Title
Stability of a_uniq.insert(i, j)
Status
open
Section
[associative.reqmts][unord.req]
Submitter
Matt Austern

Created on 2016-12-14.00:00:00 last changed 50 months ago

Messages

Date: 2020-02-15.00:00:00

[ 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.

Daniel K. volunteered to provide such wording.

Date: 2017-01-27.00:00:00

[ 2017-01-27 Telecon ]

Priority 3; Nico to provide wording.

Date: 2016-12-14.00:00:00

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?

On my implementation we got "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.

History
Date User Action Args
2020-02-14 15:48:57adminsetmessages: + msg11129
2020-02-14 15:48:57adminsetstatus: new -> open
2017-01-30 15:36:02adminsetmessages: + msg8820
2016-12-14 00:00:00admincreate