P2905R2
Runtime format strings

"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 fixes the safety issue and its companion paper [P2918] proposes direct support for runtime format strings which has been long available in the {fmt} library.

2. Problems

[P2216] "std::format improvements" introduced compile-time format string checks for std::format. 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::vformat:

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 LEWG poll results for R1.

4. Changes since R0

• Moved the API for direct runtime format string support to a separate paper [P2918] per LEWG feedback.

• Removed forwarding in print per LEWG feedback.

5. Polls

LEWG poll results for R0:

POLL: In P2905R0 split runtime_format and make_format_args (add fix for print) into separate papers, the latter being a DR against C++23.

SF  F  N  A SA
 5 10  4  0  0

Outcome: Strong Consensus in Favor

LEWG poll results for R1:

POLL: Send P2905R1 (Runtime Format Strings) to electronic balloting to be forwarded to library for C++26, with the intention to retroactively apply the paper to C++23.

SF  F  N  A SA
 4  7  0  0  0

Outcome: Unanimous consent in favor.

6. Proposal

This paper proposes changing make_format_args to take lvalue references instead of forwarding references, rejecting problematic code:

std::filesystem::path path = "path/etic/experience";
auto args = std::make_format_args(path.string()); // ill-formed

This has been implemented in {fmt} catching some bugs even though the pattern of using make_format_args has never been suggested as a way to pass runtime format strings there. If left unchanged this will be a major safety hole in the standard formatting facility.

In the standard itself make_format_args is already called with lvalue references in format, e.g. [format.functions]:

template<class... Args>
  string format(format_string<Args...> fmt, Args&&... args);

Effects: Equivalent to:

return vformat(fmt.str, make_format_args(args...));

Notice that there is intentionally no forwarding of args so the switch from forwarding to lvalue references is effectively a noop there.

There is forwarding in the definitions on recently added print functions (e.g. [print.fun]) which is unnecessary and inconsistent with format. Removing fowarding there is not observable.

7. Impact on existing code

Rejecting temporaries in make_format_args is an (intentionally) breaking change.

Searching GitHub for calls of std::make_format_args using the following query

"std::make_format_args" language:c++ -path:libstdc -path:libcxx -path:include/c++ 

returned only 844 results at the time of writing. For comparison, similar search returned 165k results for fmt::format and 7.3k for std::format. Such low usage is not very surprising because std::format is not widely available yet.

At least 452 of these call sites use make_format_args as intended and will require no changes:

std::vformat_to(std::back_inserter(c), fmt.get(), std::make_format_args(args...));

72 of remaining calls can be trivially fixed by removing unnecessary forwarding.

This leaves only 320 cases most of which will continue to work and the ones that pass temporaries can be easily fixed by either switching to std::runtime_format or by storing a temporary in a variable.

8. Wording

Change in [format.syn]:

namespace std {
  ...

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

  ...
}

Change in [format.arg.store]:

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

2 Preconditions: The type typename Context::template formatter_type<remove_cvrefconst_t<T>i> meets the BasicFormatter requirements ([formatter.requirements]) for each Ti in Args.

...

template<class... Args>
  format-arg-store<wformat_context, Args...> make_wformat_args(Args&&... args);

Change in [print.fun]:

template<class... Args>
  void print(FILE* stream, format_string<Args...> fmt, Args&&... args);

2 Effects: If the ordinary literal encoding ([lex.charset]) is UTF-8, equivalent to:

vprint_unicode(stream, fmt.str, make_format_args(std::forward<Args>(args)...args...));

Otherwise, equivalent to:

vprint_nonunicode(stream, fmt.str, make_format_args(std::forward<Args>(args)...args...));

Change in [ostream.formatted.print]:

template<class... Args>
  void print(ostream& os, format_string<Args...> fmt, Args&&... args);

1 Effects: If the ordinary literal encoding ([lex.charset]) is UTF-8, equivalent to:

vprint_unicode(os, fmt.str, make_format_args(std::forward<Args>(args)...args...));

Otherwise, equivalent to:

vprint_nonunicode(os, fmt.str, make_format_args(std::forward<Args>(args)...args...));

9. Implementation

The proposed API has been implemented in the {fmt} library ([FMT]).