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### 688. reference_wrapper, cref unsafe, allow binding to rvalues

Section: 20.14.6.2 [refwrap.const] Status: C++11 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

A `reference_wrapper` can be constructed from an rvalue, either by using the constructor, or via `cref` (and `ref` in some corner cases). This leads to a dangling reference being stored into the `reference_wrapper` object. Now that we have a mechanism to detect an rvalue, we can fix them to disallow this source of undefined behavior.

Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.

[ 2009-05-09 Alisdair adds: ]

> Now that `ref/cref` are constained that `T` must be an `ObjectType`, I do not believe there is any risk of binding `ref` to a temporary (which would rely on deducing `T` to be an rvalue reference type)

> However, the problem for `cref` remains, so I recommend retaining that deleted overload.

[ 2009-05-10 Howard adds: ]

> Without:

> >     template <class T> void ref(const T&& t) = delete;

> I believe this program will compile:

> >     #include <functional>

> >     struct A {};

> >     const A source() {return A();}

> >     int main()
> >     {
> >        std::reference_wrapper<const A> r = std::ref(source());
> >     }

> I.e. in:

> >     template <ObjectType T> reference_wrapper<T> ref(T& t);

> this:

> >     ref(source())

> deduces `T` as `const A`, and so:

> >     ref(const A& t)

> will bind to a temporary (tested with a pre-concepts rvalue-ref enabled compiler).

> Therefore I think we still need the ref-protection. I respectfully disagree with Alisdair's comment and am in favor of the proposed wording as it stands. Also, CWG 606 (noted below) has now been "favorably" resolved.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 20.14 [function.objects], add the following two signatures to the synopsis:

>     template <class T> void ref(const T&& t) = delete;
>     template <class T> void cref(const T&& t) = delete;

[ N2292 addresses the first part of the resolution but not the second. ]

[ Bellevue: Doug noticed problems with the current wording. ]

[ post Bellevue: Howard and Peter provided revised wording. ]

[ This resolution depends on a "favorable" resolution of CWG 606: that is, the "special deduction rule" is disabled with the const T&& pattern. ]