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Last update Thu Sep 16 11:25:58 2010

# std.format

This module implements the formatting functionality for strings and I/O. It's comparable to C99's vsprintf() and uses a similar format encoding scheme.

License:
Boost License 1.0.

Authors:
Walter Bright, Andrei Alexandrescu

class FormatError: object.Error;
  Signals a mismatch between a format and its corresponding argument.

void formattedWrite(Writer, Char, A...)(Writer w, in Char[] fmt, A args);
  Interprets variadic argument list args, formats them according to fmt, and sends the resulting characters to w. The encoding of the output is the same as Char. type Writer must satisfy std.range.isOutputRange!(Writer, Char).

  The variadic arguments are normally consumed in order. POSIX-style positional parameter syntax is also supported. Each argument is formatted into a sequence of chars according to the format specification, and the characters are passed to w. As many arguments as specified in the format string are consumed and formatted. If there are fewer arguments than format specifiers, a FormatError is thrown. If there are more remaining arguments than needed by the format specification, they are ignored but only if at least one argument was formatted.

  Parameters:

  w | Output is sent do this writer. Typical output writers include std.range.Appender!string and std.stdio.BlockingTextWriter.
  fmt | Format string.
  args | Variadic argument list.

  Throws:
  Mismatched arguments and formats result in a FormatError being thrown.

  Format String:
  Format strings consist of characters interspersed with format specifications. Characters are simply copied to the output (such as putc) after any necessary conversion to the corresponding UTF-8 sequence.

  The format string has the following grammar:

  Flags affect formatting depending on the specifier as follows.
  Flag | Types affected | Semantics
  --- | --- | ---

  '-' | numeric | Left justify the result in the field. It overrides any 0 flag.

  '+' | numeric | Prefix positive numbers in a signed conversion with a +. It overrides any space flag.

  '#' | integral ('o') | Add to precision as necessary so that the first digit of the octal formatting is a '0', even if both the argument and the Precision are zero.

  '#' | integral ('x', 'X') | If non-zero, prefix result with 0x (0X).

  '#' | floating | Always insert the decimal point and print trailing zeros.

  '#' | numeric ('0') | Use leading zeros to pad rather than spaces (except for the floating point values nan and infinity). Ignore if there's a Precision.

  ' ' | integral ('d') | Prefix positive numbers in a signed conversion with a space.

Width
  Specifies the minimum field width. If the width is a *, the next argument, which must be of type int, is taken as the width. If the width is negative, it is as if the - was given as a Flags character.

Precision
  Gives the precision for numeric conversions. If the precision is a *, the next argument, which must be of type int, is taken as the precision. If it is negative, it is as if there was no Precision.

FormatChar
  's'
    The corresponding argument is formatted in a manner consistent with its type:
    bool
      The result is `'true'` or `'false'`.
    integral types
      The %d format is used.
    floating point types
      The %g format is used.
    string types
      The result is the string converted to UTF-8. A Precision specifies the maximum number of characters to use in the result.
    classes derived from Object
      The result is the string returned from the class instance's .toString() method. A Precision specifies the maximum number of characters to use in the result.
    non-string static and dynamic arrays
      The result is [s0, s1, ...] where sk is the kth element formatted with the default format.

  'b','d','o','x','X'
    The corresponding argument must be an integral type and is formatted as an integer. If the argument is a signed type and the FormatChar is d it is converted to a signed string of characters, otherwise it is treated as unsigned. An argument of type bool is formatted as '1' or '0'. The base used is binary for b, octal for o, decimal for d, and hexadecimal for x or X. x formats using lower case letters, X uppercase. If there are fewer resulting digits than the Precision, leading zeros are used as necessary. If the Precision is 0 and the number is 0, no digits result.

  'e','E'
    A floating point number is formatted as one digit before the decimal point, Precision digits after, the FormatChar, ±, followed by at least a two digit exponent: d.dddddde±dd. If there is no Precision, six digits are generated after the decimal point. If the Precision is 0, no decimal point is generated.

  'f','F'
    A floating point number is formatted in decimal notation. The Precision specifies the number of digits generated after the decimal point. It defaults to six. At least one digit is generated before the decimal point. If the Precision is zero, no decimal point is generated.

  'g','G'
    A floating point number is formatted in either e or f format for g; E or F format for G. The f format is used if the exponent for an e format is greater than -5 and less than the Precision. The Precision specifies the number of significant digits, and defaults to six. Trailing zeros are elided after the decimal point, if the fractional part is zero then no decimal point is generated.

  'a','A'
    A floating point number is formatted in hexadecimal exponential notation 0xh.hhhhhhp±d. There is one hexadecimal digit before the decimal point, and as many after as specified by the Precision. If the Precision is zero, no decimal point is generated. If there is no Precision, as many hexadecimal digits as necessary to exactly represent the mantissa are generated. The exponent is written in as few digits as possible, but at least one, is in decimal, and represents a power of 2 as in h.hhhhhh*2±d. The exponent for zero is zero. The hexadecimal digits, x and p are in upper case if the FormatChar is upper case.

  Floating point NaN's are formatted as nan if the FormatChar is lower case, or NAN if upper. Floating point infinities are formatted as inf or infinity if the FormatChar is lower case, or INF or INFINITY if upper.

Example:

    import std.c.stdio;
    import std.format;

    void main()
    {
        auto writer = appender!string();
        formattedWrite(writer, "%s is the ultimate %s.", 42, "answer");
        assert(writer.data == "42 is the ultimate answer.");
        // Clear the writer
        writer = appender!string();
        formattedWrite(writer, "Date: %2$s %1$s", "October", 5);
        assert(writer.data == "Date: 5 October");
    }

The positional and non-positional styles can be mixed in the same format string. (POSIX leaves this behavior undefined.) The internal counter for non-positional parameters tracks the next parameter after the largest positional parameter already used.

void formattedRead(R, Char, S...)(ref R r, const(Char)[] fmt, S args);
  Reads characters from input range r, converts them according to fmt, and writes them to args.

  Example:

      string s = "hello!124:34.5";
      string a;
      int b;
      double c;
      formattedRead(s, "%s!%s:%s", &a, &b, &c);
      assert(a == "hello" && b == 124 && c == 34.5);

struct FormatSpec(Char);
  A compiled version of an individual format specifier, backwards compatible with printf specifiers.

  int width;
    Minimum width, default 0.

  int precision;
    Precision. Its semantics depends on the argument type. For floating point numbers, precision dictates the number of decimals printed.

  int DYNAMIC;
    Special value for width and precision. DYNAMIC width or precision means that they were specified with '*' in the format string and are passed at runtime through the varargs.

  int UNSPECIFIED;
    Special value for precision, meaning the format specifier contained no explicit precision.

  char spec;
    The actual format specifier, 's' by default.

  ubyte index;
    Index of the argument for positional parameters, from 1 to ubyte.max. (0 means not used).

  bool flDash;
    The format specifier contained a '-' (printf compatibility).

  bool flZero;
    The format specifier contained a '0' (printf compatibility).

  bool flSpace;
    The format specifier contained a ' ' (printf compatibility).

  bool flPlus;
    The format specifier contained a '+' (printf compatibility).

  bool flHash;
    The format specifier contained a '#' (printf compatibility).

  const(Char)[] nested;
    In case of a compound format specifier starting with "%(" and ending with "%)", nested contains the string contained within the two separators.

  const(Char)[] trailing;
    trailing contains the rest of the format string.

  this(in Char[] fmt);
    Given a string format specification fmt, parses a format specifier. The string is assumed to start with the character immediately following the '%'. The string is advanced to right after the end of the format specifier.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  void[] is formatted like ubyte[].

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  enum is formatted like its base value.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  Integrals are formatted like printf does.

void formatValue(Writer, D, Char)(Writer w, D obj, ref FormatSpec!(Char) f);
  Floating-point values are formatted like printf does.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  bool is formatted as "true" or "false" with %s and as "1" or "0" with integral-specific format specs.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  Individual characters (char, wchar, or dchar) are formatted as Unicode characters with %s and as integers with integral-specific format specs.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  Strings are formatted like printf does.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  Input ranges are formatted like arrays.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  void[0] is formatted as "[]".

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  Pointers are formatted as hex integers.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  Objects are formatted by calling toString.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  Interfaces are formatted by casting to Object and then calling toString.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  Associative arrays are formatted by using ':' and ' ' as separators.

void formatValue(Writer, T, Char)(Writer w, T val, ref FormatSpec!(Char) f);
  Associative arrays are formatted by using ':' and ' ' as separators.

void formatValue(Writer, T, Char)(Writer w, ref T val, ref FormatSpec!(Char) f);
  Static-size arrays are formatted just like arrays.

T unformatValue(T, Range, Char)(ref Range input, ref FormatSpec!(Char) spec);
  Reads an array (except for string types) and returns it.

T unformatValue(T, Range, Char)(ref Range input, ref FormatSpec!(Char) spec);
  Reads a string and returns it.

T unformatValue(T, Range, Char)(ref Range input, ref FormatSpec!(Char) spec);
  Reads an integral value and returns it.

T unformatValue(T, Range, Char)(ref Range input, ref FormatSpec!(Char) spec);
  Reads a floating-point value and returns it.

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