Title
Needless UB for basic_string and basic_string_view
Status
wp
Section
[char.traits][string.require][string.view.template]
Submitter
Gennaro Prota

Created on 2017-07-03.00:00:00 last changed 9 months ago

Messages

Date: 2023-06-19.14:50:03

Proposed resolution:

This wording is relative to N4950.

  1. Edit [string.require] as indicated:

    -3- In every specialization basic_string<charT, traits, Allocator>, the type allocator_traits<Allocator>::value_type shall name the same type as charT. Every object of type basic_string<charT, traits, Allocator> shall use an object of type Allocator to allocate and free storage for the contained charT objects as needed. The In every specialization basic_string<charT, traits, Allocator>, the type traits shall satisfy the character traits requirements ([char.traits]).

    [Note 1: Every specialization basic_string<charT, traits, Allocator> is an allocator-aware container, but does not use the allocator’s construct and destroy member functions ([container.requirements.general]). The program is ill-formed if Allocator::value_type is not the same type as charT. end note]

    [Note 2: The program is ill-formed if traits::char_type is not the same type as charT. — end note]

Date: 2023-06-17.00:00:00

[ 2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP. ]

Date: 2023-06-15.00:00:00

[ 2023-06-01; Reflector poll ]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

There was a request to editorially move the added text in Note 1 into Note 2 after this is approved.

Date: 2023-05-15.00:00:00

[ 2023-05-15; Jonathan Wakely provides improved wording ]

As noted above, most of this issue was resolved by P1148R0. The remainder is the change to make it ill-formed if the allocator has the wrong value_type, but since P1463R1 that is already part of the Allocator-aware container requirements, which apply to basic_string.

Date: 2023-05-15.16:05:34

[ 2017-07 Toronto Tuesday PM issue prioritization ]

Priority 3; need to check general container requirements

Partially by the adoption of P1148 in San Diego.

Tim opines: "the remainder deals with allocator value type mismatch, which I think is NAD."

This wording is relative to N4659.

  1. Edit [char.traits] as indicated:

    -3- To specialize those templates to generate a string or iostream class to handle a particular character container type CharT, that and its related character traits class Traits are passed as a pair of parameters to the string or iostream template as parameters charT and traits. If Traits::char_type shall be the same is not the same type as CharT, the program is ill-formed.

  2. Edit [string.require] as indicated:

    -3- In every specialization basic_string<charT, traits, Allocator>, if the type allocator_traits<Allocator>::value_type shall name the same type is not the same type as charT, the program is ill-formed. Every object of type basic_string<charT, traits, Allocator> shall use an object of type Allocator to allocate and free storage for the contained charT objects as needed. The Allocator object used shall be obtained as described in [container.requirements.general]. In every specialization basic_string<charT, traits, Allocator>, the type traits shall satisfy the character traits requirements ([char.traits]). If, and the type traits::char_type shall name the same type is not the same type as charT, the program is ill-formed.

  3. Edit [string.view.template] as indicated:

    -1- In every specialization basic_string_view<charT, traits>, the type traits shall satisfy the character traits requirements ([char.traits]). If, and the type traits::char_type shall name the same type is not the same type as charT, the program is ill-formed.

Date: 2017-07-06.16:57:00

basic_string and basic_string_view involve undefined behavior in a few cases where it's simple for the implementation to add a static_assert and make the program ill-formed.

With regards to basic_string, [char.traits]/3 states:

Traits::char_type shall be the same as CharT.

Here, the implementation can add a static_assert using the is_same type trait. Similar issues exist in [string.require] and, for basic_string_view, in [string.view.template]/1.

History
Date User Action Args
2023-06-19 14:50:03adminsetmessages: + msg13642
2023-06-19 14:50:03adminsetstatus: voting -> wp
2023-06-12 08:52:25adminsetstatus: ready -> voting
2023-06-01 13:58:08adminsetmessages: + msg13595
2023-06-01 13:58:08adminsetstatus: open -> ready
2023-05-15 16:05:34adminsetmessages: + msg13546
2018-11-25 18:34:32adminsetstatus: new -> open
2017-07-12 01:58:24adminsetmessages: + msg9357
2017-07-04 18:36:03adminsetmessages: + msg9317
2017-07-03 00:00:00admincreate