This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of TC1 status.

### 151. Can't currently clear() empty container

Section: 22.2.3 [sequence.reqmts] Status: TC1 Submitter: Ed Brey Opened: 1999-06-21 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [sequence.reqmts].

View all other issues in [sequence.reqmts].

View all issues with TC1 status.

Discussion:

For both sequences and associative containers, a.clear() has the semantics of erase(a.begin(),a.end()), which is undefined for an empty container since erase(q1,q2) requires that q1 be dereferenceable (23.1.1,3 and 23.1.2,7). When the container is empty, a.begin() is not dereferenceable.
The requirement that q1 be unconditionally dereferenceable causes many operations to be intuitively undefined, of which clearing an empty container is probably the most dire.
Since q1 and q2 are only referenced in the range [q1, q2), and [q1, q2) is required to be a valid range, stating that q1 and q2 must be iterators or certain kinds of iterators is unnecessary.

Proposed resolution:

In 23.1.1, paragraph 3, change:

> p and q2 denote valid iterators to a, q and q1 denote valid dereferenceable iterators to a, [q1, q2) denotes a valid range

to:

> p denotes a valid iterator to a, q denotes a valid dereferenceable iterator to a, [q1, q2) denotes a valid range in a

In 23.1.2, paragraph 7, change:

> p and q2 are valid iterators to a, q and q1 are valid dereferenceable iterators to a, [q1, q2) is a valid range

to

> p is a valid iterator to a, q is a valid dereferenceable iterator to a, [q1, q2) is a valid range into a