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.
3651. Unspecified lifetime guarantees for the format stringSection: 28.5 [format] Status: New Submitter: Barry Revzin Opened: 2021-12-08 Last modified: 2022-01-30
Priority: 3
View other active issues in [format].
View all other issues in [format].
View all issues with New status.
Discussion:
Is this program guaranteed to be valid:
#include <format>
#include <algorithm>
struct Thing { };
template <>
struct std::formatter<Thing> {
std::string_view spec;
constexpr auto parse(std::format_parse_context& ctx) {
auto end = std::find(ctx.begin(), ctx.end(), '}');
spec = std::string_view(ctx.begin(), end);
return end;
}
auto format(Thing, std::format_context& ctx) {
return std::ranges::copy(spec, ctx.out()).out;
}
};
int main() {
std::print("{:lwg issue}\n", Thing{});
}
In parse(), the formatter for Thing holds onto a string view of its specifiers. And then in format(), it just prints them. I don't think we say anywhere that this works. Does this code print "lwg issue" because there's no issue or does it print some garbage memory somewhere because there is one?
string_view's into the format string (for named argument support), which implies that it should work. But it'd be nice to come out and say that.
[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
"Presumably we need to say in [formatter.requirements] that [pc.begin(), pc.end()) is guaranteed to be a valid range until the next call to parse() or f is destroyed, whichever comes first."
Proposed resolution:
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4