Text Formatting

Revision History

Changes since R6

Rebase wording onto the pre-Kona C++ working draft N4800.
Use placeholder for section and table numbers and drop "utilities" from the section name.
Clarify the parse wording in Table � — Formatter requirements.
Add an entry to the "General utilities library summary" table.
Replace overload resolution Remarks with Constraints per p1369.
Remove "shall" from Expects wording.
Add Kona LEWG poll results for R6.
Minor improvements to the wording and editing notes.

Changes since R5

Separate parsing context from formatting context and remove basic_format_context::parse_context().
Require that id in basic_format_parse_context::check_arg_id is a valid argument id.
Replace Requires with Expects per p1369.

Remove basic_format_context<>::parse_context().

SF	F	N	A	SA
7	2	0	0	0

Unanimous consent to all other design changes presented.

Forward back to LWG as written for C++20.

SF	F	N	A	SA
8	4	0	0	0

Changes since R4

Clarify that s in basic_format_arg(const char_type* s) shall point to a NTCTS.
Remove is_arithmetic and is_integral.
Clarify that the formatter output should be deterministic in Table � — Formatter requirements.
Minor wording corrections.

Changes since R3

Use OutputIterator concept in formatting functions that take output iterators and replace the Size template parameter with iter_difference_t.
Rename *parse_context to *format_parse_context.
Disallow consecutive zeros in arg-id and width.
Specify formatting of arithmetic and pointer types in terms of to_chars.
Clarify what is the current locale.
Use the shortest round-trip format as the default floating-point formatting.
Add missing specializations of formatter.
Clarify that the sign option applies to floating-point infinity and NaN.
Propose adding a new entry <format> to table 18 in section 15.5.1.2 [headers].
Add section numbers in Proposed wording.
Add a feature test macro.
Remove tautological Ensures clauses.
Merge two make_format_args overloads.
Replace args() with arg(size_t) in basic_format_context.
Make format_arg_store exposition-only.

Changes since R2

Add section Argument visitation with an example of how to use the visitation API to implement dynamic format specifiers.
Rename visit to visit_format_arg to distinguish from variant's visit.
Merge [format.syntax] into [format.functions].
Remove the restriction that '\0' cannot be used as a fill character.
Replace Postconditions with Ensures.

Changes since R1

Add the format_to_n function taking an output iterator and a size.
Add a note that compile-time processing of format strings applies to user-defined types with custom parsers to section Compile-time processing of format strings.
Rename count to formatted_size.
Drop nested namespace fmt.
Add format prefix or infix to class and function names to avoid potential name collision after removing the nested namespace.
Improve wording.
Replace the requirement of implementing a formatter via ostream insertion operator if the latter is provided with a note that it can be implemented in such way.
Expand the Acknowledgements section and remove "Reply to".

Changes since R0

Add section Compile-time processing of format strings.
Separate parsing and formatting in the extension API replacing format_value function template with class template formatter to allow compile-time processing of format strings.
Change return type of format_to and vformat_to to OutputIterator in synopsis.
Remove sections Null-terminated string view and Format string, and replace basic_cstring_view with basic_string_view.
Add a link to the implementation in Introduction.
Add a note regarding time formatting and compatibility with D0355 "Extending <chrono> to Calendars and Time Zones" [16] to section Extensibility.
Rename basic_args to basic_format_args.
Rename is_numeric to is_arithmetic.
Add the count function that counts the number of characters and use it to define output ranges.
Remove basic_buffer and section Formatting buffer and replace buffers with output iterators.
Add Appendix A: Benchmarks.
Explain the purpose of the type-erased API in more details in the Binary footprint section.
Add Appendix B: Binary code comparison.
Add formatting function overloads for wchar_t strings.

Even with proliferation of graphical and voice user interfaces, text remains one of the main ways for humans to interact with computer programs and programming languages provide a variety of methods to perform text formatting. The first thing we do when learning a new programming language is often write a "Hello, World!" program that performs simple formatted output.

C++ has not one but two standard APIs for producing formatted output, the printf family of functions inherited from C and the I/O streams library (iostreams). While iostreams are usually the recommended way of producing formatted output in C++ for safety and extensibility reasons, printf offers some advantages, such as an arguably more natural function call API, the separation of formatted message and arguments, possibly with argument reordering as a POSIX extension, and often more compact source and binary code.

This paper proposes a new text formatting library that can be used as a safe and extensible alternative to the printf family of functions. It is intended to complement the existing C++ I/O streams library and reuse some of its infrastructure such as overloaded insertion operators for user-defined types.

Example:

string message = format("The answer is {}.", 42);

A full implementation of this proposal is available at https://github.com/fmtlib/fmt/tree/std.

Design

Format string syntax

Variations of the printf format string syntax are arguably the most popular among the programming languages and C++ itself inherits printf from C [1]. The advantage of the printf syntax is that many programmers are familiar with it. However, in its current form it has a number of issues:

Many format specifiers like hh, h, l, j, etc. are used only to convey type information. They are redundant in type-safe formatting and would unnecessarily complicate specification and parsing.
There is no standard way to extend the syntax for user-defined types.
There are subtle differences between different implementations. For example, POSIX positional arguments [2] are not supported on some systems [6].
Using '%' in a custom format specifier poses difficulties, e.g. for put_time-like time formatting.

Although it is possible to address these issues while maintaining resemblance to the original printf format, this will still break compatibility and can potentially be more confusing to users than introducing a different syntax.

Therefore we propose a new syntax based on the ones used in Python [3], the .NET family of languages [4], and Rust [5]. This syntax employs '{' and '}' as replacement field delimiters instead of '%' and it is described in detail in [format.functions]. Some advantages of the proposal are:

A consistent and easy to parse mini-language focused on formatting rather than conveying type information
Extensibility and support for custom format strings for user-defined types
Positional arguments
Support for both locale-specific and locale-independent formatting (see Locale support)
Formatting improvements such as better alignment control, fill character, and binary representation

The syntax is expressive enough to enable translation, possibly automated, of most printf format strings. The correspondence between printf and the new syntax is given in the following table:

printf	new
-	<
+	+
space	space
#	#
0	0
hh	unused
h	unused
l	unused
ll	unused
j	unused
z	unused
t	unused
L	unused
c	c (optional)
s	s (optional)
d	d (optional)
i	d (optional)
o	o
x	x
X	X
u	d (optional)
f	f
F	F
e	e
E	E
a	a
A	A
g	g (optional)
G	G
n	unused
p	p (optional)

Width and precision are represented similarly in printf and the proposed syntax with the only difference that runtime value is specified by '*' in the former and '{}' in the latter, possibly with the index of the argument inside the braces:

printf("%*s", 10, "foo");
format("{:{}}", "foo", 10);

As can be seen from the table above, most of the specifiers remain the same which simplifies migration from printf. A notable difference is in the alignment specification. The proposed syntax allows left, center, and right alignment represented by '<', '^', and '>' respectively which is more expressive than the corresponding printf syntax. The latter only supports left and right alignment.

The following example uses center alignment and '*' as a fill character:

format("{:*^30}", "centered");

resulting in "***********centered***********". The same formatting cannot be easily achieved with printf.

In addition to positional arguments, the grammar can be easily extended to support named arguments.

Extensibility

Both the format string syntax and the API are designed with extensibility in mind. The mini-language can be extended for user-defined types and users can provide functions that implement parsing, possibly at compile time, and formatting for such types.

The general syntax of a replacement field in a format string is

replacement-field ::= '{' [arg-id] [':' format-spec] '}'

where format-spec is predefined for built-in types, but can be customized for user-defined types. For example, the syntax can be extended for put_time-like date and time formatting

time_t t = time(nullptr);
string date = format("The date is {0:%Y-%m-%d}.", *localtime(&t));

by providing a specialization of formatter for tm:

template<>
  struct formatter<tm> {
    constexpr format_parse_context::iterator parse(format_parse_context& ctx);

    template<class FormatContext>
      typename FormatContext::iterator format(const tm& tm, FormatContext& ctx);
  };

The formatter<tm>::parse function parses the format-spec portion of the format string corresponding to the current argument and formatter<tm>::format formats the value and writes the output via the iterator ctx.begin().

Note that date and time formatting is not covered by this proposal but formatting facilities provided by D0355 "Extending <chrono> to Calendars and Time Zones" [16] can be easily implemented using this extension API.

An implementation of formatter<T>::format can use ostream insertion operator<< for user-defined type T if available.

The extension API is based on specialization instead of the argument-dependent lookup (ADL), because the parse function doesn't take the object to be formatted as an argument and therefore some other way of parameterizing it on the argument type T such as introducing a dummy argument has to be used, e.g.

constexpr auto parse(type<T>, format_parse_context& ctx);

Another problem with ADL-based approach is compile-time performance as pointed out in [20]:

Overload resolution on operator<< tends to get expensive for larger projects with hundreds or thousands of candidates in the overload set. This seems hard to resolve, since choosing a different name for operator<< simply shifts the expense of overload resolution to a differently-named function.

Safety

Formatting functions rely on variadic templates instead of the mechanism provided by <cstdarg>. The type information is captured automatically and passed to formatters guaranteeing type safety and making many of the printf specifiers redundant (see Format String Syntax). Memory management is automatic to prevent buffer overflow errors common to printf.

Locale support

As pointed out in P0067 "Elementary string conversions"[17] there is a number of use cases that do not require internationalization support, but do require high throughput when produced by a server. These include various text-based interchange formats such as JSON or XML. The need for locale-independent functions for conversions between integers and strings and between floating-point numbers and strings has also been highlighted in [20]. Therefore a user should be able to easily control whether to use locales or not during formatting.

We follow Python's approach [3] and designate a separate format specifier 'n' for locale-aware numeric formatting. It applies to all integral and floating-point types. All other specifiers produce output unaffected by locale settings. This can also have positive effect on performance because locale-independent formatting can be implemented more efficiently.

Positional arguments

An important feature for localization is the ability to rearrange formatting arguments as the word order may vary in different languages [7]. For example:

printf("String `%s' has %d characters\n", string, length(string)));

A possible German translation of the format string might be:

"%2$d Zeichen lang ist die Zeichenkette `%1$s'\n"

using POSIX positional arguments [2]. Unfortunately these positional specifiers are not portable [6]. The C++ I/O streams don't support such rearranging of arguments by design because they are interleaved with the portions of the literal string:

cout << "String `" << string << "' has " << length(string) << " characters\n";

The current proposal allows both positional and automatically numbered arguments, for example:

format("String `{}' has {} characters\n", string, length(string)));

with the German translation of the format string being:

"{1} Zeichen lang ist die Zeichenkette `{0}'\n"

Performance

The formatting library has been designed with performance in mind. It tries to minimize the number of virtual function calls and dynamic memory allocations done per formatting operation. In particular, if formatting output can fit into a fixed-size array allocated on stack, it should be possible to avoid both of them altogether by using a suitable API.

The format_to function takes an arbitrary output iterator and, for performance reasons, can be specialized for random-access and contiguous iterators as shown in the reference implementation [14].

The locale-independent formatting can also be implemented more efficiently than the locale-aware one. However, the main goal for the former is to support specific use cases (see Locale support) rather than to improve performance.

See Appendix A: Benchmarks for a small performance comparison of the reference implementation of this proposal versus the standard formatting facilities.

Binary footprint

In order to minimize binary code size each formatting function that uses variadic templates can be implemented as a small inline wrapper around its non-variadic counterpart. This wrapper creates a basic_format_args object, representing an array of type-erased argument references, with make_format_args and calls the non-variadic function to do the actual work. For example, the format variadic function calls vformat:

string vformat(string_view fmt, format_args args);

template<class... Args>
  inline string format(string_view fmt, const Args&... args) {
    return vformat(fmt, make_format_args(args...));
  }

basic_format_args can be implemented as an array of tagged unions. If the number of arguments is small then the tags that indicate the arguments types can be combined and passed into a formatting function as a single integer. This single integer representing all argument types is computed at compile time and only needs to be stored, resulting in smaller binary code.

Given a reasonable optimizing compiler, this will result in a compact per-call binary code, effectively consisting of placing argument pointers (or, possibly, copies for primitive types) and packed tags on the stack and calling a formatting function. See Appendix B: Binary code comparison for a specific example.

Exposing the type-erased API rather than making it an implementation detail and only providing variadic functions allows applying the same technique to the user code. For example, consider a simple logging function that writes a formatted error message to clog:

template<class... Args>
  void log_error(int code, string_view fmt, const Args&... args) {
    clog << "Error " << code << ": " << format(fmt, args...);
  }

The code for this function will be generated for every combination of argument types which can be undesirable. However, if we use the type-erased API, there will be only one instance of the logging function while the wrapper function can be trivially inlined:

void vlog_error(int code, string_view fmt, format_args args) {
  clog << "Error " << code << ": " << vformat(fmt, args);
}

template<class... Args>
  inline void log_error(int code, string_view fmt, const Args&... args) {
    vlog_error(code, fmt, make_format_args(args...));
  }

The current design allows users to easily switch between the two approaches.

For similar reasons formatting functions take format strings as instances of basic_string_view instead of being parameterized on a string type. This allows implicit conversions from null-terminated character strings and basic_string without affecting code size.

Format strings and `char_traits`

Format strings intentionally use the default character traits in the formatting functions API because making traits customizable creates potential problems with handling format specifications.

Consider the following example:

using ci_string_view = std::basic_string_view<char, ci_char_traits>;
auto s = format(ci_string_view("{:X}"), 0xCAFE);

where ci_char_traits is a character trait with case-insensitive comparison.

Since the format specification grammar is case-sensitive, the behavior of such code is no longer intuitive. There are several options:

ill-formed (current proposal),
well-formed and s == "cafe",
well-formed and s == "CAFE".

Option 2 is a potential pessimization because comparisons will have to go through traits while option 3 effectively ignores traits.

Existing standard functions that take format strings (printf, put_time, strftime) do not allow passing character traits.

Compile-time processing of format strings

It is possible to parse format strings at compile time with constexpr functions which has been demonstrated in the reference implementation [14] and in [18]. Unfortunately a satisfactory API cannot be provided using existing C++17 features. Ideally we would like to use a function call API similar to the one proposed in this paper:

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

where String is some type representing a compile-time format string, for example

struct MyString {
  static constexpr string_view value() { return string_view("{}", 2); }
};

However, requiring a user to create a new type either manually or via a macro for every format string is not acceptable. P0424R1 "Reconsidering literal operator templates for strings" [15] provides a solution to this problem based on user-defined literal operators. If this or other similar proposal for compile-time strings is accepted into the standard, it should be easy to provide additional formatting APIs that make use of this feature. Obviously runtime checks will still be needed in cases where the format string is not known at compile time, but as shown in Appendix A: Benchmarks even with runtime parsing performance can be on par with or better than that of existing methods.

Compile-time processing of format strings can work with a user-defined type T if the formatter<T>::parse function is constexpr.

Argument visitation

The argument visitation API can be used to implement dynamic format specifiers, for example, width and precision passed as additional formatting arguments as opposed to being encoded in the format string itself:

int width = 10;
int precision = 3;
auto s = format("{0:{1}.{2}f}", 12.345678, width, precision);
// s == "    12.346"

An example of a user-defined formatter with dynamic width:

struct Answer {};

template<>
  struct formatter<Answer> {
    int width_arg_index = 0;

    // Parses dynamic width in the format "{<digit>}".
    auto parse(format_parse_context& parse_ctx) {
      auto iter = parse_ctx.begin();
      auto get_char = [&]() { return iter != parse_ctx.end() ? *iter : 0; };

      if (get_char() != '{')
        return iter;
      ++iter;
      char c = get_char();
      if (!std::isdigit(c) || (++iter, get_char()) != '}')
        throw format_error("invalid format");
      width_arg_index = c - '0';
      return ++iter;
    }

    auto format(Answer, format_context& format_ctx) {
      auto arg = format_ctx.arg(width_arg_index);
      int width = visit_format_arg([](auto value) -> int {
        if constexpr (!std::is_integral_v<decltype(value)>)
          throw format_error("width is not integral");
        else if (value < 0 || value > std::numeric_limits<int>::max())
          throw format_error("invalid width");
        else
          return value;
        }, arg);
      return format_to(format_ctx.out(), "{:{}}", 42, width);
    }
  };

std::string s = format("{0:{1}}", Answer(), 10);
// s == "        42"

In many cases formatting can be delegated to standard formatters which makes manual handling of dynamic specifiers unnecessary, but the latter is still important for more complex cases.

This API can also be used to implement a custom formatting engine, such as the one compatible with the printf syntax, to provide some of the benefits of the current proposal to the legacy code.

Impact on existing code

The proposed formatting API is defined in the new header <format> and should have no impact on existing code.

Proposed wording

Editing notes

All wording is relative to the pre-Kona C++ working draft N4800.

The � character is used to denote a placeholder section number, table number, or paragraph number which the editor shall determine.

Add the header <format> to the "C++ library headers" table in [headers], in a place that respects the table’s current alphabetic order.

Add an entry for __cpp_lib_format to the "Standard library feature-test macros" table in [support.limits.general], in a place that respects the table’s current alphabetic order:

Macro name Value Header(s)

… … …

__cpp_lib_format 201902L ** placeholder ** <format>

… … …

Macro name	Value	Header(s)
…	…	…
__cpp_lib_format	201902L placeholder	<format>
…	…	…

Add a new entry to the "General utilities library summary" table in [utilities.general]:

Subclause Header(s)

… …

19.� Formatting <format>

… …

Subclause	Header(s)
…	…
19.� Formatting	<format>
…	…

Add a new section in 19 [utilities].

19.� Formatting [format]

19.�.1 Header `<format>` synopsis [format.syn]

namespace std {
  // [format.error], class format_error
  class format_error;

  // [format.formatter], formatter
  template<class charT> class basic_format_parse_context;
  using format_parse_context = basic_format_parse_context<char>;
  using wformat_parse_context = basic_format_parse_context<wchar_t>;
  
  template<class O, class charT> requires OutputIterator<O, const charT&>
    class basic_format_context;
  using format_context = basic_format_context<unspecified, char>;
  using wformat_context = basic_format_context<unspecified, wchar_t>;

  template<class T, class charT = char> struct formatter;
  
  // [format.arguments], arguments
  template<class Context> class basic_format_arg;

  template<class Visitor, class Context>
    see below visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);

  template<class Context, class... Args> struct format_arg_store; // exposition only

  template<class Context> class basic_format_args;
  using format_args = basic_format_args<format_context>;
  using wformat_args = basic_format_args<wformat_context>;

  template<class O, class charT>
    using format_args_t = basic_format_args<basic_format_context<O, charT>>;

  template<class Context = format_context, class... Args>
    format_arg_store<Context, Args...>
      make_format_args(const Args&... args);
  template<class... Args>
    format_arg_store<wformat_context, Args...>
      make_wformat_args(const Args&... args);

  // [format.functions], formatting functions
  template<class... Args>
    string format(string_view fmt, const Args&... args);
  template<class... Args>
    wstring format(wstring_view fmt, const Args&... args);

  string vformat(string_view fmt, format_args args);
  wstring vformat(wstring_view fmt, wformat_args args);

  template<OutputIterator<const char&> O, class... Args>
    O format_to(O out, string_view fmt, const Args&... args);
  template<OutputIterator<const wchar_t&> O, class... Args>
    O format_to(O out, wstring_view fmt, const Args&... args);

  template<OutputIterator<const char&> O>
    O vformat_to(O out, string_view fmt, format_args_t<O, char> args);
  template<OutputIterator<const wchar_t&> O>
    O vformat_to(O out, wstring_view fmt, format_args_t<O, wchar_t> args);

  template<class O>
    struct format_to_n_result {
      O out;
      iter_difference_t<O> size;
    };
  
  template<OutputIterator<const char&> O, class... Args>
    format_to_n_result<O> format_to_n(O out, iter_difference_t<O> n,
                                      string_view fmt, const Args&... args);
  template<OutputIterator<const wchar_t&> O, class... Args>
    format_to_n_result<O> format_to_n(O out, iter_difference_t<O> n,
                                      wstring_view fmt, const Args&... args);

  template<class... Args>
    size_t formatted_size(string_view fmt, const Args&... args);
  template<class... Args>
    size_t formatted_size(wstring_view fmt, const Args&... args);
}

19.�.2 Formatting functions [format.functions]

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

Returns: vformat(fmt, make_format_args(args...)).

template<class... Args> wstring format(wstring_view fmt, const Args&... args);

Returns: vformat(fmt, make_wformat_args(args...)).

string vformat(string_view fmt, format_args args); wstring vformat(wstring_view fmt, wformat_args args);

Returns: A string object holding the character representation of formatting arguments provided by args formatted according to specifications given in fmt.

Throws: format_error if fmt is not a valid format string.

template<OutputIterator<const char&> O, class... Args> O format_to(O out, string_view fmt, const Args&... args);

Returns: vformat_to(out, fmt, {make_format_args<basic_format_context<O, char>>(args...)}).

template<OutputIterator<const wchar_t&> O, class... Args> O format_to(O out, wstring_view fmt, const Args&... args);

Returns: vformat_to(out, fmt, {make_format_args<basic_format_context<O, wchar_t>>(args...)}).

template<OutputIterator<const char&> O> O vformat_to(O out, string_view fmt, format_args_t<O, char> args); template<OutputIterator<const wchar_t&> O> O vformat_to(O out, wstring_view fmt, format_args_t<O, wchar_t> args);

Effects: Places the character representation of formatting arguments provided by args formatted according to specifications given in fmt into the range [out, out + N), where N is the formatted output size.

Returns: out + N.

Throws: format_error if fmt is not a valid format string.

template<OutputIterator<const char&> O, class... Args> format_to_n_result<O> format_to_n(O out, iter_difference_t<O> n, string_view fmt, const Args&... args); template<OutputIterator<const wchar_t&> O, class... Args> format_to_n_result<O> format_to_n(O out, iter_difference_t<O> n, wstring_view fmt, const Args&... args);

Let N be the formatted output size and N be min(max(n, 0), N).

Effects: Places the character representation of formatting arguments provided by args formatted according to specifications given in fmt into the range [out, out + M).

Returns: {out + M, N}.

Throws: format_error if fmt is not a valid format string.

template<class... Args> size_t formatted_size(string_view fmt, const Args&... args); template<class... Args> size_t formatted_size(wstring_view fmt, const Args&... args);

Returns: The number of characters in the character representation of formatting arguments args formatted according to specifications given in fmt.

Throws: format_error if fmt is not a valid format string.

The fmt string consists of zero or more replacement fields, escape sequences, and other characters. Each character that is not part of a replacement field or an escape sequence is copied unchanged to the output. An escape sequence is one of {{ or }}. It is replaced with { or } respectively in the output. The syntax of replacement fields is as follows:

replacement-field ::= '{' [arg-id] [':' format-spec] '}'
arg-id            ::= '0' | nonzero-digit [integer]
integer           ::= digit [integer]
nonzero-digit     ::= '1'...'9'
digit             ::= '0'...'9'

The arg-id field specifies the index of the argument in args whose value is to be formatted and inserted into the output instead of the replacement field. The optional format-spec field specifies a non-default format for the replacement value.

[ Example:

  string s = format("{0}-{{", 8); // s == "8-{"

— end example ]

If the numeric arg-ids in a format string are 0, 1, 2, ... in sequence, they can all be omitted (not just some) and the numbers 0, 1, 2, ... will be automatically used in that order. A format string is invalid if it contains a mix of automatic and manual indexing.

[ Example:

  string s0 = format("{} to {}",   "a", "b"); // OK: automatic indexing
  string s1 = format("{1} to {0}", "a", "b"); // OK: manual indexing
  string s2 = format("{0} to {}",  "a", "b"); // Error: mixing automatic and manual indexing
  string s3 = format("{} to {1}",  "a", "b"); // Error: mixing automatic and manual indexing

— end example ]

The format-spec field contains format specifications that define how the value should be presented, including such details as field width, alignment, padding, and decimal precision. Each type can define its own formatting mini-language or interpretation of the format-spec field. The syntax of format specifications is as follows:

format-spec     ::= std-format-spec | custom-format-spec
std-format-spec ::= [[fill] align] [sign] ['#'] ['0'] [width] ['.' precision] [type]
fill            ::= <a character other than '{' or '}'>
align           ::= '<' | '>' | '=' | '^'
sign            ::= '+' | '-' | ' '
width           ::= nonzero-digit [integer] | '{' arg-id '}'
precision       ::= integer | '{' arg-id '}'
type            ::= 'a' | 'A' | 'b' | 'B' | 'c' | 'd' | 'e' | 'E' | 'f' | 'F' |
                    'g' | 'G' | 'n' | 'o' | 'p' | 's' | 'x' | 'X'

where std-format-spec defines a common formatting mini-language supported by fundamental and string types, while custom-format-spec is a placeholder for user-defined mini-languages. Some of the formatting options are only supported by arithmetic types.

The fill character can be any character other than '{' or '}'. The presence of a fill character is signaled by the character following it, which must be one of the alignment options. If the second character of std-format-spec is not a valid alignment option, then it is assumed that both the fill character and the alignment option are absent.

Let charT be decltype(fmt)::value_type.

The meaning of the various alignment options is as follows:

Option	Meaning
`'<'`	Forces the field to be left-aligned within the available space. This is the default for non-arithmetic types, `charT`, and `bool`, unless an integer presentation type is specified.
`'>'`	Forces the field to be right-aligned within the available space. This is the default for arithmetic types other than `charT` and `bool` or when an integer presentation type is specified.
`'='`	Forces the padding to be placed after the sign (if any) but before the digits. This is used for printing fields in the form `+000000120`. This alignment option is only valid for arithmetic types other than `charT` and `bool` or when an integer presentation type is specified.
`'^'`	Forces the field to be centered within the available space by inserting `N / 2` and `N - N / 2` fill characters before and after the value respectively, where `N` is the total number of fill characters to insert.

[ Example:

  char c = 120;
  string s0 = format("{:6}", 42);      // s0 == "    42"
  string s1 = format("{:6}", 'x');     // s1 == "x     "
  string s2 = format("{:*<6}", 'x');   // s2 == "x*****"
  string s3 = format("{:*>6}", 'x');   // s3 == "*****x"
  string s4 = format("{:*^6}", 'x');   // s4 == "**x***"
  string s5 = format("{:=6}", 'x');    // Error: '=' with charT and no integer presentation type
  string s6 = format("{:6d}", c);      // s6 == "   120"
  string s7 = format("{:=+06d}", c);   // s7 == "+00120"
  string s8 = format("{:6}", true);    // s8 == "true  "

— end example ]

Unless a minimum field width is defined, the field width will be determined by the size of the content and the alignment option will have no effect.

The sign option is only valid for arithmetic types other than charT and bool or when an integer presentation type is specified. It can be one of the following:

Option	Meaning
`'+'`	Indicates that a sign should be used for both non-negative and negative numbers.
`'-'`	Indicates that a sign should be used only for negative numbers (this is the default behavior).
space	Indicates that a leading space should be used for non-negative numbers, and a minus sign for negative numbers.

The sign option applies to floating-point infinity and NaN.

[ Example:

  double inf = std::numeric_limits<double>::infinity();
  double nan = std::numeric_limits<double>::quiet_NaN();
  string s0 = format("{0:} {0:+} {0:-} {0: }", 1);   // s0 == "1 +1 1  1"
  string s1 = format("{0:} {0:+} {0:-} {0: }", -1);  // s1 == "-1 -1 -1 -1"
  string s2 = format("{0:} {0:+} {0:-} {0: }", inf); // s2 == "inf +inf inf  inf"
  string s3 = format("{0:} {0:+} {0:-} {0: }", nan); // s3 == "nan +nan nan  nan"

— end example ]

The '#' option causes the alternate form to be used for the conversion. This option is only valid for arithmetic types other than charT and bool or when an integer presentation type is specified. For integers, when binary, octal, or hexadecimal output is used, this option adds the respective prefix "0b" ("0B"), "0", or "0x" ("0X") to the output value. Whether the prefix is lower-case or upper-case is determined by the case of the type format specifier. For floating-point numbers the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it. Normally, a decimal-point character appears in the result of these conversions only if a digit follows it. In addition, for 'g' and 'G' conversions, trailing zeros are not removed from the result.

width is a decimal integer defining the minimum field width. If not specified, then the field width will be determined by the content.

Preceding the width field by a zero ('0') character enables sign-aware zero-padding for arithmetic types. This is equivalent to a fill character of '0' with an alignment type of '='.

The precision field is a decimal integer defining the precision or maximum field size. It can only be used with floating-point and string types. For floating-point types this field specifies the formatting precision. For string types it specifies how many characters will be used from the string.

Finally, the type determines how the data should be presented.

The available string presentation types are:

Type	Meaning
`'s'`	Copies the string to the output.
none	The same as `'s'`.

The available charT presentation types are:

Type	Meaning
`'c'`	Copies the character to the output.
none	The same as `'c'`.

Formatting of objects of arithmetic types and const void* is done as if by calling to_chars and copying the output through the output iterator of the format context with additional padding and adjustments as per format specifiers.

Let [first, last) be a range large enough to hold the to_chars output and value be the formatting argument value.

The available integer presentation types and their mapping to to_chars are:

Type	Meaning
`'b'`	`to_chars(first, last, value, 2)`; using the `'#'` option with this type adds the prefix `"0b"` to the output.
`'B'`	The same as `'b'`, except that the `'#'` option adds the prefix `"0B"` to the output.
`'d'`	`to_chars(first, last, value)`.
`'o'`	`to_chars(first, last, value, 8)`; using the `'#'` option with this type adds the prefix `"0"` to the output.
`'x'`	`to_chars(first, last, value, 16)`; using the `'#'` option with this type adds the prefix `"0x"` to the output.
`'X'`	The same as `'x'`, except that it uses uppercase letters for digits above 9 and the `'#'` option adds the prefix `"0X"` to the output.
`'n'`	The same as `'d'`, except that it uses the current global locale to insert the appropriate number separator characters.
none	The same as `'d'` if the formatting argument type is not `charT` or `bool`.

Integer presentation types can also be used with charT and bool values. Values of type bool are formatted using textual representation, either "true" or "false", if the presentation type is not specified.

[ Example:

  string s0 = format("{}", 42);                      // s0 == "42"
  string s1 = format("{0:b} {0:d} {0:o} {0:x}", 42); // s1 == "101010 42 52 2a"
  string s2 = format("{0:#x} {0:#X}", 42);           // s2 == "0x2a 0X2A"
  string s3 = format("{:n}", 1234);                  // s3 == "1,234" (depends on the locale)

— end example ]

The available floating-point presentation types and their mapping to to_chars are:

Type	Meaning
`'a'`	`to_chars(first, last, value, chars_format::hex, precision)` if `precision` is specified, `to_chars(first, last, value, chars_format::hex)` otherwise.
`'A'`	The same as `'a'`, except that it uses uppercase letters for digits above 9, `"P"` to indicate the exponent, `"INF"` for infinity, and `"NAN"` for NaN.
`'e'`	`to_chars(first, last, value, chars_format::scientific, precision)`; `precision` defaults to 6 if not specified.
`'E'`	The same as `'e'`, except that it uses `"E"` to indicate exponent, `"INF"` for infinity, and `"NAN"` for NaN.
`'f'`	`to_chars(first, last, value, chars_format::fixed, precision)`; `precision` defaults to 6 if not specified.
`'F'`	The same as `'f'`, except that it uses `"INF"` for infinity, and `"NAN"` for NaN.
`'g'`	`to_chars(first, last, value, chars_format::general, precision)`; `precision` defaults to 6 if not specified.
`'G'`	The same as `'g'`, except that it uses `"E"` to indicate exponent, `"INF"` for infinity, and `"NAN"` for NaN.
`'n'`	The same as `'g'`, except that it uses the current global locale to insert the appropriate number separator characters.
none	`to_chars(first, last, value, chars_format::general, precision)` if `precision` is specified, `to_chars(first, last, value)` otherwise.

The available pointer presentation types and their mapping to to_chars are:

Type	Meaning
`'p'`	`to_chars(first, last, reinterpret_cast<uintptr_t>(value), 16)` with the prefix `"0x"` added to the output.
none	The same as `'p'`.

A format string that doesn't conform to the current specification is invalid.

19.�.3 Formatter [format.formatter]

The functions defined in [format.functions] use specializations of the class template formatter to format individual arguments.

Each specialization of formatter is either enabled or disabled, as described below. [ Note: Enabled specializations meet the Formatter requirements, and disabled specializations do not. — end note ] Each header that declares the template formatter provides enabled specializations

template<> struct formatter<charT, charT>;

template<> struct formatter<char, wchar_t>;

template<> struct formatter<charT*, charT>;

template<> struct formatter<const charT*, charT>;

template<size_t N> struct formatter<const charT[N], charT>;

template<class traits, class Allocator>
  struct formatter<std::basic_string<charT, traits, Allocator>, charT>;

template<class traits>
  struct formatter<std::basic_string_view<charT, traits>, charT>;

specializations of formatter for nullptr_t, void*, const void*, bool, and all cv-unqualified standard integer types, extended integer types, and floating-point types as the first template argument and charT as the second template argument,

where charT is char or wchar_t (both sets of specializations are provided).

[ Note: Specializations such as formatter<wchar_t, char> and formatter<const char*, wchar_t> that require implicit multibyte / wide string or character conversion are intentionally disabled. — end note ]

For any types T and charT for which neither the library nor the user provides an explicit or partial specialization of the class template formatter, formatter<T, charT> is disabled.

If the library provides an explicit or partial specialization of formatter<T, charT>, that specialization is enabled except as noted otherwise.

If F is a disabled specialization of formatter, these values are false: is_default_constructible_v<F>, is_copy_constructible_v<F>, is_move_constructible_v<F>, is_copy_assignable_v<F>, is_move_assignable_v<F>.

An enabled specialization formatter<T, charT> will satisfy the Formatter requirements.

[ Example:

  #include <format>

  enum color { red, green, blue };

  const char* color_names[] = { "red", "green", "blue" };

  template<> struct std::formatter<color> : std::formatter<const char*> {
    auto format(color c, format_context& ctx) {
      return formatter<const char*>::format(color_names[c], ctx);
    }
  };

  struct err {};

  std::string s0 = std::format("{}", 42);     // OK: library-provided formatter
  std::string s1 = std::format("{}", L"foo"); // Ill-formed: disabled formatter
  std::string s2 = std::format("{}", red);    // OK: user-provided formatter
  std::string s3 = std::format("{}", err{});  // Ill-formed: disabled formatter

— end example ]

19.�.3.1 Formatter requirements [formatter.requirements]

A type F meets the Formatter requirements if:

it satisfies the Cpp17DefaultConstructible (Table 25), Cpp17CopyConstructible (Table 27), Cpp17CopyAssignable (Table 29), and Cpp17Destructible (Table 30) requirements,
it is swappable ([swappable.requirements]) for lvalues, and
the expressions shown in Table � are valid and have the indicated semantics.

Given character type charT, output iterator type O, and formatting argument type T, in Table � f is a value of type F, u is an lvalue of type T, t is a value of a type convertible to (possibly const) T, pc is an lvalue of type basic_format_parse_context<charT> (denoted by PC), and fc is an lvalue of type basic_format_context<O, charT> (denoted by FC). pc.begin() points to the beginning of the format-spec ([format.syntax]) portion of the format string. If format-spec is empty then either pc.begin() == pc.end() or *pc.begin() == '}'.

Table � — *Formatter* requirements
Expression	Return type	Requirement
`f.parse(pc)`	`PC::iterator`	Shall parse the range `[pc.begin(), pc.end())` until the first unmatched character according to the format specifications for type `T`. Shall throw `format_error` unless the whole range is parsed or the unmatched character is `'}'`. [ Note: This allows formatters to emit meaningful error messages. — end note ] Shall store the parsed format specifiers in `*this` and return an iterator past the end of the parsed range.
`f.format(t, fc)`	`FC::iterator`	Shall format `t` according to the specifiers stored in `*this`, write the output to `fc.out()` and return an iterator past the end of the output range. The output shall only depend on `t`, the current global locale, and the format string range `[pc.begin(), pc.end())` from the last call to `f.parse(pc)`.
`f.format(u, fc)`	`FC::iterator`	Shall not modify u.

19.�.3.2 Class template `basic_format_parse_context` [format.parse_context]

namespace std {
  template<class charT>
  class basic_format_parse_context {
  public:
    using char_type = charT;
    using const_iterator = typename basic_string_view<charT>::const_iterator;
    using iterator = const_iterator;

  private:
    iterator begin_;                              // exposition only
    iterator end_;                                // exposition only
    enum indexing { unknown, manual, automatic }; // exposition only
    indexing indexing_;                           // exposition only
    size_t next_arg_id_;                          // exposition only

  public:
    explicit constexpr basic_format_parse_context(basic_string_view<charT> fmt) noexcept;
    basic_format_parse_context(const basic_format_parse_context&) = delete;
    basic_format_parse_context& operator=(const basic_format_parse_context&) = delete;

    constexpr const_iterator begin() const noexcept;
    constexpr const_iterator end() const noexcept;
    constexpr void advance_to(const_iterator it);

    constexpr size_t next_arg_id();
    constexpr void check_arg_id(size_t id);
  };
}

An instance of basic_format_parse_context holds the format string parsing state consisting of the format string range being parsed and the argument counter for automatic indexing.

explicit constexpr basic_format_parse_context(basic_string_view<charT> fmt) noexcept;

Effects: Initializes begin_ with fmt.begin(), end_ with fmt.end(), indexing_ with unknown, and next_arg_id_ with 0.

constexpr const_iterator begin() const noexcept;

Returns: begin_.

constexpr const_iterator end() const noexcept;

Returns: end_.

constexpr void advance_to(const_iterator it);

Expects: end() is reachable from it.

Effects: Equivalent to: begin_ = it;

constexpr size_t next_arg_id();

Effects: If indexing_ != manual, equivalent to:

  if (indexing_ == unknown)
    indexing_ = automatic;
  return next_arg_id_++;

Throws: format_error if indexing_ == manual which indicates mixing of automatic and manual argument indexing.

constexpr void check_arg_id(size_t id);

Expects: id is a valid argument id.

Effects: If indexing_ != automatic, equivalent to:

  if (indexing_ == unknown)
    indexing_ = manual;

Throws: format_error if indexing_ == automatic which indicates mixing of automatic and manual argument indexing.

19.�.3.3 Class template `basic_format_context` [format.context]

namespace std {
  template<class O, class charT> requires OutputIterator<O, const charT&>
  class basic_format_context {
    basic_format_args<basic_format_context> args_; // exposition only
    O out_;                                        // exposition only

  public:
    using iterator = O;
    using char_type = charT;
    template<class T> using formatter_type = formatter<T>;

    basic_format_arg<basic_format_context> arg(size_t id) const;

    iterator out();
    void advance_to(iterator it);
  };
}

An instance of basic_format_context holds formatting state consisting of the formatting arguments and the output iterator.

basic_format_arg<basic_format_context> arg(size_t id) const;: Returns: args_.get(id).
iterator out();: Returns: out_.
void advance_to(iterator it);: Effects: Equivalent to: out_ = it;

[ Example:

  struct S {
    int value;
  };

  template<> struct std::formatter<S> {
    int width_arg_id = 0;

    // Parses a width argument id in the format { <digit> }.
    constexpr auto parse(format_parse_context& ctx) {
      auto iter = ctx.begin();
      auto get_char = [&]() { return iter != ctx.end() ? *iter : 0; };
      if (get_char() != '{')
        return iter;
      ++iter;
      char c = get_char();
      if (!std::isdigit(c) || (++iter, get_char()) != '}')
        throw format_error("invalid format");
      width_arg_id = c - '0';
      ctx.check_arg_id(width_arg_id);
      return ++iter;
    }

    // Formats S with width given by the argument width_arg_id.
    auto format(S s, format_context& ctx) {
      int width = visit_format_arg([](auto value) -> int {
        if constexpr (!std::is_integral_v<decltype(value)>)
          throw format_error("width is not integral");
        else if (value < 0 || value > std::numeric_limits<int>::max())
          throw format_error("invalid width");
        else
          return value;
        }, ctx.arg(width_arg_id));
      return format_to(ctx.out(), "{0:{1}}", s.value, width);
    }
  };

  std::string s = std::format("{0:{1}}", S{42}, 10); // s == "        42"

— end example ]

19.�.4 Arguments [format.arguments]

19.�.4.1 Class template `basic_format_arg` [format.arg]

namespace std {
  template<class Context>
  class basic_format_arg {
  public:
    class handle;

    using char_type = typename Context::char_type;                     // exposition only

    variant<monostate, bool, char_type,
            int, unsigned int, long long int, unsigned long long int,
            double, long double,
            const char_type*, basic_string_view<char_type>,
            const void*, handle> value;                                // exposition only

    basic_format_arg() noexcept;

    template<Integral I> explicit basic_format_arg(I n) noexcept;      // exposition only
    explicit basic_format_arg(float n) noexcept;                       // exposition only
    explicit basic_format_arg(double n) noexcept;                      // exposition only
    explicit basic_format_arg(long double n) noexcept;                 // exposition only
    explicit basic_format_arg(const char_type* s) noexcept;            // exposition only

    template<class traits>
      explicit basic_format_arg(
        basic_string_view<char_type, traits> s) noexcept;              // exposition only

    template<class traits, class Allocator>
      explicit basic_format_arg(
        const basic_string<char_type, traits, Allocator>& s) noexcept; // exposition only

    explicit basic_format_arg(nullptr_t) noexcept;                     // exposition only
    template<class T>
      explicit basic_format_arg(const T* p) noexcept;                  // exposition only
    template<class T>
      explicit basic_format_arg(const T& v) noexcept;                  // exposition only

    explicit operator bool() const noexcept;
  };
}

An instance of basic_format_arg provides access to a formatting argument for user-defined formatters.

basic_format_arg() noexcept;

Ensures: !(*this).

template<Integral I> explicit basic_format_arg(I n) noexcept;

Effects:

if I is bool or char_type, initializes value with n,
if I is char and char_type is wchar_t, initializes value with static_cast<wchar_t>(n),
if I is a standard signed integer type or an extended signed integer type and sizeof(I) <= sizeof(int), initializes value with static_cast<int>(n),
if I is a standard unsigned integer type or an extended unsigned integer type and sizeof(I) <= sizeof(unsigned int), initializes value with static_cast<unsigned int>(n),
if I is a standard signed integer type or an extended signed integer type and sizeof(I) <= sizeof(long long int), initializes value with static_cast<long long int>(n),
if I is a standard unsigned integer type or an extended unsigned integer type and sizeof(I) <= sizeof(unsigned long long int), initializes value with static_cast<unsigned long long int>(n),
otherwise the program is ill-formed.

explicit basic_format_arg(float n) noexcept;

Effects: Initializes value with static_cast<double>(n).

explicit basic_format_arg(double n) noexcept; explicit basic_format_arg(long double n) noexcept;

Effects: Initializes value with n.

explicit basic_format_arg(const char_type* s) noexcept;

Expects: s points to a NTCTS ([defns.ntcts]).

Effects: Initializes value with s.

template<class traits> explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;

Effects: Initializes value with s.

template<class traits, class Allocator> explicit basic_format_arg( const basic_string<char_type, traits, Allocator>& s) noexcept;

Effects: Initializes value with basic_string_view<char_type>(s.data(), s.size()).

explicit basic_format_arg(nullptr_t) noexcept;

Effects: Initializes value with static_cast<const void*>(nullptr).

template<class T> explicit basic_format_arg(T* p) noexcept;

Constraints: is_void_v<T> is true.

Effects: Initializes value with p.

template<class T> explicit basic_format_arg(const T& v) noexcept;

Constraints: typename Context::template formatter_type<T> is enabled.

Effects: Initializes value with handle(v).

[ Note: Constructing basic_format_arg from a pointer to a member is ill-formed unless the user provides an enabled specialization of formatter for this pointer to member type. — end note ]

explicit operator bool() const noexcept;

Returns: !holds_alternative<monostate>(value).

namespace std {
  template<class Context>
  class basic_format_arg<Context>::handle {
    const void* ptr_;                                         // exposition only
    void (*format_)(basic_format_parse_context<char_type>&,
                    Context&, const void*);                   // exposition only

  public:
    template<class T> explicit handle(const T& val) noexcept; // exposition only

    void format(basic_format_parse_context<char_type>&, Context& ctx) const;
  };
}

The class handle allows formatting an object of a user-defined type.

template<class T> explicit handle(const T& val) noexcept;

Effects: Initializes ptr_ with &val and format_ with

[](basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx, const void* ptr) {
    typename Context::template formatter_type<T> f;
    parse_ctx.advance_to(f.parse(parse_ctx));
    format_ctx.advance_to(f.format(*static_cast<const T*>(ptr), format_ctx));
  }

void format(basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx) const;

Effects: Equivalent to: format_(parse_ctx, format_ctx, ptr_);

19.�.4.2 Argument visitation [format.visit]

template<class Visitor, class Context> see below visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);

Returns: visit(vis, arg.value).

Remarks: The return type is the type of the expression in the Returns section.

19.�.4.3 Class template `format_arg_store` [format.arg_store]

namespace std {
  template<class Context, class... Args>
  struct format_arg_store { // exposition only
    array<basic_format_arg<Context>, sizeof...(Args)> args;
  };
}

19.�.4.4 Class template `basic_format_args` [format.basic_args]

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;
  };
}

An instance basic_format_args provides access to formatting arguments.

basic_format_args() noexcept;: Effects: Initializes size_ with 0.

template<class... Args> basic_format_args(const format_arg_store<Context, Args...>& store) noexcept;: Effects: Initializes size_ with sizeof...(Args) and data_ with store.args.data().

basic_format_arg<Context> get(size_t i) const noexcept;: Returns: i < size_ ? data_[i] : basic_format_arg<Context>().

[ Note: Implementations are encouraged to optimize the representation of basic_format_args for small number of formatting arguments by storing indices of type alternatives separately from values and packing the former. — end note ]

19.�.4.5 Function template `make_format_args` [format.make_args]

template<class Context = format_context, class... Args> format_arg_store<Context, Args...> make_format_args(const Args&... args);: Returns: {basic_format_arg<Context>(args)...}.

19.�.4.6 Function template `make_wformat_args` [format.make_wargs]

template<class... Args> format_arg_store<wformat_context, Args...> make_wformat_args(const Args&... args);: Returns: {basic_format_arg<wformat_context>(args)...}.

19.�.5 Class `format_error` [format.error]

namespace std {
  class format_error : public runtime_error {
  public:
    explicit format_error(const string& what_arg);
    explicit format_error(const char* what_arg);
  };
}

The class format_error defines the type of objects thrown as exceptions to report errors from the formatting library.

format_error(const string& what_arg);: Ensures: strcmp(what(), what_arg.c_str()) == 0.
format_error(const char* what_arg);: Ensures: strcmp(what(), what_arg) == 0.

Related work

The Boost Format library [8] is an established formatting library that uses printf-like format string syntax with extensions. The main differences between this library and the current proposal are:

Syntax: for the reasons described in section Format String Syntax this proposal uses a new syntax instead of extending the printf one. This allows much simpler and easier to parse grammar, not burdened by legacy specifiers used to convey type information. For example, Boost Format has two ways to refer to an argument by index and allows but ignores some format specifiers.
API: Boost Format uses operator% to pass formatting arguments while this proposal uses variadic function templates.
Performance: the implementation of this proposal is several times faster that the implementation of Boost Format on tinyformat benchmarks [9], generates smaller binary code and is faster to compile.

A printf-like Interface for the Streams Library [10] is similar to the Boost Format library but uses variadic templates instead of operator%. Unfortunately it hasn't been updated since 2013 and the same arguments about format string syntax apply to it.

The FastFormat library [11] is another well-known formatting library. Similarly to this proposal, FastFormat uses brace-delimited format specifiers, but otherwise the format string syntax is different and the library has significant limitations [12]:

Three features that have no hope of being accommodated within the current design are:

Leading zeros (or any other non-space padding)
Octal/hexadecimal encoding
Runtime width/alignment specification

Formatting facilities of the Folly library [13] are the closest to the current proposal. Folly also uses Python-like format string syntax nearly identical to the one described here. However, the API details are quite different. The current proposal tries to address performance and code bloat issues that are largely ignored by Folly Format. For instance formatting functions in Folly Format are parameterized on all argument types while in this proposal, only the inlined wrapper functions are, which results in much smaller binary code and better compile times.

Implementation

An implementation of this proposal is available in the std branch of the open-source fmt library [14].

Acknowledgements

Thanks to Alberto Barbati, Antony Polukhin, Beman Dawes, Bengt Gustafsson, Daniel Krügler, Eric Niebler, Jason McKesson, Jeffrey Yasskin, Joël Lamotte, Lars Gullik Bjønnes, Lee Howes, Louis Dionne, Matt Clarke, Michael Park, Sergey Ignatchenko, Thiago Macieira, Tomasz Kamiński, Zach Laine, Zhihao Yuan and participants of the Library Evolution Working Group and the Library Working Group for their feedback, support, constructive criticism and contributions to the proposal. Special thanks to Howard Hinnant who encouraged me to write the proposal and gave useful early advice on how to go about it.

The format string syntax is based on the Python documentation [3].

References

[1] The fprintf function. ISO/IEC 9899:2011. 7.21.6.1.
[2] fprintf, printf, snprintf, sprintf - print formatted output. The Open Group Base Specifications Issue 6 IEEE Std 1003.1, 2004 Edition.
[3] 6.1.3. Format String Syntax. Python 3.5.2 documentation.
[4] String.Format Method. .NET Framework Class Library.
[5] Module std::fmt. The Rust Standard Library.
[6] Format Specification Syntax: printf and wprintf Functions. C++ Language and Standard Libraries.
[7] 10.4.2 Rearranging printf Arguments. The GNU Awk User's Guide.
[8] Boost Format library. Boost 1.63 documentation.
[9] Speed Test. The fmt library repository.
[10] A printf-like Interface for the Streams Library (revision 1).
[11] The FastFormat library website.
[12] An Introduction to Fast Format (Part 1): The State of the Art. Overload Journal #89 - February 2009
[13] The folly library repository.
[14] The fmt library repository.
[15] P0424: Reconsidering literal operator templates for strings.
[16] D0355: Extending <chrono> to Calendars and Time Zones.
[17] P0067: Elementary string conversions.
[18] MPark.Format: Compile-time Checked, Type-Safe Formatting in C++14.
[19] Google Benchmark: A microbenchmark support library.
[20] N4412: Shortcomings of iostreams.

Appendix A: Benchmarks

To demonstrate that the formatting functions described in this paper can be implemented efficiently, we compare the reference implementation [14] of format and format_to to sprintf, ostringstream and to_string on the following benchmark. This benchmark generates a set of integers with random numbers of digits, applies each method to convert each integer into a string (either std::string or a char buffer depending on the API) and uses the Google Benchmark library [19] to measure timings:

  #include <algorithm>
  #include <cmath>
  #include <cstdio>
  #include <limits>
  #include <sstream>
  #include <string>
  #include <utility>
  #include <vector>

  #include <benchmark/benchmark.h>
  #include <fmt/format.h>

  // Returns a pair with the smallest and the largest value of integral type T
  // with the given number of digits.
  template<typename T>
  std::pair<T, T> range(int num_digits) {
    T first = std::pow(T(10), num_digits - 1);
    int max_digits = std::numeric_limits<T>::digits10 + 1;
    T last = num_digits < max_digits ? first * 10 - 1 :
                                 std::numeric_limits<T>::max();
    return {num_digits > 1 ? first : 0, last};
  }

  // Generates values of integral type T with random number of digits.
  template<typename T>
  std::vector<T> generate_random_data(int numbers_per_digit) {
    int max_digits = std::numeric_limits<T>::digits10 + 1;
    std::vector<T> data;
    data.reserve(max_digits * numbers_per_digit);
    for (int i = 1; i <= max_digits; ++i) {
      auto r = range<T>(i);
      auto value = r.first;
      std::generate_n(std::back_inserter(data), numbers_per_digit, [=]() mutable {
        T result = value;
        value = value < r.second ? value + 1 : r.first;
        return result;
      });
    }
    std::random_shuffle(data.begin(), data.end());
    return data;
  }

  auto data = generate_random_data<int>(1000);

  void sprintf(benchmark::State& s) {
    size_t result = 0;
    while (s.KeepRunning()) {
      for (auto i: data) {
        char buffer[12];
        result += std::sprintf(buffer, "%d", i);
      }
    }
    benchmark::DoNotOptimize(result);
  }
  BENCHMARK(sprintf);

  void ostringstream(benchmark::State& s) {
    size_t result = 0;
    while (s.KeepRunning()) {
      for (auto i: data) {
        std::ostringstream ss;
        ss << i;
        result += ss.str().size();
      }
    }
    benchmark::DoNotOptimize(result);
  }
  BENCHMARK(ostringstream);

  void to_string(benchmark::State& s) {
    size_t result = 0;
    while (s.KeepRunning()) {
      for (auto i: data)
        result += std::to_string(i).size();
    }
    benchmark::DoNotOptimize(result);
  }
  BENCHMARK(to_string);

  void format(benchmark::State& s) {
    size_t result = 0;
    while (s.KeepRunning()) {
      for (auto i: data)
        result += fmt::format("{}", i).size();
    }
    benchmark::DoNotOptimize(result);
  }
  BENCHMARK(format);

  void format_to(benchmark::State& s) {
    size_t result = 0;
    while (s.KeepRunning()) {
      for (auto i: data) {
        char buffer[12];
        result += fmt::format_to(buffer, "{}", i) - buffer;
      }
    }
    benchmark::DoNotOptimize(result);
  }
  BENCHMARK(format_to);

  BENCHMARK_MAIN();

The benchmark was compiled with clang (Apple LLVM version 9.0.0 clang-900.0.39.2) with -O3 -DNDEBUG and run on a macOS system. Below are the results:

  Run on (4 X 3100 MHz CPU s)
  2018-01-27 07:12:00
  Benchmark              Time           CPU Iterations
  ----------------------------------------------------
  sprintf           882311 ns     881076 ns        781
  ostringstream    2892035 ns    2888975 ns        242
  to_string        1167422 ns    1166831 ns        610
  format            675636 ns     674382 ns       1045
  format_to         499376 ns     498996 ns       1263

The format and format_to functions show much better performance than the other methods. The format function that constructs std::string is even 30% faster than the system's version of sprintf that uses stack-allocated char buffer. format_to with a stack-allocated buffer is ~60% faster than sprintf.

Appendix B: Binary code comparison

In this section we compare per-call binary code size between the reference implementation that uses techniques described in section Binary footprint and standard formatting facilities. All the code snippets are compiled with clang (Apple LLVM version 9.0.0 clang-900.0.39.2) with -O3 -DNDEBUG -c -std=c++14 and the resulted binaries are disassembled with objdump -S:

  void consume(const char*);

  void sprintf_test() {
    char buffer[100];
    sprintf(buffer, "The answer is %d.", 42);
    consume(buffer);
  }

  __Z12sprintf_testv:
         0:       55      pushq   %rbp
         1:       48 89 e5        movq    %rsp, %rbp
         4:       53      pushq   %rbx
         5:       48 83 ec 78     subq    $120, %rsp
         9:       48 8b 05 00 00 00 00    movq    (%rip), %rax
        10:       48 8b 00        movq    (%rax), %rax
        13:       48 89 45 f0     movq    %rax, -16(%rbp)
        17:       48 8d 35 37 00 00 00    leaq    55(%rip), %rsi
        1e:       48 8d 5d 80     leaq    -128(%rbp), %rbx
        22:       ba 2a 00 00 00  movl    $42, %edx
        27:       31 c0   xorl    %eax, %eax
        29:       48 89 df        movq    %rbx, %rdi
        2c:       e8 00 00 00 00  callq   0 <__Z12sprintf_testv+0x31>
        31:       48 89 df        movq    %rbx, %rdi
        34:       e8 00 00 00 00  callq   0 <__Z12sprintf_testv+0x39>
        39:       48 8b 05 00 00 00 00    movq    (%rip), %rax
        40:       48 8b 00        movq    (%rax), %rax
        43:       48 3b 45 f0     cmpq    -16(%rbp), %rax
        47:       75 07   jne     7 <__Z12sprintf_testv+0x50>
        49:       48 83 c4 78     addq    $120, %rsp
        4d:       5b      popq    %rbx
        4e:       5d      popq    %rbp
        4f:       c3      retq
        50:       e8 00 00 00 00  callq   0 <__Z12sprintf_testv+0x55>

  void format_test() {
    consume(format("The answer is {}.", 42).c_str());
  }

  __Z11format_testv:
         0:       55      pushq   %rbp
         1:       48 89 e5        movq    %rsp, %rbp
         4:       53      pushq   %rbx
         5:       48 83 ec 28     subq    $40, %rsp
         9:       48 c7 45 d0 2a 00 00 00         movq    $42, -48(%rbp)
        11:       48 8d 35 f4 83 01 00    leaq    99316(%rip), %rsi
        18:       48 8d 7d e0     leaq    -32(%rbp), %rdi
        1c:       4c 8d 45 d0     leaq    -48(%rbp), %r8
        20:       ba 11 00 00 00  movl    $17, %edx
        25:       b9 02 00 00 00  movl    $2, %ecx
        2a:       e8 00 00 00 00  callq   0 <__Z11format_testv+0x2F>
        2f:       f6 45 e0 01     testb   $1, -32(%rbp)
        33:       48 8d 7d e1     leaq    -31(%rbp), %rdi
        37:       48 0f 45 7d f0  cmovneq -16(%rbp), %rdi
        3c:       e8 00 00 00 00  callq   0 <__Z11format_testv+0x41>
        41:       f6 45 e0 01     testb   $1, -32(%rbp)
        45:       74 09   je      9 <__Z11format_testv+0x50>
        47:       48 8b 7d f0     movq    -16(%rbp), %rdi
        4b:       e8 00 00 00 00  callq   0 <__Z11format_testv+0x50>
        50:       48 83 c4 28     addq    $40, %rsp
        54:       5b      popq    %rbx
        55:       5d      popq    %rbp
        56:       c3      retq
        57:       48 89 c3        movq    %rax, %rbx
        5a:       f6 45 e0 01     testb   $1, -32(%rbp)
        5e:       74 09   je      9 <__Z11format_testv+0x69>
        60:       48 8b 7d f0     movq    -16(%rbp), %rdi
        64:       e8 00 00 00 00  callq   0 <__Z11format_testv+0x69>
        69:       48 89 df        movq    %rbx, %rdi
        6c:       e8 00 00 00 00  callq   0 <__Z11format_testv+0x71>
        71:       66 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00    nopw    %cs:(%rax,%rax)

  void ostringstream_test() {
    std::ostringstream ss;
    ss << "The answer is " << 42 << ".";
    consume(ss.str().c_str());
  }

  __Z18ostringstream_testv:
         0:	55 	pushq	%rbp
         1:	48 89 e5 	movq	%rsp, %rbp
         4:	41 57 	pushq	%r15
         6:	41 56 	pushq	%r14
         8:	41 55 	pushq	%r13
         a:	41 54 	pushq	%r12
         c:	53 	pushq	%rbx
         d:	48 81 ec 38 01 00 00 	subq	$312, %rsp
        14:	4c 8d b5 18 ff ff ff 	leaq	-232(%rbp), %r14
        1b:	4c 8d a5 b0 fe ff ff 	leaq	-336(%rbp), %r12
        22:	48 8b 05 00 00 00 00 	movq	(%rip), %rax
        29:	48 8d 48 18 	leaq	24(%rax), %rcx
        2d:	48 89 8d a8 fe ff ff 	movq	%rcx, -344(%rbp)
        34:	48 83 c0 40 	addq	$64, %rax
        38:	48 89 85 18 ff ff ff 	movq	%rax, -232(%rbp)
        3f:	4c 89 f7 	movq	%r14, %rdi
        42:	4c 89 e6 	movq	%r12, %rsi
        45:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x4A>
        4a:	48 c7 45 a0 00 00 00 00 	movq	$0, -96(%rbp)
        52:	c7 45 a8 ff ff ff ff 	movl	$4294967295, -88(%rbp)
        59:	48 8b 1d 00 00 00 00 	movq	(%rip), %rbx
        60:	4c 8d 6b 18 	leaq	24(%rbx), %r13
        64:	4c 89 ad a8 fe ff ff 	movq	%r13, -344(%rbp)
        6b:	48 83 c3 40 	addq	$64, %rbx
        6f:	48 89 9d 18 ff ff ff 	movq	%rbx, -232(%rbp)
        76:	4c 89 e7 	movq	%r12, %rdi
        79:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x7E>
        7e:	4c 8b 3d 00 00 00 00 	movq	(%rip), %r15
        85:	49 83 c7 10 	addq	$16, %r15
        89:	4c 89 bd b0 fe ff ff 	movq	%r15, -336(%rbp)
        90:	48 c7 85 08 ff ff ff 00 00 00 00 	movq	$0, -248(%rbp)
        9b:	48 c7 85 00 ff ff ff 00 00 00 00 	movq	$0, -256(%rbp)
        a6:	48 c7 85 f8 fe ff ff 00 00 00 00 	movq	$0, -264(%rbp)
        b1:	48 c7 85 f0 fe ff ff 00 00 00 00 	movq	$0, -272(%rbp)
        bc:	c7 85 10 ff ff ff 10 00 00 00 	movl	$16, -240(%rbp)
        c6:	0f 57 c0 	xorps	%xmm0, %xmm0
        c9:	0f 29 45 b0 	movaps	%xmm0, -80(%rbp)
        cd:	48 c7 45 c0 00 00 00 00 	movq	$0, -64(%rbp)
        d5:	48 8d 75 b0 	leaq	-80(%rbp), %rsi
        d9:	4c 89 e7 	movq	%r12, %rdi
        dc:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0xE1>
        e1:	f6 45 b0 01 	testb	$1, -80(%rbp)
        e5:	74 09 	je	9 <__Z18ostringstream_testv+0xF0>
        e7:	48 8b 7d c0 	movq	-64(%rbp), %rdi
        eb:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0xF0>
        f0:	48 8d 35 dd 10 00 00 	leaq	4317(%rip), %rsi
        f7:	48 8d bd a8 fe ff ff 	leaq	-344(%rbp), %rdi
        fe:	ba 0e 00 00 00 	movl	$14, %edx
       103:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x108>
       108:	be 2a 00 00 00 	movl	$42, %esi
       10d:	48 89 c7 	movq	%rax, %rdi
       110:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x115>
       115:	48 8d 35 c7 10 00 00 	leaq	4295(%rip), %rsi
       11c:	ba 01 00 00 00 	movl	$1, %edx
       121:	48 89 c7 	movq	%rax, %rdi
       124:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x129>
       129:	48 8d 7d b0 	leaq	-80(%rbp), %rdi
       12d:	4c 89 e6 	movq	%r12, %rsi
       130:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x135>
       135:	f6 45 b0 01 	testb	$1, -80(%rbp)
       139:	48 8d 7d b1 	leaq	-79(%rbp), %rdi
       13d:	48 0f 45 7d c0 	cmovneq	-64(%rbp), %rdi
       142:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x147>
       147:	f6 45 b0 01 	testb	$1, -80(%rbp)
       14b:	74 09 	je	9 <__Z18ostringstream_testv+0x156>
       14d:	48 8b 7d c0 	movq	-64(%rbp), %rdi
       151:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x156>
       156:	4c 89 ad a8 fe ff ff 	movq	%r13, -344(%rbp)
       15d:	48 89 9d 18 ff ff ff 	movq	%rbx, -232(%rbp)
       164:	4c 89 bd b0 fe ff ff 	movq	%r15, -336(%rbp)
       16b:	f6 85 f0 fe ff ff 01 	testb	$1, -272(%rbp)
       172:	74 0c 	je	12 <__Z18ostringstream_testv+0x180>
       174:	48 8b bd 00 ff ff ff 	movq	-256(%rbp), %rdi
       17b:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x180>
       180:	4c 89 e7 	movq	%r12, %rdi
       183:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x188>
       188:	48 8b 35 00 00 00 00 	movq	(%rip), %rsi
       18f:	48 83 c6 08 	addq	$8, %rsi
       193:	48 8d bd a8 fe ff ff 	leaq	-344(%rbp), %rdi
       19a:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x19F>
       19f:	4c 89 f7 	movq	%r14, %rdi
       1a2:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x1A7>
       1a7:	48 81 c4 38 01 00 00 	addq	$312, %rsp
       1ae:	5b 	popq	%rbx
       1af:	41 5c 	popq	%r12
       1b1:	41 5d 	popq	%r13
       1b3:	41 5e 	popq	%r14
       1b5:	41 5f 	popq	%r15
       1b7:	5d 	popq	%rbp
       1b8:	c3 	retq
       1b9:	48 89 45 d0 	movq	%rax, -48(%rbp)
       1bd:	f6 45 b0 01 	testb	$1, -80(%rbp)
       1c1:	74 3b 	je	59 <__Z18ostringstream_testv+0x1FE>
       1c3:	48 8b 7d c0 	movq	-64(%rbp), %rdi
       1c7:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x1CC>
       1cc:	eb 30 	jmp	48 <__Z18ostringstream_testv+0x1FE>
       1ce:	eb 2a 	jmp	42 <__Z18ostringstream_testv+0x1FA>
       1d0:	48 89 45 d0 	movq	%rax, -48(%rbp)
       1d4:	f6 45 b0 01 	testb	$1, -80(%rbp)
       1d8:	74 39 	je	57 <__Z18ostringstream_testv+0x213>
       1da:	48 8b 7d c0 	movq	-64(%rbp), %rdi
       1de:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x1E3>
       1e3:	f6 85 f0 fe ff ff 01 	testb	$1, -272(%rbp)
       1ea:	75 30 	jne	48 <__Z18ostringstream_testv+0x21C>
       1ec:	eb 3a 	jmp	58 <__Z18ostringstream_testv+0x228>
       1ee:	48 89 45 d0 	movq	%rax, -48(%rbp)
       1f2:	eb 3c 	jmp	60 <__Z18ostringstream_testv+0x230>
       1f4:	48 89 45 d0 	movq	%rax, -48(%rbp)
       1f8:	eb 4d 	jmp	77 <__Z18ostringstream_testv+0x247>
       1fa:	48 89 45 d0 	movq	%rax, -48(%rbp)
       1fe:	4c 89 ad a8 fe ff ff 	movq	%r13, -344(%rbp)
       205:	48 89 9d 18 ff ff ff 	movq	%rbx, -232(%rbp)
       20c:	4c 89 bd b0 fe ff ff 	movq	%r15, -336(%rbp)
       213:	f6 85 f0 fe ff ff 01 	testb	$1, -272(%rbp)
       21a:	74 0c 	je	12 <__Z18ostringstream_testv+0x228>
       21c:	48 8b bd 00 ff ff ff 	movq	-256(%rbp), %rdi
       223:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x228>
       228:	4c 89 e7 	movq	%r12, %rdi
       22b:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x230>
       230:	48 8b 35 00 00 00 00 	movq	(%rip), %rsi
       237:	48 83 c6 08 	addq	$8, %rsi
       23b:	48 8d bd a8 fe ff ff 	leaq	-344(%rbp), %rdi
       242:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x247>
       247:	4c 89 f7 	movq	%r14, %rdi
       24a:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x24F>
       24f:	48 8b 7d d0 	movq	-48(%rbp), %rdi
       253:	e8 00 00 00 00 	callq	0 <__Z18ostringstream_testv+0x258>
       258:	0f 1f 84 00 00 00 00 00 	nopl	(%rax,%rax)

The code generated for the format_test function that uses the reference implementation of the format function described in this proposal is several times smaller than the ostringstream code and only 40% larger than the one generated for sprintf which is a moderate price to pay for full type and memory safety.

The following factors contribute to the difference in binary code size between format and sprintf:

Passing format string as string_view instead of const char*.
Using string instead of a char buffer.
Preparing the array of formatting arguments.

We can exclude the first two factors from the experiment by mimicking parts of the sprintf API:

int vraw_format(char* buffer, const char* format, format_args args);

template<typename... Args>
inline int raw_format(char* buffer, const char* format, const Args&... args) {
  return vraw_format(buffer, format, make_format_args(args...));
}

void raw_format_test() {
  char buffer[100];
  raw_format(buffer, "The answer is {}.", 42);
}

__Z15raw_format_testv:
       0:       55      pushq   %rbp
       1:       48 89 e5        movq    %rsp, %rbp
       4:       48 81 ec 80 00 00 00    subq    $128, %rsp
       b:       48 8b 05 00 00 00 00    movq    (%rip), %rax
      12:       48 8b 00        movq    (%rax), %rax
      15:       48 89 45 f8     movq    %rax, -8(%rbp)
      19:       48 c7 45 80 2a 00 00 00         movq    $42, -128(%rbp)
      21:       48 8d 35 24 12 00 00    leaq    4644(%rip), %rsi
      28:       48 8d 7d 90     leaq    -112(%rbp), %rdi
      2c:       48 8d 4d 80     leaq    -128(%rbp), %rcx
      30:       ba 02 00 00 00  movl    $2, %edx
      35:       e8 00 00 00 00  callq   0 <__Z15raw_format_testv+0x3A>
      3a:       48 8b 05 00 00 00 00    movq    (%rip), %rax
      41:       48 8b 00        movq    (%rax), %rax
      44:       48 3b 45 f8     cmpq    -8(%rbp), %rax
      48:       75 09   jne     9 <__Z15raw_format_testv+0x53>
      4a:       48 81 c4 80 00 00 00    addq    $128, %rsp
      51:       5d      popq    %rbp
      52:       c3      retq
      53:       e8 00 00 00 00  callq   0 <__Z15raw_format_testv+0x58>
      58:       0f 1f 84 00 00 00 00 00         nopl    (%rax,%rax)

This shows that passing formatting arguments adds very little overhead and is comparable with sprintf.

Text Formatting

Changes since R6

Changes since R5

Changes since R4

Changes since R3

Changes since R2

Changes since R1

Changes since R0

19.� Formatting [format]

19.�.3.1 Formatter requirements [formatter.requirements]

19.�.4.2 Argument visitation [format.visit]

19.�.4.3 Class template format_arg_store [format.arg_store]

19.�.4.4 Class template basic_format_args [format.basic_args]

19.�.4.6 Function template make_wformat_args [format.make_wargs]

19.�.4.3 Class template `format_arg_store` [format.arg_store]

19.�.4.4 Class template `basic_format_args` [format.basic_args]

19.�.4.6 Function template `make_wformat_args` [format.make_wargs]