Re: [Boost-users] [intrusive_ptr] Strange ternary op behavior with MSVC14

Subject: Re: [Boost-users] [intrusive_ptr] Strange ternary op behavior with MSVC14
From: TONGARI J (tongari95_at_[hidden])
Date: 2015-08-11 21:07:46

2015-08-12 7:29 GMT+08:00 Gavin Lambert <gavinl_at_[hidden]>:

> On 11/08/2015 21:39, Stephan Bergmann wrote:
>
>> On 08/11/2015 10:27 AM, TONGARI J wrote:
>>
>>> template<class T>
>>> struct wrapper
>>> {
>>>      wrapper() = default;
>>>
>>>      wrapper(boost::intrusive_ptr<T> const& p) : _p(p) {}
>>>
>>>      boost::intrusive_ptr<T> const& get_ptr() const
>>>      {
>>> #if 0
>>>          // This is OK.
>>>          if (_p)
>>>              return _p;
>>>          return nullptr;
>>> #else
>>>          // Strange behavior.
>>>          return _p ? _p : nullptr;
>>> #endif
>>>      }
>>>
>>
>> In both cases, you return a reference to a temporary, incurring
>> undefined behavior.
>>
>
> _p is a field, so it's not a temporary as long as its instance isn't.
> get_ptr() is invoked on named instances that live longer than the test()
> function, so they're not temporaries either.
>
> While granted it's probably not a good idea to create a tuple from
> references, I don't actually see any undefined behaviour in this particular
> example.  What am I missing?

The nullptr -> boost::intrusive_ptr<T> makes it a temporary.