"Temporary solutions often become permanent problems." — Craig Bruce
1. Introduction
[P2216] "std::format improvements" introduced compile-time format string checks which, quoting Barry Revzin, "is a fantastic feature" ([P2757]). However, due to resource constraints it didn’t provide a good API for using formatting functions with format strings not known at compile time. As a workaround one could use type-erased API which has never been designed for that. This severely undermined safety and led to poor user experience. This paper proposes direct support for runtime format strings which has been long available in the {fmt} library and its companion paper ([P2905]) fixes the safety issue.
2. Problems
[P2216] "std::format improvements" introduced compile-time format string
checks for
. This obviously requires format strings be known at
compile time. However, there are some use cases where format strings are only
known at runtime, e.g. when translated through gettext ([GETTEXT]).
One possible workaround is using type-erased formatting functions such as
:
std :: string str = translate ( "The answer is {}." ); std :: string msg = std :: vformat ( str , std :: make_format_args ( 42 ));
This is not a great user experience because the type-erased API was designed to avoid template bloat and should only be used by formatting function writers and not by end users.
Such misuse of the API also introduces major safety issues illustrated in the following example:
std :: string str = "{}" ; std :: filesystem :: path path = "path/etic/experience" ; auto args = std :: make_format_args ( path . string ()); std :: string msg = std :: vformat ( str , args );
This innocent-looking code exhibits undefined behavior because format arguments store a reference to a temporary which is destroyed before use. This has been discovered and fixed in [FMT] which now rejects such code at compile time.
3. Changes since R1
-
Added instructions to bump the
feature test macro per LWG feedback.__cpp_lib_format -
Made the exposition-only member
ofstr
private per LWG feedback.runtime - format - string -
Added a constructor for
per LWG feedback.runtime - format - string -
Made functions added in this paper
per LWG feedback.noexcept -
Immobilized
per LWG feedback.runtime - format - string
4. Changes since R0
-
Added poll results.
5. Polls
LEWG poll results:
POLL: Send P2918R1 (Runtime Format Strings II) to Library for C++26.
SF F N A SA 4 6 0 0 0
Outcome: Unanimous consent in favor.
6. Proposal
This paper proposes adding the
function to explicitly mark
a format string as a runtime one and opt out of compile-time format string
checks.
Before | After |
---|---|
|
|
This improves usability and makes the intent more explicit. It can also enable
detection of some lifetime errors for arguments ([P2418]). This API has been
available in {fmt} since
-based format string checks were introduced
~2 years ago and usage experience was very positive. In a large codebase with
> 100k calls of
only ~0.1% use
.
This was previously part of [P2905] but moved to a separate paper per LEWG feedback with the original paper focusing on the safety fix only.
7. Impact on existing code
This paper adds a new API and has no impact on existing code.
8. Wording
Update the value of the feature-testing macro
to the date of
adoption in [version.syn].
Change in [format.syn]:
namespace std { ... // [format.fmt.string], class template basic_format_string template < class charT , class ... Args > struct basic_format_string ; template < class charT > struct runtime - format - string { // exposition-only private : basic_string_view < charT > str ; // exposition-only public : runtime - format - string ( basic_string_view < charT > s ) noexcept : str ( s ) {} runtime - format - string ( const runtime - format - string & ) = delete ; const runtime - format - string & operator = ( const runtime - format - string & ) = delete ; }; runtime - format - string < char > runtime_format ( string_view fmt ) noexcept { return fmt ; } runtime - format - string < wchar_t > runtime_format ( wstring_view fmt ) noexcept { return fmt ; } ... }
Change in [format.fmt.string]:
namespace std { template < class charT , class ... Args > struct basic_format_string { private : basic_string_view < charT > str ; // exposition only public : template < class T > consteval basic_format_string ( const T & s ); basic_format_string ( runtime - format - string < charT > s ) noexcept : str ( s . str ) {} constexpr basic_string_view < charT > get () const noexcept { return str ; } }; }
9. Implementation
The proposed API has been implemented in the {fmt} library ([FMT]).
10. Acknowledgements
Thanks to Mateusz Pusz for the suggestion to pass runtime-format-string by rvalue reference that improves safety of the API.