Text Formatting

2016-08-19

Introduction

This paper proposes a new text formatting functionality 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:

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

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, this will 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.

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:

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

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

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 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

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

by providing an overload of std::format_arg for std::tm:

TODO: example

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). Buffer management is also automatic to prevent buffer overflow errors common to printf.

Locale Support

As pointed out in P0067R1: Elementary string conversions 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 peformance effect 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 [3]. 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:

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

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

std::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

TODO

Binary Footprint

TODO

Proposed Wording

Header <format> synopsis

namespace std {
  class format_error;

  class format_args;

  template <class Char>
  basic_string<Char> format(const Char* fmt, format_args args);

  template <class Char, class ...Args>
  basic_string<Char> format(const Char* fmt, const Args&... args);
}

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 numerical 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 the numeric 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:

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 number types, and can be one of the following:

TODO

Class format_error

class format_error : public std::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);

Effects: Constructs an object of class format_error.

Postcondition: strcmp(what(), what_arg.c_str()) == 0.

format_error(const char* what_arg);

Effects: Constructs an object of class format_error.

Postcondition: strcmp(what(), what_arg) == 0.

Class format_args

TODO

Function template format

template <class Char>
  basic_string<Char> format(const Char* fmt, format_args args);

template <class Char, class ...Args>
  basic_string<Char> format(const Char* fmt, const Args&... args);

Requires: fmt shall not be a null pointer.

Effects: Each function returns a basic_string object constructed from the format string argument fmt 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.

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

Implementation

The ideas proposed in this paper have been implemented in the open-source fmt library. TODO: link and mention other implementations (Boost Format, FastFormat)

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.