[P2216R3], in order to improve type safety around the
std::format API, introduced the type
std::basic-format-string<charT, Args...> as a way to check to make sure that the format string actually matches the arguments provided. The example used in that paper is:
Originally, this would throw an exception at runtime. With the change, this now is a compile error. Which is a very nice improvement - catching bugs at compile time is awesome.
Unfortunately, the ability to validate format strings at compile time is currently limited to specifically those functions provided by the standard library. In particular,
std::formatted_size. But users cannot take advantage of this in their own code.
As Joseph Thompson pointed out in [std-discussion]:
However, consider the common use case:
Without a specified
std::format_stringtype, is it possible to forward arguments to
std::formatwith compile-time format string checks? I’m thinking about this as an average user of the standard library, so I’m not looking for workarounds. Is this a defect?
Today, if users want to statically verify their format strings… what can they do? Originally, I thought that you could write a macro to wrap the call to
log, so that you can attempt to pass the expressions as they are to try to statically verify them. But this compiles just fine:
Because… of course it compiles. The way that this check works in format today is that
basic-format-string<charT, Args...> has a
consteval constructor that just isn’t a constant-expression if the value of the format string doesn’t match the arguments.
But constant-expression-ness (or, in general, values) don’t affect overload resolution, so there’s no way to check this externally (there were some papers that considered value-based constraints which would’ve let this work [P1733R0] [P2049R0], which aren’t without potential issue [P2089R0], but we don’t have something like that today).
The only way for Joseph (or you or me or …) to get this behavior in
log is to basically manually re-implement
basic-format-string. That seems like an unreasonable burden to place for such a useful facility. The standard library already has to provide it, why not provide it under a guaranteed name that we can use?
There are two reasons why
basic-format-string<charT, Args...> (and its more specialized alias templates,
wformat-string<Args...>) are exposition-only.
The first is that, as a late DR against C++20, [P2216R3] was trying to limit its scope. Totally understandable.
The second is that it is possible with future language features to be able to do this better. For example, if we had
constexpr function parameters [P1045R1] or some kind of hygienic macro facility [P1221R1], then the whole shape of this API would be different. For
constexpr function parameters, for instance, the first argument to
std::format would probably be a
constexpr std::string (or some such) and then there would just be a
consteval function you could call like
validate_format_string<Args...>(str). In which case, we could expose
validate_format_string and the implementation of
log could call that directly too. And that would be pretty nice!
But we won’t have
constexpr function parameters or a hygienic macros in C++23, so the solution that we have to this problem today is
basic-format-string<charT, Args...>. Since we have this solution, and it works, we shouldn’t just restrict it to be used by the standard library internally. If eventually we adopt a better way of doing this checking, we can expose that too. Better give users a useful facility today that’s possibly worse than a facility we might be able to provide in C++26, rather than having them have to manually write their own version (which is certainly doable, though tedious and error-prone).
This doesn’t strictly have to be a DR, and could certainly just be a C++23 feature. Although it would be nice to have sooner rather than later.
Even if an implementation already ships with a
std::_Ugly::_Basic_Format__Construct__Additional__Underscores<charT, Args...> (which Microsoft is close to doing, with a slightly more reasonable ugly name), they can still provide the new names (
std::wformat_string) without having to worry about ABI, since these are templates. Or we could ask Charlie very nicely to simply rename
basic_format_string as that PR is going out the door.
Either way, it would be nice to avoid specifying
std::basic_format_string as an alias to
std::basic-format-string (which, technically, is an option, and obviously has no ABI impact or take much implementation effort at all). That would be weird, but probably not even particularly notably weird. Note that specifying
std::basic_format_string as a class template does prohibit implementing it as an alias template to
std::_Basic_format_string, as this difference is observable.
I leave it up to the discretion of LEWG, Charlie, and the ghost of C++20 Past whether or not we ever actually want to declare C++20 complete.
In 20.20.1 [format.syn], replace the exposition-only names
wformat-string with the non-exposition-only names
In 20.20.4 [format.fmt.string], the member should still be exposition only. The full subclause should now read (the only change is the name of the class template, which no longer is exposition only):
2 Effects: Direct-non-list-initializes
3 Remarks: A call to this function is not a core constant expression ([expr.const]) unless there exist
stris a format string for
It is definitely important to provide a feature-test macro for this, which will allow
log above to conditionally opt in to this feature if possible:
#if __cpp_lib_format >= whatever template <typename... Args> using my_format_string = std::format_string<std::type_identity_t<Args>...>; #else template <typename... Args> using my_format_string = std::string_view; #endif template <typename... Args> void log(my_format_string<Args...> s, Args&&... args);
format feature-test macro in 17.3.2 [version.syn]:
[P1045R1] David Stone. 2019-09-27. constexpr Function Parameters.
[P1221R1] Jason Rice. 2018-10-03. Parametric Expressions.
[P1733R0] David Sankel, Daveed Vandevoorde. 2019-06-17. User-friendly and Evolution-friendly Reflection: A Compromise.
[P2049R0] Andrew Sutton, Wyatt Childers. 2020-01-13. Constraint refinement for special-cased functions.
[P2089R0] Barry Revzin. 2020-02-17. Function parameter constraints are too fragile.
[P2216R3] Victor Zverovich. 2021-02-15. std::format improvements.
[std-discussion] Joseph Thomson. 2021. Should a
std::basic_format_string be specified?