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### 227. std::swap() should require CopyConstructible or DefaultConstructible arguments

Section: 27.7.3 [alg.swap] Status: TC1 Submitter: Dave Abrahams Opened: 2000-04-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

25.2.2 reads:

> `template<class T> void swap(T& a, T& b);`
> Requires: Type T is Assignable (_lib.container.requirements_).
> Effects: Exchanges values stored in two locations.

The only reasonable** generic implementation of swap requires construction of a new temporary copy of one of its arguments:

>     template<class T> void swap(T& a, T& b);
>       {
>           T tmp(a);
>           a = b;
>           b = tmp;
>       }

But a type which is only Assignable cannot be swapped by this implementation.

**Yes, there's also an unreasonable implementation which would require T to be DefaultConstructible instead of CopyConstructible. I don't think this is worthy of consideration:

>     template<class T> void swap(T& a, T& b);
>     {
>         T tmp;
>         tmp = a;
>         a = b;
>         b = tmp;
>     }

Proposed resolution:

Change 25.2.2 paragraph 1 from:

> Requires: Type T is Assignable (23.1).

to:

> Requires: Type T is CopyConstructible (20.1.3) and Assignable (23.1)