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folly/Format.h

folly/Format.h provides a fast, powerful, type-safe, flexible facility for formatting text, using a specification language similar to Python's str.format. By default, it can format strings, numbers (integral and floating point), and dynamically-typed folly::dynamic objects, and can extract values from random-access containers and string-keyed maps. In many cases, format is faster than sprintf as well as being fully type-safe.

Overview

Here are some code samples to get started:

using folly::format;
using folly::sformat;

// Objects produced by format() can be streamed without creating
// an intermediary string; {} yields the next argument using default
// formatting.
std::cout << format("The answers are {} and {}", 23, 42);
// => "The answers are 23 and 42"

// If you just want the string, though, you're covered.
std::string result = sformat("The answers are {} and {}", 23, 42);
// => "The answers are 23 and 42"

// To insert a literal '{' or '}', just double it.
std::cout << format("{} {{}} {{{}}}", 23, 42);
// => "23 {} {42}"

// Arguments can be referenced out of order, even multiple times
std::cout << format("The answers are {1}, {0}, and {1} again", 23, 42);
// => "The answers are 42, 23, and 42 again"

// It's perfectly fine to not reference all arguments
std::cout << format("The only answer is {1}", 23, 42);
// => "The only answer is 42"

// Values can be extracted from indexable containers
// (random-access sequences and integral-keyed maps), and also from
// string-keyed maps
std::vector<int> v {23, 42};
std::map<std::string, std::string> m { {"what", "answer"} };
std::cout << format("The only {1[what]} is {0[1]}", v, m);
// => "The only answer is 42"

// format works with pairs and tuples
std::tuple<int, std::string, int> t {42, "hello", 23};
std::cout << format("{0} {2} {1}", t);
// => "42 23 hello"

// Format supports width, alignment, arbitrary fill, and various
// format specifiers, with meanings similar to printf
// "X<10": fill with 'X', left-align ('<'), width 10
std::cout << format("{:X<10} {}", "hello", "world");
// => "helloXXXXX world"

// Field width may be a runtime value rather than part of the format string
int x = 6;
std::cout << format("{:-^*}", x, "hi");
// => "--hi--"

// Explicit arguments work with dynamic field width, as long as indexes are
// given for both the value and the field width.
std::cout << format("{2:+^*0}",
9, "unused", 456); // => "+++456+++"

// Format supports printf-style format specifiers
std::cout << format("{0:05d} decimal = {0:04x} hex", 42);
// => "00042 decimal = 002a hex"

// Formatter objects may be written to a string using folly::to or
// folly::toAppend (see folly/Conv.h), or by calling their appendTo(),
// and str() methods
std::string s = format("The only answer is {}", 42).str();
std::cout << s;
// => "The only answer is 42"

// Decimal precision usage
std::cout << format("Only 2 decimals is {:.2f}", 23.34134534535);
// => "Only 2 decimals is 23.34"

Format string syntax

Format string (format): "{" [arg_index] ["[" key "]"] [":" format_spec] "}"

  • arg_index: index of argument to format; default = next argument. Note that a format string may have either default argument indexes or non-default argument indexes, but not both (to avoid confusion).
  • key: if the argument is a container (C-style array or pointer, std::array, vector, deque, map), you may use this to select the element to format; works with random-access sequences and integer- and string-keyed maps. Multiple level keys work as well, with components separated with "."; for example, given map<string, map<string, string>> m, {[foo.bar]} selects m["foo"]["bar"].
  • format_spec: format specification, see below

Format specification: [[fill] align] [sign] ["#"] ["0"] [width] [","] ["." precision] ["."] [type]

  • fill (may only be specified if align is also specified): pad with this character (' ' (space) or '0' (zero) might be useful; space is default)
  • align: one of '<', '>', '=', '^':
    • '<': left-align (default for most objects)
    • '>': right-align (default for numbers)
    • '=': pad after sign, but before significant digits; used to print -0000120; only valid for numbers
    • '^': center
  • sign: one of '+', '-', ' ' (space) (only valid for numbers)
    • '+': output '+' if positive or zero, '-' if negative
    • '-': output '-' if negative, nothing otherwise (default)
    • ' ' (space): output ' ' (space) if positive or zero, '-' if negative
  • '#': output base prefix (0 for octal, 0b or 0B for binary, 0x or 0X for hexadecimal; only valid for integers)
  • '0': 0-pad after sign, same as specifying "0=" as the fill and align parameters (only valid for numbers)
  • width: minimum field width. May be '*' to indicate that the field width is given by an argument. Defaults to the next argument (preceding the value to be formatted) but an explicit argument index may be given following the '*'.
  • ',' (comma): output comma as thousands' separator (only valid for integers, and only for decimal output)
  • precision (not allowed for integers):
    • for floating point values, number of digits after decimal point ('f' or 'F' presentation) or number of significant digits ('g' or 'G')
    • for others, maximum field size (truncate subsequent characters)
  • '.' (when used after precision or in lieu of precison): Forces a trailing decimal point to make it clear this is a floating point value.
  • type: presentation format, see below

Presentation formats:

  • Strings (folly::StringPiece, std::string, folly::fbstring, const char*):
    • 's' (default)
  • Integers:
    • 'b': output in binary (base 2) ("0b" prefix if '#' specified)
    • 'B': output in binary (base 2) ("0B" prefix if '#' specified)
    • 'c': output as a character (cast to char)
    • 'd': output in decimal (base 10) (default)
    • 'o': output in octal (base 8)
    • 'O': output in octal (base 8) (same as 'o')
    • 'x': output in hexadecimal (base 16) (lower-case digits above 9)
    • 'X': output in hexadecimal (base 16) (upper-case digits above 9)
    • 'n': locale-aware output (currently same as 'd')
  • bool:
    • default: output "true" or "false" as strings
    • integer presentations allowed as well
  • char:
    • same as other integers, but default is 'c' instead of 'd'
  • Floating point (float, double; long double is not implemented):
    • 'e': scientific notation using 'e' as exponent character
    • 'E': scientific notation using 'E' as exponent character
    • 'f': fixed point
    • 'F': fixed point (same as 'f')
    • 'g': general; use either 'f' or 'e' depending on magnitude (default)
    • 'G': general; use either 'f' or 'E' depending on magnitude
    • 'n': locale-aware version of 'g' (currently same as 'g')
    • '%': percentage: multiply by 100 then display as 'f'

Extension

You can extend format for your own class by providing a specialization for folly::FormatValue. See folly/Format.h and folly/FormatArg.h for details, and the existing specialization for folly::dynamic in folly/dynamic-inl.h for an implementation example.