Annex C (informative) Compatibility [diff]

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.

For example: auto s = std::format("{:d}", "I am not a number"); // ill-formed, // previously threw format_error

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.

For example: struct tiny { int bit: 1; }; auto t = tiny(); std::format("{}", t.bit); // ill-formed, previously returned "0"

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.

For example: std::format("{:*^{}}", "", true); // ill-formed, previously returned "*" std::format("{:*^{}}", "", '1'); // ill-formed, previously returned an // implementation-defined number of '*' characters

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.