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

Section: 26.7.3[alg.swap]Status:TC1Submitter: Dave AbrahamsOpened: 2000-04-09Last 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)