Re: [boost] [optional] get() misses optional r-value overload in contrast to operator* and value()

23 Mar 2018

      Hi Viktor,
Thank you for reporting this. This is an omission (a bug) that I will need
to fix. I have created an issue in GitHub to track it:
https://github.com/boostorg/optional/issues/51.

On the other hand, I do not think that the bug is causing an UB any more
than function `value()` in your example. Unless something has changed in
the way temporaries' life is prolonged in C++17. I tested it for `int`s and
all tests work fine. I tested it on `unique_ptr<int>`:

```
#include <memory>
#include <iostream>
#include 

using boost::optional;

optional func() {
  return boost::make_optional(std::unique_ptr<int>(new int{3}));
}

int main()
{
  auto const& p = func().value();
  std::cout << (bool)p << std::endl; // outputs: 0
}
```

And even when I use `value()` I do not get the intuitive result. I would
recommend catching the result by value rather than by reference, and all
surprises will be gone, including the one from your example.

Regards,
&rzej;

2018-03-23 2:04 GMT+01:00 Viktor Sehr via Boost :
...
Both *value()* and *operator** has an overload to handle if the
*optional<T>* object is an r-value. However *get()* does not have the
r-value overload.
This leads to undefined behavior using *get()*, but not with *operator**
 and *value()*, in the following example:
auto func0() {
  return optional<int>{3};
}
auto func1() {
  const auto& x = func0().get(); // Undefined behavior
  const auto& y = func0().value(); // Correct
  const auto& z = *func0(); // Correct
}
Is this intended, I can't see any reason why the r-value overload of .get()
would be missing?
Otherwise, the following overload of .get() should be added:
reference_type_of_temporary_wrapper get() && {
  BOOST_ASSERT(this->is_initialized());
  return boost::move(this->get_impl());
}
/Viktor Sehr, Software Developer at Toppluva
_______________________________________________
Unsubscribe & other changes: http://lists.boost.org/
mailman/listinfo.cgi/boost

Re: [boost] [optional] get() misses optional r-value overload in contrast to operator* and value()

Andrzej Krzemienski