# std.format

This module implements the workhorse functionality for string and I/O formatting. It's comparable to C99's vsprintf().**Source:**

std/format.d

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

- void
__doFormat__(void delegate(dchar)*putc*, TypeInfo[]*arguments*, va_list*argptr*); - Interprets variadic argument list pointed to by
*argptr*whose types are given by*arguments*[], formats them according to embedded format strings in the variadic argument list, and sends the resulting characters to*putc*.

The variadic*arguments*are consumed in order. Each is formatted into a sequence of chars, using the default format specification for its type, and the characters are sequentially passed to*putc*. If a char[], wchar[], or dchar[] argument is encountered, it is interpreted as a format string. As many*arguments*as specified in the format string are consumed and formatted according to the format specifications in that string and passed to*putc*. If there are too few remaining*arguments*, a FormatError is thrown. If there are more remaining*arguments*than needed by the format specification, the default processing of*arguments*resumes until they are all consumed.

**Params:**

void delegate(dchar) *putc*Output is sent do this delegate, character by character. TypeInfo[] *arguments*Array of TypeInfo's, one for each argument to be formatted. va_list *argptr*Points to 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.

A*format specification*starts with a '%' character, and has the following grammar:

*FormatSpecification*:**'%%'****'%'***Flags**Width**Precision**FormatChar*

*Flags*:*empty***'-'***Flags***'+'***Flags***'#'***Flags***'0'***Flags***' '***Flags*

*Width*:*empty**Integer***'*'**

*Precision*:*empty***'.'****'.'***Integer***'.*'**

*Integer*:*Digit**Digit**Integer*

*Digit*:**'0'****'1'****'2'****'3'****'4'****'5'****'6'****'7'****'8'****'9'**

*FormatChar*:**'s'****'b'****'d'****'o'****'x'****'X'****'e'****'E'****'f'****'F'****'g'****'G'****'a'****'A'***Flags***'-'**-
Left justify the result in the field.
It overrides any
**0**flag.

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

**'#'**- Use alternative formatting:
- For
**'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. - For
**'x'**(**'X'**): - If non-zero, prefix result with
**0x**(**0X**). - For floating point formatting:
- Always insert the decimal point.
- For
**'g'**(**'G'**): - Do not elide trailing zeros.

- For
**'0'**- For integer and floating point formatting when not nan or
infinity, use leading zeros
to pad rather than spaces.
Ignore if there's a
*Precision*.

**' '**- 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
`'`or**true**'`'`.**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 [s
_{0}, s_{1}, ...] where s_{k}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.dddddd*e*±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 0x
*h.hhhhhh*p*±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 formattedPrint(...) { void putc(char c) { fputc(c, stdout); } std.format.doFormat(&putc, _arguments, _argptr); } ... int x = 27; // prints 'The answer is 27:6' formattedPrint("The answer is %s:", x, 6);