This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-08-11
2514. Modifying const subobjectsSection: 6.8.4 [basic.life] Status: open Submitter: Jiang An Date: 2021-11-07 Liaison: SG12The change in C++20 for RU007 allows transparently replacing a const subobject whose complete object is not const, and the new object can be non-const. However, if the reuse of the object has not happened, modifying such subobjects is still undefined behavior.
This restriction causes problems in the implementation of std::map and std::unordered_map; see this bug report. Here, the key_type objects in map containers are const, and implementations generally can't replace these objects after construction.
Perhaps these restrictions can be relaxed to assist in this case: if
the complete object of a key_type subobject in a container is not const (as the mapped_type subobject is not const), or
the complete object has dynamic storage duration
a const subobject could be modified.
(Is it meaningful to allow a new-expression like new const int(42) to create cv-qualified objects? Perhaps such objects should be unqualified, while maintaining the cv-qualification in the type of the expression?)
Notes from the November, 2022 meeting
The advice of SG12 is solicited; see cplusplus/papers#1395.
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