2698. Effect of assign() on iterators/pointers/references

Section: 26.2.3 [sequence.reqmts] Status: C++17 Submitter: Tim Song Opened: 2016-04-25 Last modified: 2017-07-30

Priority: 0

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Discussion:

The sequence container requirements table says nothing about the effect of assign() on iterators, pointers or references into the container. Before LWG 2209 (and LWG 320 for std::list), assign() was specified as "erase everything then insert", which implies wholesale invalidation from the "erase everything" part. With that gone, the blanket "no invalidation" wording in 26.2.1 [container.requirements.general]/12 would seem to apply, which makes absolutely no sense.

The proposed wording below simply spells out the invalidation rule implied by the previous "erase everything" wording.

[2016-05 Issues Telecon]

This is related to 2256

Proposed resolution:

This wording is relative to N4582.

  1. In 26.2.3 [sequence.reqmts], edit Table 107 (Sequence container requirements) as indicated:

    Table 107 — Sequence container requirements (in addition to container)
    Expression Return type Assertion/note
    pre-/post-condition
    […]
    a.assign(i, j) void Requires: T shall be EmplaceConstructible into X from *i and assignable from *i.
    For vector, if the iterator does not meet the forward iterator requirements (24.2.5),
    T shall also be MoveInsertable into X.
    Each iterator in the range [i, j) shall be dereferenced exactly once.
    pre: i, j are not iterators into a.
    Replaces elements in a with a copy of [i, j).
    Invalidates all references, pointers and iterators referring to the elements of a.
    For vector and deque, also invalidates the past-the-end iterator.
    […]
    a.assign(n, t) void Requires: T shall be CopyInsertable into X and CopyAssignable.
    pre: t is not a reference into a.
    Replaces elements in a with n copies of t.
    Invalidates all references, pointers and iterators referring to the elements of a.
    For vector and deque, also invalidates the past-the-end iterator.