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### 987. `reference_wrapper` and function types

Section: 22.10.6 [refwrap] Status: C++11 Submitter: Howard Hinnant Opened: 2009-02-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The synopsis in 22.10.6 [refwrap] says:

>     template <ObjectType T> class reference_wrapper
>     ...

And then paragraph 3 says:

> The template instantiation `reference_wrapper<T>` shall be derived from `std::unary_function<T1, R>` only if the type `T` is any of the following:

> - a function type or a pointer to function type taking one argument of type `T1` and returning `R`

But function types are not `ObjectType`s.

Paragraph 4 contains the same contradiction.

[ Post Summit: ]

> Jens: restricted reference to ObjectType

> Recommend Review.

[ Post Summit, Peter adds: ]

> In https://svn.boost.org/trac/boost/ticket/1846 however Eric Niebler makes the very reasonable point that `reference_wrapper<F>`, where `F` is a function type, represents a reference to a function, a legitimate entity. So `boost::ref` was changed to allow it.

> http://svn.boost.org/svn/boost/trunk/libs/bind/test/ref_fn_test.cpp

> Therefore, I believe an alternative proposed resolution for issue 987 could simply allow `reference_wrapper` to be used with function types.

[ Post Summit, Howard adds: ]

> I agree with Peter (and Eric). I got this one wrong on my first try. Here is code that demonstrates how easy (and useful) it is to instantiate `reference_wrapper` with a function type:

> >     #include <functional>

> >     template <class F>
> >     void test(F f);

> >     void f() {}

> >     int main()
> >     {
> >         test(std::ref(f));
> >     }

> Output (link time error shows type of `reference_wrapper` instantiated with function type):

> >     Undefined symbols:
> >       "void test<std::reference_wrapper<void ()()> >(std::reference_wrapper<void ()()>)",...

> I've taken the liberty of changing the proposed wording to allow function types and set to Open. I'll also freely admit that I'm not positive `ReferentType` is the correct concept.

[ Batavia (2009-05): ]

> Howard observed that `FunctionType`, a concept not (yet?) in the Working Paper, is likely the correct constraint to be applied. However, the proposed resolution provides an adequate approximation.

> Move to Review.

[ 2009-05-23 Alisdair adds: ]

> By constraining to `PointeeType` we rule out the ability for `T` to be a reference, and call in reference-collapsing. I'm not sure if this is correct and intended, but would like to be sure the case was considered.

> Is dis-allowing reference types and the implied reference collapsing the intended result?

[ 2009-07 Frankfurt ]

> Moved from Review to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-10-14 Daniel provided de-conceptified wording. ]

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

Change 22.10.6 [refwrap]/1 as indicated:

> `reference_wrapper<T>` is a `CopyConstructible` and `CopyAssignable` wrapper around a reference to an object or function of type `T`.