`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](http://docs.python.org/library/string.html#formatstrings).
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.

To use `format`, you need to be using gcc 4.6 or later.  You'll want to include
`folly/Format.h`.

### Overview
***

Here are some code samples to get started:

``` Cpp
using folly::format;
using folly::sformat;
using folly::vformat;
using folly::svformat;

// 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"

// If you only have one container argument, vformat makes the syntax simpler
std::map<std::string, std::string> m { {"what", "answer"}, {"value", "42"} };
std::cout << vformat("The only {what} is {value}", m);
// => "The only answer is 42"
// same as
std::cout << format("The only {0[what]} is {0[value]}", m);
// => "The only answer is 42"
// And if you just want the string,
std::string result = svformat("The only {what} is {value}", m);
// => "The only answer is 42"
std::string result = svformat("The only {0[what]} is {0[value]}", m);
// => "The only answer is 42"

// {} works for vformat too
std::vector<int> v {42, 23};
std::cout << vformat("{} {}", v);
// => "42 23"

// format and vformat work with pairs and tuples
std::tuple<int, std::string, int> t {42, "hello", 23};
std::cout << vformat("{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"

// 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(),
// str(), and fbstr() methods
std::string s = format("The only answer is {}", 42).str();
std::cout << s;
// => "The only answer is 42"
```


### 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 string (`vformat`):
`"{" [ key ] [":" format_spec] "}"`

- `key`: select the argument to format from the container argument;
  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
- '`,`' (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.
