a_uniq.insert(i, j)Section: 23.2.7 [associative.reqmts], 23.2.8 [unord.req] Status: Open Submitter: Matt Austern Opened: 2016-12-14 Last modified: 2020-02-14
Priority: 3
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Discussion:
If we write a_uniq.insert(i, j) and [i, j) has multiple elements with keys that compare equivalent,
which ones get inserted? Consider, for example, inserting into a map<string, int> with
m.insert({{"red", 5}, {"green", 3}, {"red", 7}, {"blue", 2}, {"pink", 6}});
Which value for "red" will the map have?
"red" -> 5, and I suspect that's true on most or all implementations, but I don't believe
that's guaranteed by anything in the requirements. The wording in Table 90 just says that it "inserts each element from
the range [i, j) if and only if there is no element with key equivalent to the key of that element", but that
doesn't tell us what happens if [i, j) contains duplicate keys because it doesn't say what order the insertions
are performed in. The standard should either guarantee that the first value is the one that gets inserted, or explicitly
say that this is unspecified.
The same issue applies to the range constructor, and to the unordered associative containers.
[2017-01-27 Telecon]
Priority 3; Nico to provide wording.
[2020-02-14, Prague]
LWG discussion. Suggestion to specify that we like the direction of the wording for insert of
unordered containers, but would also like to clarify that the loop is meant to be "in order" of the
sequence elements.
Proposed resolution: