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### 3651. Unspecified lifetime guarantees for the format string

Section: 20.20 [format] Status: New Submitter: Barry Revzin Opened: 2021-12-08 Last modified: 2021-12-11

Priority: 3

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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?

libfmt's implementation internally stores `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: