Text Formatting

Revision History

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 compatiblity 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 writing a "Hello, World!" program that performs simple formatted output.

C++ has not one but two standard APIs for doing 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 doing formatted output in C++ for safety and extensibility reasons, printf offers some advantages, such as arguably more natural function call API, separation of formatted message and arguments, possibly with argument reordering as a POSIX extension, and often more compact code, both source and binary.

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 = fmt::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, e.g. for put_time-like time formatting, poses difficulties.

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 details in the syntax reference. Here are some of the advantages:

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 format

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.

As can be seen from the table above, most of the specifiers remain the same which simplifies migration from printf. 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 (the default) alignment.

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

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

by providing a specialization of formatter for tm:

template <>
struct formatter<tm> {
  constexpr fmt::parse_context::iterator parse(fmt::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.

The default implementation of formatter<T> uses ostream insertion operator<< for type T if available.

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 N4412: Shortcomings of iostreams. 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 because 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:

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

with the German translation of the format string:

"{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 a formatting operation. In particular, if formatting output can fit into a fixed-size array allocated on stack, it should be possible to avoid them altogether by using a suitable API.

The format_to function takes an arbitrary output iterator and can be specialized for random access and contiguous iterators for performance reasons as it is done 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 to 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_args and calls the non-variadic function to do the actual work. For example, the format variadic function calls vformat:

string vformat(string_view format_str, format_args args);

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

basic_format_args can be implemented as an array of tagged unions. Tags that indicate types of aguments can be combined and passed into a formatting function as a single integer if the number of arguments is small. Since argument types are known at compile time this can be an integer constant and there will be no code generated to compute it at runtime, only to store according to calling conventions.

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 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 format_str, const Args&... args) {
    clog << "Error " << code << ": " << format(format_str, 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 format_str, format_args args) {
  clog << "Error " << code << ": " << vformat(format_str, args);
}

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

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

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 format_str, 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] gives 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 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.

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

Insert a new section in 27.7 [iostream.format].

Header `<format>` synopsis

namespace std {
namespace fmt {

template <class Context>
  class basic_arg;

template <class Visitor, class Context>
  see below visit(Visitor&& vis, basic_arg<Context> arg);

template <class Context, class... Args>
  class arg_store;

template <class Context>
  class basic_format_args;

template <class charT>
  class basic_parse_context;

template <class OutputIterator, class charT>
  class basic_context;

using parse_context = basic_parse_context<char>;

using context = basic_context<implementation-defined>;
using wcontext = basic_context<implementation-defined>;

using format_args = basic_format_args<context>;
using wformat_args = basic_format_args<wcontext>;

template <class OutputIterator, class charT>
  using format_args_t = basic_format_args<implementation-defined>

template <class Context, class... Args>
  arg_store<Context, Args...> make_args(const Args&... args);

template <class... Args>
  arg_store<context, Args...> make_args(const Args&... args);

class format_error;

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

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

template <class OutputIterator, class... Args>
  OutputIterator format_to(OutputIterator out, string_view format_str,
                           const Args&... args);
template <class OutputIterator, class... Args>
  OutputIterator format_to(OutputIterator out, wstring_view format_str,
                           const Args&... args);

template <class OutputIterator>
  OutputIterator vformat_to(OutputIterator out, string_view format_str,
                            format_args_t<OutputIterator, char> args);
template <class OutputIterator>
  OutputIterator vformat_to(OutputIterator out, wstring_view format_str,
                            format_args_t<OutputIterator, wchar_t> args);

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

template <class T, class charT = char>
struct formatter {
  constexpr typename basic_parse_context<charT>::iterator
    parse(basic_parse_context<charT>& ctx);

  template <class FormatContext>
    typename FormatContext::iterator format(const T& value,
                                            FormatContext& ctx);
};
}  // namespace fmt
}  // namespace std

Format string syntax

Format strings contain replacement fields surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. A brace character can be included in the literal text by doubling: {{ and }}.

The grammar for a replacement field is as follows:

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

In less formal terms, the replacement field can start with an arg-id that specifies the argument whose value is to be formatted and inserted into the output instead of the replacement field. The arg-id is optionally followed by a format-spec, which is preceded by a colon ':'. These specify a non-default format for the replacement value.

See also the Format specification mini-language section.

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 inserted in that order.

Some simple format string examples:

"First, thou shalt count to {0}" // References the first argument
"Bring me a {}"                  // Implicitly references the first argument
"From {} to {}"                  // Same as "From {0} to {1}"

The format-spec field contains a specification of how the value should be presented, including such details as field width, alignment, padding, decimal precision and so on. Each value type can define its own formatting mini-language or interpretation of the format-spec.

Most built-in types support a common formatting mini-language, which is described in the next section.

A format-spec field can also include nested replacement fields in certain position within it. These nested replacement fields can contain only an argument index; format specifications are not allowed. This allows the formatting of a value to be dynamically specified.

Format specification mini-language

Format specifications are used within replacement fields contained within a format string to define how individual values are presented (see Format string syntax). Each formattable type may define how the format specification is to be interpreted.

Most built-in types implement the following options for format specifications, although some of the formatting options are only supported by arithmetic types.

The general form of a standard format specifier is:

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

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 format-spec is not a valid alignment option, then it is assumed that both the fill character and the alignment option are absent.

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 most objects).
`'>'`	Forces the field to be right-aligned within the available space (this is the default for arithmetic types).
`'='`	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.
`'^'`	Forces the field to be centered within the available space.

Note that unless a minimum field width is defined, the field width will always be the same size as the data to fill it, so that the alignment option has no meaning in this case.

The sign option is only valid for arithmetic types, and can be one of the following:

Option	Meaning
`'+'`	Indicates that a sign should be used for both positive as well as 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 on positive numbers, and a minus sign on negative numbers.

The '#' option causes the alternate form to be used for the conversion. The alternate form is defined differently for different types. This option is only valid for integer and floating-point types. For integers, when binary, octal, or hexadecimal output is used, this option adds the prefix respective "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 specifier, for example, the prefix "0x" is used for the type 'x' and "0X" is used for 'X'. 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 is a decimal number indicating how many digits should be displayed after the decimal point for a floating-point value formatted with 'f' and 'F', or before and after the decimal point for a floating-point value formatted with 'g' or 'G'. For non-arithmetic types the field indicates the maximum field size - in other words, how many characters will be used from the field content. The precision is not allowed for integer, character, Boolean, and pointer values.

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

The available string presentation types are:

Type	Meaning
`'s'`	String format. This is the default type for strings and may be omitted.
none	The same as `'s'`.

The available character presentation types are:

Type	Meaning
`'c'`	Character format. This is the default type for characters and may be omitted.
none	The same as `'c'`.

The available integer presentation types are:

Type	Meaning
`'b'`	Binary format. Outputs the number in base 2. Using the `'#'` option with this type adds the prefix `"0b"` to the output value.
`'B'`	Binary format. Outputs the number in base 2. Using the `'#'` option with this type adds the prefix `"0B"` to the output value.
`'d'`	Decimal integer. Outputs the number in base 10.
`'o'`	Octal format. Outputs the number in base 8.
`'x'`	Hex format. Outputs the number in base 16, using lower-case letters for the digits above 9. Using the `'#'` option with this type adds the prefix `"0x"` to the output value.
`'X'`	Hex format. Outputs the number in base 16, using upper-case letters for the digits above 9. Using the `'#'` option with this type adds the prefix `"0X"` to the output value.
`'n'`	Number. This is the same as `'d'`, except that it uses the locale to insert the appropriate number separator characters.
none	The same as `'d'`.

Integer presentation types can also be used with character and Boolean values. Boolean values are formatted using textual representation, either true or false, if the presentation type is not specified.

The available presentation types for floating-point values are:

Type	Meaning
`'a'`	Hexadecimal floating point format. Prints the number in base 16 with prefix `"0x"` and lower-case letters for digits above 9. Uses `'p'` to indicate the exponent.
`'A'`	Same as `'a'` except it uses upper-case letters for the prefix, digits above 9 and to indicate the exponent.
`'e'`	Exponent notation. Prints the number in scientific notation using the letter `'e'` to indicate the exponent.
`'E'`	Exponent notation. Same as `'e'` except it uses an upper-case `'E'` as the separator character.
`'f'`	Fixed point. Displays the number as a fixed-point number.
`'F'`	Fixed point. Same as `'f'`, but converts `nan` to `NAN` and `inf` to `INF`.
`'g'`	General format. For a given precision `p >= 1`, this rounds the number to `p` significant digits and then formats the result in either fixed-point format or in scientific notation, depending on its magnitude. A precision of `0` is treated as equivalent to a precision of `1`.
`'n'`	Number. This is the same as `'g'`, except that it uses the locale to insert the appropriate number separator characters.
none	The same as `'g'`.

The available presentation types for pointers are:

Type	Meaning
`'p'`	Pointer format. This is the default type for pointers and may be omitted.
none	The same as `'p'`.

Formatting functions

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

Effects: The function returns a string object constructed from the format string argument format_str with each replacement field substituted with the character representation of the argument it refers to, formatted according to the specification given in the field.

Returns: The formatted string.