Annex C (informative) Compatibility [diff]

C.2 C++ and ISO C++ 2020 [diff.cpp20]

C.2.10 [utilities]: general utilities library [diff.cpp20.utilities]

1
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Affected subclause: [format]
Change: Signature changes: format, format_to, vformat_to, format_to_n, formatted_size.
Removal of format_args_t.

Rationale: Improve safety via compile-time format string checks, avoid unnecessary template instantiations.

Effect on original feature: Valid C++ 2020 code that contained errors in format strings or relied on previous format string signatures or format_args_t may become ill-formed.
[Example 1: auto s = std::format("{:d}", "I am not a number"); // ill-formed, // previously threw format_error — end example]
2
#
Affected subclause: [format]
Change: Signature changes: format, format_to, format_to_n, formatted_size.

Rationale: Enable formatting of views that do not support iteration when const-qualified and that are not copyable.

Effect on original feature: Valid C++ 2020 code that passes bit-fields to formatting functions may become ill-formed.
[Example 2: struct tiny { int bit: 1; }; auto t = tiny(); std::format("{}", t.bit); // ill-formed, previously returned "0" — end example]
3
#
Affected subclause: [format.string.std]
Change: Restrict types of formatting arguments used as width or precision in a std-format-spec.

Rationale: Disallow types that do not have useful or portable semantics as a formatting width or precision.

Effect on original feature: Valid C++ 2020 code that passes a boolean or character type as arg-id becomes invalid.
[Example 3: std::format("{:*^{}}", "", true); // ill-formed, previously returned "*" std::format("{:*^{}}", "", '1'); // ill-formed, previously returned an // implementation-defined number of '*' characters — end example]
4
#
Affected subclause: [format.formatter.spec]
Change: Removed the formatter specialization: template<size_t N> struct formatter<const charT[N], charT>;
Rationale: The specialization is inconsistent with the design of formatter, which is intended to be instantiated only with cv-unqualified object types.

Effect on original feature: Valid C++ 2020 code that instantiated the removed specialization can become ill-formed.