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

Subject: Re: [boost] [optional] get() misses optional r-value overload in contrast to operator* and value()
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2018-03-23 07:36:46

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 <boost/optional.hpp>

using boost::optional;

optional<std::unique_ptr<int>> 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 <boost_at_[hidden]>:

> 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
>
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