P3235R3
std::print more types faster with less memory

"No work is less work than some work" - Andrei Alexandrescu

1. Introduction

[P3107] enabled an efficient implementation of std::print and applied the optimization to fundamental and string types. The current paper applies this important optimization to the remaining standard types.

2. Changes since R2

• Added instructions to update the __cpp_lib_print feature testing macro.

• Conditionally opted in durations based on whether their Rep is opted in per LWG feedback since Rep can be a user-defined arithmetic-like type.

3. Changes since R1

• Made ranges not opt into the optimization per LEWG feedback.

• Fixed a markdown issue in wording.

4. Changes since R0

• Fixed a rendering issue in the definition of range formatter.

5. Proposal

[P3107] "Permit an efficient implementation of std::print" brought significant speedups (from 20% in the original benchmarks to 2x in [SO-LARGE-DATA]) to std::print and eliminated the need for dynamic memory allocations in the common case by enabling direct writes into the stream buffer. To expedite the adoption of the fix, [P3107] limited the scope to fundamental and string types but it is, of course, beneficial to enable this optimization for other standard types that have formatters. This was discussed in LEWG that encouraged writing such paper (for ranges):

LEWG requests for an additional paper to fix formatters for ranges

The current paper proposes opting in formatters for ranges and other standard types into this optimization.

Here is a list of standard formatters that are not yet opted into the std::print optimization.

Date and time formatters [time.syn]:

template<class Rep, class Period, class charT>
  struct formatter<chrono::duration<Rep, Period>, charT>;
template<class Duration, class charT>
  struct formatter<chrono::sys_time<Duration>, charT>;
template<class Duration, class charT>
  struct formatter<chrono::utc_time<Duration>, charT>;
template<class Duration, class charT>
  struct formatter<chrono::tai_time<Duration>, charT>;
template<class Duration, class charT>
  struct formatter<chrono::gps_time<Duration>, charT>;
template<class Duration, class charT>
  struct formatter<chrono::file_time<Duration>, charT>;
template<class Duration, class charT>
  struct formatter<chrono::local_time<Duration>, charT>;
template<class Duration, class charT>
  struct formatter<chrono::local-time-format-t<Duration>, charT>;
template<class charT> struct formatter<chrono::day, charT>;
template<class charT> struct formatter<chrono::month, charT>;
template<class charT> struct formatter<chrono::year, charT>;
template<class charT> struct formatter<chrono::weekday, charT>;
template<class charT> struct formatter<chrono::weekday_indexed, charT>;
template<class charT> struct formatter<chrono::weekday_last, charT>;
template<class charT> struct formatter<chrono::month_day, charT>;
template<class charT> struct formatter<chrono::month_day_last, charT>;
template<class charT> struct formatter<chrono::month_weekday, charT>;
template<class charT> struct formatter<chrono::month_weekday_last, charT>;
template<class charT> struct formatter<chrono::year_month, charT>;
template<class charT> struct formatter<chrono::year_month_day, charT>;
template<class charT> struct formatter<chrono::year_month_day_last, charT>;
template<class charT> struct formatter<chrono::year_month_weekday, charT>;
template<class charT> struct formatter<chrono::year_month_weekday_last, charT>;
template<class Rep, class Period, class charT>
  struct formatter<chrono::hh_mm_ss<duration<Rep, Period>>, charT>;
template<class charT> struct formatter<chrono::sys_info, charT>;
template<class charT> struct formatter<chrono::local_info, charT>;
template<class Duration, class TimeZonePtr, class charT>
  struct formatter<chrono::zoned_time<Duration, TimeZonePtr>, charT>;

Rep is an arithmetic type, Period is std::ratio<...>, Duration is std::duration<...> and charT is char or wchar_t so all chrono formatters except the one for std::zoned_time can be unconditionally opted into the optimization. The formatter for std::zoned_time can be opted in for the default TimeZonePtr (const std::chrono::time_zone*) but not arbitrary user-provided TimeZonePtr that can be potentially locking.

std::thread::id formatter [thread.thread.id]:

template<class charT> struct formatter<thread::id, charT>;

Stacktrace formatters [stacktrace.syn]:

// [stacktrace.format], formatting support
template<> struct formatter<stacktrace_entry>;
template<class Allocator> struct formatter<basic_stacktrace<Allocator>>;

std::vector<bool> formatter [vector.syn]:

// [vector.bool.fmt], formatter specialization for vector<bool>
template<class T, class charT> requires is-vector-bool-reference<T>
  struct formatter<T, charT>;

std::filesystem::path formatter added in [P2845] and, as of 14 Apr 2024, in the process of being merged into the standard draft:

// [fs.path.fmt], formatter
template<class charT> struct formatter<filesystem::path, charT>;

std::thread::id, stacktrace, std::vector<bool> and std::filesystem::path formatters don’t invoke any user code and can be opted into the optimization.

Tuple formatter [format.tuple]:

template<class charT, formattable<charT>... Ts>
struct formatter<pair-or-tuple<Ts...>, charT> {
  ...
};

The tuple formatter can be opted in if all the element formatters are opted in.

Range formatter [format.syn]:

// [format.range.fmtmap], [format.range.fmtset], [format.range.fmtstr], specializations for maps, sets, and strings
template<ranges::input_range R, class charT>
  requires (format_kind<R> != range_format::disabled) &&
           formattable<ranges::range_reference_t<R>, charT>
struct formatter<R, charT> : range-default-formatter<format_kind<R>, R, charT> { };

std::queue and std::priority_queue formatters [queue.syn]:

// [container.adaptors.format], formatter specialization for queue
template<class charT, class T, formattable<charT> Container>
  struct formatter<queue<T, Container>, charT>;

...

// [container.adaptors.format], formatter specialization for priority_queue
template<class charT, class T, formattable<charT> Container, class Compare>
  struct formatter<priority_queue<T, Container, Compare>, charT>;

std::stack formatter [stack.syn]:

// [container.adaptors.format], formatter specialization for stack
template<class charT, class T, formattable<charT> Container>
  struct formatter<stack<T, Container>, charT>;

Range and container adaptor formatters are the most interesting case because formatting requires iterating and user-defined iterators can be locking, at least in principle. None of the standard containers, ranges and container adaptors and even common concurrent containers such as concurrent_vector from [TBB] provide locking iterators. For this reason, the current paper proposes opting range and adaptor formatters into the optimization by default. Other languages such as Java (see [P3107]) and even Rust don’t try to prevent deadlocks when printing any user-defined types to a C stream and for iterators those are very unlikely. As shown in [P3107] examples of such deadlocks are pretty contrived and may indicate other issues (bugs) in the program such as incorrect lock scope.

And finally this paper proposes renaming vprint_(non)unicode and vprint_(non)unicode_locking to vprint_(non)unicode_buffered and vprint_(non)unicode respectively. The current naming is misleading because all of these functions are locking and "nonlocking" overloads confusingly call "locking" ones. In POSIX and other languages the default is locking so the new naming is more consistent with standard practice. The new naming reflects the fact that the main difference is buffering of all of the output.

6. Wording

Update the value of the feature-testing macro __cpp_lib_print to the date of adoption in [version.syn].

Modify [format.formatter.spec] as indicated:

...

The parse member functions of these formatters interpret the format specification as a std-format-spec as described in [format.string.std]. In addition, for each type T for which a formatter specialization is provided above, each of the headers provides the following specialization:

template<> inline constexpr bool enable_nonlocking_formatter_optimization<T> = true;

...

Modify [time.format] as indicated:

...

If the chrono-specs is omitted, the chrono object is formatted as if by streaming it to basic_ostringstream<charT> os with the formatting locale imbued and copying os.str() through the output iterator of the context with additional padding and adjustments as specified by the format specifiers.

[Example 3:

string s = format("{:=>8}", 42ms);      // value of s is "====42ms"

— end example]

template<class Rep, class Period>
  inline constexpr bool enable_nonlocking_formatter_optimization<
    chrono::duration<Rep, Period>> =
      enable_nonlocking_formatter_optimization<Rep>;

template<class Duration>
  inline constexpr bool enable_nonlocking_formatter_optimization<
    chrono::zoned_time<Duration, const std::chrono::time_zone*>> = true;

template<class Duration, class charT>
  struct formatter<chrono::sys_time<Duration>, charT>;

...

Modify [format.tuple] as indicated:

For each of pair and tuple, the library provides the following formatter specialization where pair-or-tuple is the name of the template:

namespace std {
  template<class charT, formattable<charT>... Ts>
  struct formatter<pair-or-tuple<Ts...>, charT> {
    ...
  };

  template<class... Ts>
    inline constexpr bool enable_nonlocking_formatter_optimization<pair-or-tuple<Ts...>> = 
      (enable_nonlocking_formatter_optimization<Ts> && ...);
}

Modify [format.syn] as indicated:

...

// [format.range.fmtmap], [format.range.fmtset], [format.range.fmtstr], specializations for maps, sets, and strings
template<ranges::input_range R, class charT>
  requires (format_kind<R> != range_format::disabled) &&
           formattable<ranges::range_reference_t<R>, charT>
struct formatter<R, charT> : range-default-formatter<format_kind<R>, R, charT> { };

template<ranges::input_range R>
    requires (format_kind<R> != range_format::disabled)
  inline constexpr bool enable_nonlocking_formatter_optimization<R> = false;

// [format.arguments], arguments
// [format.arg], class template basic_format_arg
template<class Context> class basic_format_arg;

...

Modify [print.fun] as indicated:

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

Effects: Let locksafe be (enable_nonlocking_formatter_optimization< remove_cvref_t<Args>> && ...). If the ordinary literal encoding ([lex.charset]) is UTF-8, equivalent to:

locksafe
  ? vprint_unicode_locking(stream, fmt.str, make_format_args(args...))
  : vprint_unicode_buffered(stream, fmt.str, make_format_args(args...));

Otherwise, equivalent to:

locksafe
  ? vprint_nonunicode_locking(stream, fmt.str, make_format_args(args...))
  : vprint_nonunicode_buffered(stream, fmt.str, make_format_args(args...));

...

void vprint_unicode_buffered(FILE* stream, string_view fmt, format_args args);

Effects: Equivalent to:

string out = vformat(fmt, args);
vprint_unicode_locking(stream, "{}", make_format_args(out));

void vprint_unicode_locking(FILE* stream, string_view fmt, format_args args);

...

void vprint_nonunicode_buffered(FILE* stream, string_view fmt, format_args args);

Effects: Equivalent to:

string out = vformat(fmt, args);
vprint_nonunicode_locking("{}", make_format_args(out));

void vprint_nonunicode_locking(FILE* stream, string_view fmt, format_args args);

...