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### 811. `pair` of pointers no longer works with literal 0

Section: 20.4 [pairs] Status: C++11 Submitter: Doug Gregor Opened: 2008-03-14 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [pairs].

View all issues with C++11 status.

Discussion:

>     #include <utility>

>     int main()
>     {
>        std::pair<char *, char *> p (0,0);
>     }

I just got a bug report about that, because it's valid C++03, but not C++0x. The important realization, for me, is that the emplace proposal---which made `push_back` variadic, causing the `push_back(0)` issue---didn't cause this break in backward compatibility. The break actually happened when we added this pair constructor as part of adding rvalue references into the language, long before variadic templates or emplace came along:

>     template<class U, class V> pair(U&& x, V&& y);

Now, concepts will address this issue by constraining that `pair` constructor to only `U`'s and `V`'s that can properly construct "first" and "second", e.g. (from N2322):

>     template<class U , class V >
>     requires Constructible<T1, U&&> && Constructible<T2, V&&>
>     pair(U&& x , V&& y );

[ San Francisco: ]

> Suggested to resolve using pass-by-value for that case.

> Side question: Should pair interoperate with tuples? Can construct a tuple of a pair, but not a pair from a two-element tuple.

> Related to 885.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> Leave as open. Howard to provide wording.

[ 2010-02-06 Howard provided wording. ]

[ 2010-02-09 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Rationale:

[ San Francisco: ]

> Solved by N2770.

[ The rationale is obsolete. ]

Proposed resolution:

Add a paragraph to 20.4 [pairs]:

>     template<class U, class V> pair(U&& x, V&& y);

> > 6 Effects: The constructor initializes `first` with `std::forward<U>(x)` and second with `std::forward<V>(y)`.

> > Remarks: `U` shall be implicitly convertible to `first_type` and `V` shall be implicitly convertible to `second_type`, else this constructor shall not participate in overload resolution.