[cfe-dev] Broken type deduction for pair of reference_wrappers?

[Richard Smith]

On Wed, Aug 8, 2012 at 7:41 PM, Michael Price <michael.b.price.dev at gmail.com
> wrote:

> Given two objects, one and two that are references to some pure virtual
> type MyType, the following code compiles fine:
>
>     std::pair<std::reference_wrapper<MyType>,std::reference_wrapper<
> MyType >> mypair = std::make_pair(std::ref(one), std::ref(two));
>     std::swap(mypair.first, mypair.second);
>
> But if I replace the declaration of the pair object type with 'auto' as in:
>
>     auto mypair = std::make_pair(std::ref(one), std::ref(two));
>

mypair's type is std::pair<MyType&, MyType&>. See the make_pair
specification:

template <class T1, class T2> pair<V1, V2> make_pair(T1&& x, T2&& y);

Returns: pair<V1, V2>(std::forward<T1>(x), std::forward<T2>(y));
where V1 and V2 are determined as follows: Let Ui be decay<Ti>::type for
each Ti. Then each Vi is
X& if Ui equals reference_wrapper<X>, otherwise Vi is Ui.

Then...


> I get the following errors with std::swap:
>
> GameController.cpp:43:9: error: no matching function for call to 'swap'
>         std::swap(mypair.first, mypair.second);
>         ^~~~~~~~~
> /usr/include/c++/v1/type_traits:2902:5: note: candidate template ignored:
> disabled by 'enable_if' [with _Tp = MyType]
>     is_move_constructible<_Tp>::value &&
>     ^
>

your values can't be swapped, because MyType is not move constructible,
because it's an abstract class type. The specification for 'swap' says:

Requires: Type T shall be MoveConstructible (Table 20) and MoveAssignable
(Table 22).

I'm not sure whether it's valid to implement this "Requires:" clause as an
enable_if, but in any case, I would expect that better diagnostics would be
produced if it were instead a static_assert within the definition of swap.
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