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3544. format-arg-store::args is unintentionally not exposition-only

Section: 28.5.8.2 [format.arg.store] Status: C++23 Submitter: Casey Carter Opened: 2021-04-22 Last modified: 2023-11-22

Priority: 3

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Discussion:

Despite the statement in 28.5.8.3 [format.args]/1:

An instance of basic_format_args provides access to formatting arguments. Implementations should optimize the representation of basic_format_args for a small number of formatting arguments. [Note 1: For example, by storing indices of type alternatives separately from values and packing the former. — end note]

make_format_args and make_wformat_args are specified to return an object whose type is a specialization of the exposition-only class template format-arg-store which has a public non-static data member that is an array of basic_format_arg. In order to actually "optimize the representation of basic_format_args" an implementation must internally avoid using make_format_args (and make_wformat_args) and instead use a different mechanism to type-erase arguments. basic_format_args must still be convertible from format-arg-store as specified, however, so internally basic_format_args must support both the bad/slow standard mechanism and a good/fast internal-only mechanism for argument storage.

While this complicated state of affairs is technically implementable, it greatly complicates the implementation of <format> with no commensurate benefit. Indeed, naive users may make the mistake of thinking that e.g. vformat(fmt, make_format_args(args...)) is as efficient as format(fmt, args...) — that's what the "Effects: Equivalent to" in 28.5.5 [format.functions]/2 implies — and inadvertently introduce performance regressions. It would be better for both implementers and users if format-arg-store had no public data members and its member args were made exposition-only.

[2021-05-10; Reflector poll]

Priority set to 3. Tim: "The current specification of make_format_args depends on format-arg-store being an aggregate, which is no longer true with this PR."

Previous resolution [SUPERSEDED]:

This wording is relative to N4885.

1. Modify 28.5.1 [format.syn], header <format> synopsis, as indicated:

   […]
   // 28.5.8.2 [format.arg.store], class template format-arg-store
   template<class Context, class... Args> structclass format-arg-store; // exposition only
   […]
   

2. Modify 28.5.8.2 [format.arg.store] as indicated:

   namespace std {
     template<class Context, class... Args>
     structclass format-arg-store { // exposition only
       array<basic_format_arg<Context>, sizeof...(Args)> args; // exposition only
     };
   }
   

[2021-05-18; Tim updates wording.]

[2021-05-20; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 28.5.1 [format.syn], header <format> synopsis, as indicated:

   […]
   // 28.5.8.2 [format.arg.store], class template format-arg-store
   template<class Context, class... Args> structclass format-arg-store; // exposition only
   
   template<class Context = format_context, class... Args>
     format-arg-store<Context, Args...>
       make_format_args(const Args&... fmt_args);
   […]
   

2. Modify 28.5.8.2 [format.arg.store] as indicated:

   namespace std {
     template<class Context, class... Args>
     structclass format-arg-store { // exposition only
       array<basic_format_arg<Context>, sizeof...(Args)> args; // exposition only
     };
   }
   

   -1- An instance of format-arg-store stores formatting arguments.

   template<class Context = format_context, class... Args>
     format-arg-store<Context, Args...> make_format_args(const Args&... fmt_args);
   

   -2- Preconditions: The type typename Context::template formatter_type<Ti> meets the Formatter requirements (28.5.6.1 [formatter.requirements]) for each Ti in Args.

   -3- Returns: An object of type format-arg-store<Context, Args...> whose args data member is initialized with {basic_format_arg<Context>(fmt_args)...}.