Section: 22.14.6.1 [formatter.requirements] Status: New Submitter: Jiang An Opened: 2023-10-10 Last modified: 2023-10-30 17:22:20 UTC
Priority: 3
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Discussion:
Format strings need to be checkable at compile-time since P2216R3. In order to check a compile-time format string, the parse functions of the formatters need to be called. However, currently there is seemingly no requirement for the parse function to be constexpr, and hence whether a format string is correct for formatted types may be not well-defined at compile-time.
[2023-10-30; Reflector poll]
Set priority to 3 after reflector poll.
"Should be OK to provide a non-constexpr parse
if you only use it
with std::vformat
and not std::format
."
"Only the std::formatter
specializations in the standard library
should be required to have constexpr parse
."
"No need to require this, but a note with this text explaining when a call
to parse
can be a constant expression might be nice."
Proposed resolution:
This wording is relative to N4958.
Modify 22.14.6.1 [formatter.requirements], Table 73, BasicFormatter requirements [tab:formatter.basic] as indicated:
Table 73: BasicFormatter requirements [tab:formatter.basic] Expression Return type Requirement g.parse(pc) PC::iterator Parses format-spec (22.14.2 [format.string]) for type T in the
range [pc.begin(), pc.end()) until the first unmatched
character. Throws format_error unless
the whole range is parsed or the unmatched
character is }.
[Note 1: This allows formatters to emit meaningful
error messages. — end note]
Stores the parsed format specifiers in *this and
returns an iterator past the end of the parsed
range. The expression shall be a core constant expression if g,
pc.begin(), pc.end(), and lvalue-to-rvalue conversion
for each element in [pc.begin(), pc.end()) are core constant
expressions, and no exception is thrown.…