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

Subject: Re: [Boost-users] [intrusive_ptr] Strange ternary op behavior with MSVC14
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-08-11 19:29:44

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?