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### 742. Enabling `swap` for proxy iterators

Section: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

This issue was split from 672. 672 now just deals with changing the requirements of `T` in the `Swappable` requirement from `CopyConstructible` and `CopyAssignable` to `MoveConstructible` and `MoveAssignable`.

This issue seeks to widen the `Swappable` requirement to support proxy iterators. Here is example code:

>     namespace Mine {

>     template <class T>
>     struct proxy {...};

>     template <class T>
>     struct proxied_iterator
>     {
>        typedef T value_type;
>        typedef proxy<T> reference;
>        reference operator*() const;
>        ...
>     };

>     struct A
>     {
>        // heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable
>        void swap(A&);
>        ...
>     };

>     void swap(A&, A&);
>     void swap(proxy<A>, A&);
>     void swap(A&, proxy<A>);
>     void swap(proxy<A>, proxy<A>);

>     }  // Mine

>     ...

>     Mine::proxied_iterator<Mine::A> i(...)
>     Mine::A a;
>     swap(*i1, a);

The key point to note in the above code is that in the call to `swap`, `*i1` and `a` are different types (currently types can only be `Swappable` with the same type). A secondary point is that to support proxies, one must be able to pass rvalues to `swap`. But note that I am not stating that the general purpose `std::swap` should accept rvalues! Only that overloaded `swap`s, as in the example above, be allowed to take rvalues.

That is, no standard library code needs to change. We simply need to have a more flexible definition of `Swappable`.

[ Bellevue: ]

> While we believe Concepts work will define a swappable concept, we should still resolve this issue if possible to give guidance to the Concepts work.

> Would an ambiguous swap function in two namespaces found by ADL break this wording? Suggest that the phrase "valid expression" means such a pair of types would still not be swappable.

> Motivation is proxy-iterators, but facility is considerably more general. Are we happy going so far?

> We think this wording is probably correct and probably an improvement on what's there in the WP. On the other hand, what's already there in the WP is awfully complicated. Why do we need the two bullet points? They're too implementation-centric. They don't add anything to the semantics of what swap() means, which is there in the post-condition. What's wrong with saying that types are swappable if you can call swap() and it satisfies the semantics of swapping?

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

[ 2009-10 Santa Cruz: ]

> Leave as Open. Dave to provide wording.

[ 2009-11-08 Howard adds: ]

> Updated wording to sync with N3000. Also this issue is very closely related to 594.

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial Resolved. Rationale added.

Rationale:

Solved by N3048.

Proposed resolution:

Change 16.4.4.2 [utility.arg.requirements]:

> -1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables, `T` and `V` are is a types to be supplied by a C++ program instantiating a template; `a`, `b`, and `c` are values of type `const T`; `s` and `t` are modifiable lvalues of type `T`; `u` is a value of type (possibly `const`) `T`; and `rv` is a non-`const` rvalue of type `T`; `w` is a value of type `T`; and `v` is a value of type `V`.

> Table 37: `Swappable` requirements [swappable]
> expression | Return type | Post-condition
> --- | --- | ---
> `swap(sw,tv)` | `void` | `t` `w` has the value originally held by `u` `v`, and `u` `v` has the value originally held by `t` `w`
> The `Swappable` requirement is met by satisfying one or more of the following conditions: `T` is `Swappable` if `T` and `V` are the same type and `T` satisfies the `MoveConstructible` requirements (Table 33) and the `MoveAssignable` requirements (Table 35); `T` is `Swappable` with `V` if a namespace scope function named `swap` exists in the same namespace as the definition of `T` or `V`, such that the expression `swap(sw,t v)` is valid and has the semantics described in this table. `T` is `Swappable` if `T` is an array type whose element type is `Swappable`.

Rationale:

[ post San Francisco: ]

> Solved by N2758.