2015-08-12 7:29 GMT+08:00 Gavin Lambert
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.