on the global, so as I said in my
other reply a minute ago, I think it should be:

inline memory_resource*
set_default_resource(memory_resource* __r) noexcept
{
  if ( __r == nullptr)
    __r = new_delete_resource();
  return memory_resource::_S_default_resource.exchange(__r);
}