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3810. CTAD for std::basic_format_args

Section: 28.5.8.3 [format.args] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-11-03 Last modified: 2023-11-22

Priority: 3

View all other issues in [format.args].

View all issues with C++23 status.

Discussion:

It seems desirable for this should work:

auto args_store = std::make_format_args<C>(1,2,3);
// …
std::basic_format_args args = args_store;

i.e. CTAD should deduce the Context argument from the fmt-store-args<C, int, int, int> object returned by make_format_args.

Another example (from Tomasz Kamiński):

Given:

template<typename Context>
void foo(basic_format_args<Context> c);

foo(make_format_args<SomeContext>(…)); // won't work
foo(basic_format_args(make_format_args<SomeContext>(…))); // should work

Since fmt-store-args is exposition-only, it's not entirely clear that it must have exactly the form shown in 28.5.8.2 [format.arg.store]. E.g. maybe it can have different template arguments, or could be a nested type defined inside basic_format_args. I don't know how much of the exposition-only spec is actually required for conformance. If CTAD is already intended to be required, it's a bit subtle.

If we want the CTAD to work (and I think it's nice if it does) we could make that explicit by adding a deduction guide.

[Kona 2022-11-12; Set priority to 3, status to LEWG]

[2023-01-10; LEWG telecon]

Unanimous consensus in favor.

[Issaquah 2023-02-06; LWG]

Unanimous consent (9/0/0) to move to Immediate for C++23.

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 28.5.8.3 [format.args] as indicated:

   namespace std {
     template<class Context>
     class basic_format_args {
       size_t size_;                           // exposition only
       const basic_format_arg<Context>* data_; // exposition only
   
     public:
       basic_format_args() noexcept;
   
       template<class... Args>
         basic_format_args(const format-arg-store<Context, Args...>& store) noexcept;
   
       basic_format_arg<Context> get(size_t i) const noexcept;
     };
     
     template<class Context, class... Args>
       basic_format_args(format-arg-store<Context, Args...>) -> basic_format_args<Context>;
   }