This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of New status.

4263. What should std::format_to etc. behave when the output is overlong?

Section: 28.5 [format] Status: New Submitter: Jiang An Opened: 2025-05-14 Last modified: 2025-10-17

Priority: 3

View other active issues in [format].

View all other issues in [format].

View all issues with New status.

Discussion:

It seems permitted to write overly long contents via an (C++20) output iterator with std::format_to(_n), where the length isn't representable in size_t or ptrdiff_t, especially when the implementation provides integer-class types. However, currently some implementation can't properly handle content whose length is greater than PTRDIFF_MAX. Presumably we don't want UB in such cases. Should we explicitly allow throwing exception and/or silent truncation?

Additionally, given that the return type of std::formatted_size is std::size_t, even if the implementation supports formatted contents whose lengths are greater than SIZE_MAX, the length can't be correctly returned. Perhaps we need to either precisely specify the return value as modulo arithmetic seems undesired, or specify that an exception is thrown.

[2025-10-17; Reflector poll.]

Set priority to 3 after reflector poll.

Victor thinks it should be an exception.

Proposed resolution: