438 lines
20 KiB
ReStructuredText
438 lines
20 KiB
ReStructuredText
.. _syntax:
|
|
|
|
********************
|
|
Format String Syntax
|
|
********************
|
|
|
|
Formatting functions such as :ref:`fmt::format() <format>` and
|
|
:ref:`fmt::print() <print>` use the same format string syntax described in this
|
|
section.
|
|
|
|
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. If you need to include a brace character in the
|
|
literal text, it can be escaped by doubling: ``{{`` and ``}}``.
|
|
|
|
The grammar for a replacement field is as follows:
|
|
|
|
.. productionlist:: sf
|
|
replacement_field: "{" [`arg_id`] [":" `format_spec`] "}"
|
|
arg_id: `integer` | `identifier`
|
|
integer: `digit`+
|
|
digit: "0"..."9"
|
|
identifier: `id_start` `id_continue`*
|
|
id_start: "a"..."z" | "A"..."Z" | "_"
|
|
id_continue: `id_start` | `digit`
|
|
|
|
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 :ref:`formatspec` 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.
|
|
|
|
Named arguments can be referred to by their names or indices.
|
|
|
|
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
|
|
positions within it. These nested replacement fields can contain only an
|
|
argument id; format specifications are not allowed. This allows the formatting
|
|
of a value to be dynamically specified.
|
|
|
|
See the :ref:`formatexamples` section for some examples.
|
|
|
|
.. _formatspec:
|
|
|
|
Format Specification Mini-Language
|
|
==================================
|
|
|
|
"Format specifications" are used within replacement fields contained within a
|
|
format string to define how individual values are presented (see
|
|
:ref:`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:
|
|
|
|
.. productionlist:: sf
|
|
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" | "L" | "p" | "s"
|
|
int_type: "b" | "B" | "d" | "o" | "x" | "X"
|
|
|
|
The *fill* character can be any Unicode code point 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:
|
|
|
|
+---------+----------------------------------------------------------+
|
|
| Option | Meaning |
|
|
+=========+==========================================================+
|
|
| ``'<'`` | Forces the field to be left-aligned within the available |
|
|
| | space (this is the default for most objects). |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'>'`` | Forces the field to be right-aligned within the |
|
|
| | available space (this is the default for numbers). |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'^'`` | Forces the field to be centered within the available |
|
|
| | space. |
|
|
+---------+----------------------------------------------------------+
|
|
|
|
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:
|
|
|
|
+---------+----------------------------------------------------------+
|
|
| Option | Meaning |
|
|
+=========+==========================================================+
|
|
| ``'+'`` | indicates that a sign should be used for both |
|
|
| | positive as well as negative numbers. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'-'`` | indicates that a sign should be used only for negative |
|
|
| | numbers (this is the default behavior). |
|
|
+---------+----------------------------------------------------------+
|
|
| space | indicates that a leading space should be used on |
|
|
| | positive numbers, and a minus sign on negative numbers. |
|
|
+---------+----------------------------------------------------------+
|
|
|
|
The ``'#'`` option causes the "alternate form" to be used for the
|
|
conversion. The alternate form is defined differently for different
|
|
types. This option is only valid for integer and floating-point types.
|
|
For integers, when binary, octal, or hexadecimal output is used, this
|
|
option adds the prefix respective ``"0b"`` (``"0B"``), ``"0"``, or
|
|
``"0x"`` (``"0X"``) to the output value. Whether the prefix is
|
|
lower-case or upper-case is determined by the case of the type
|
|
specifier, for example, the prefix ``"0x"`` is used for the type ``'x'``
|
|
and ``"0X"`` is used for ``'X'``. For floating-point numbers the
|
|
alternate form causes the result of the conversion to always contain a
|
|
decimal-point character, even if no digits follow it. Normally, a
|
|
decimal-point character appears in the result of these conversions
|
|
only if a digit follows it. In addition, for ``'g'`` and ``'G'``
|
|
conversions, trailing zeros are not removed from the result.
|
|
|
|
.. ifconfig:: False
|
|
|
|
The ``','`` option signals the use of a comma for a thousands separator.
|
|
For a locale aware separator, use the ``'L'`` integer presentation type
|
|
instead.
|
|
|
|
*width* is a decimal integer defining the minimum field width. If not
|
|
specified, then the field width will be determined by the content.
|
|
|
|
Preceding the *width* field by a zero (``'0'``) character enables sign-aware
|
|
zero-padding for numeric types. It forces the padding to be placed after the
|
|
sign or base (if any) but before the digits. This is used for printing fields in
|
|
the form '+000000120'. This option is only valid for numeric types and it has no
|
|
effect on formatting of infinity and NaN.
|
|
|
|
The *precision* is a decimal number indicating how many digits should be
|
|
displayed after the decimal point for a floating-point value formatted with
|
|
``'f'`` and ``'F'``, or before and after the decimal point for a floating-point
|
|
value formatted with ``'g'`` or ``'G'``. For non-number types the field
|
|
indicates the maximum field size - in other words, how many characters will be
|
|
used from the field content. The *precision* is not allowed for integer,
|
|
character, Boolean, and pointer values.
|
|
|
|
Finally, the *type* determines how the data should be presented.
|
|
|
|
The available string presentation types are:
|
|
|
|
+---------+----------------------------------------------------------+
|
|
| Type | Meaning |
|
|
+=========+==========================================================+
|
|
| ``'s'`` | String format. This is the default type for strings and |
|
|
| | may be omitted. |
|
|
+---------+----------------------------------------------------------+
|
|
| none | The same as ``'s'``. |
|
|
+---------+----------------------------------------------------------+
|
|
|
|
The available character presentation types are:
|
|
|
|
+---------+----------------------------------------------------------+
|
|
| Type | Meaning |
|
|
+=========+==========================================================+
|
|
| ``'c'`` | Character format. This is the default type for |
|
|
| | characters and may be omitted. |
|
|
+---------+----------------------------------------------------------+
|
|
| none | The same as ``'c'``. |
|
|
+---------+----------------------------------------------------------+
|
|
|
|
The available integer presentation types are:
|
|
|
|
+---------+----------------------------------------------------------+
|
|
| Type | Meaning |
|
|
+=========+==========================================================+
|
|
| ``'b'`` | Binary format. Outputs the number in base 2. Using the |
|
|
| | ``'#'`` option with this type adds the prefix ``"0b"`` |
|
|
| | to the output value. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'B'`` | Binary format. Outputs the number in base 2. Using the |
|
|
| | ``'#'`` option with this type adds the prefix ``"0B"`` |
|
|
| | to the output value. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'d'`` | Decimal integer. Outputs the number in base 10. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'o'`` | Octal format. Outputs the number in base 8. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'x'`` | Hex format. Outputs the number in base 16, using |
|
|
| | lower-case letters for the digits above 9. Using the |
|
|
| | ``'#'`` option with this type adds the prefix ``"0x"`` |
|
|
| | to the output value. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'X'`` | Hex format. Outputs the number in base 16, using |
|
|
| | upper-case letters for the digits above 9. Using the |
|
|
| | ``'#'`` option with this type adds the prefix ``"0X"`` |
|
|
| | to the output value. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'L'`` | Locale-specific format. This is the same as ``'d'``, |
|
|
| | except that it uses the current locale setting to insert |
|
|
| | the appropriate number separator characters. |
|
|
+---------+----------------------------------------------------------+
|
|
| none | The same as ``'d'``. |
|
|
+---------+----------------------------------------------------------+
|
|
|
|
Integer presentation types can also be used with character and Boolean values.
|
|
Boolean values are formatted using textual representation, either ``true`` or
|
|
``false``, if the presentation type is not specified.
|
|
|
|
The available presentation types for floating-point values are:
|
|
|
|
+---------+----------------------------------------------------------+
|
|
| Type | Meaning |
|
|
+=========+==========================================================+
|
|
| ``'a'`` | Hexadecimal floating point format. Prints the number in |
|
|
| | base 16 with prefix ``"0x"`` and lower-case letters for |
|
|
| | digits above 9. Uses ``'p'`` to indicate the exponent. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'A'`` | Same as ``'a'`` except it uses upper-case letters for |
|
|
| | the prefix, digits above 9 and to indicate the exponent. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'e'`` | Exponent notation. Prints the number in scientific |
|
|
| | notation using the letter 'e' to indicate the exponent. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'E'`` | Exponent notation. Same as ``'e'`` except it uses an |
|
|
| | upper-case ``'E'`` as the separator character. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'f'`` | Fixed point. Displays the number as a fixed-point |
|
|
| | number. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'F'`` | Fixed point. Same as ``'f'``, but converts ``nan`` to |
|
|
| | ``NAN`` and ``inf`` to ``INF``. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'g'`` | General format. For a given precision ``p >= 1``, |
|
|
| | this rounds the number to ``p`` significant digits and |
|
|
| | then formats the result in either fixed-point format |
|
|
| | or in scientific notation, depending on its magnitude. |
|
|
| | |
|
|
| | A precision of ``0`` is treated as equivalent to a |
|
|
| | precision of ``1``. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'G'`` | General format. Same as ``'g'`` except switches to |
|
|
| | ``'E'`` if the number gets too large. The |
|
|
| | representations of infinity and NaN are uppercased, too. |
|
|
+---------+----------------------------------------------------------+
|
|
| ``'L'`` | Locale-specific format. This is the same as ``'g'``, |
|
|
| | except that it uses the current locale setting to insert |
|
|
| | the appropriate number separator characters. |
|
|
+---------+----------------------------------------------------------+
|
|
| none | Similar to ``'g'``, except that fixed-point notation, |
|
|
| | when used, has at least one digit past the decimal |
|
|
| | point. The default precision is as high as needed to |
|
|
| | represent the particular value. |
|
|
+---------+----------------------------------------------------------+
|
|
|
|
.. ifconfig:: False
|
|
|
|
+---------+----------------------------------------------------------+
|
|
| | The precise rules are as follows: suppose that the |
|
|
| | result formatted with presentation type ``'e'`` and |
|
|
| | precision ``p-1`` would have exponent ``exp``. Then |
|
|
| | if ``-4 <= exp < p``, the number is formatted |
|
|
| | with presentation type ``'f'`` and precision |
|
|
| | ``p-1-exp``. Otherwise, the number is formatted |
|
|
| | with presentation type ``'e'`` and precision ``p-1``. |
|
|
| | In both cases insignificant trailing zeros are removed |
|
|
| | from the significand, and the decimal point is also |
|
|
| | removed if there are no remaining digits following it. |
|
|
| | |
|
|
| | Positive and negative infinity, positive and negative |
|
|
| | zero, and nans, are formatted as ``inf``, ``-inf``, |
|
|
| | ``0``, ``-0`` and ``nan`` respectively, regardless of |
|
|
| | the precision. |
|
|
| | |
|
|
+---------+----------------------------------------------------------+
|
|
|
|
The available presentation types for pointers are:
|
|
|
|
+---------+----------------------------------------------------------+
|
|
| Type | Meaning |
|
|
+=========+==========================================================+
|
|
| ``'p'`` | Pointer format. This is the default type for |
|
|
| | pointers and may be omitted. |
|
|
+---------+----------------------------------------------------------+
|
|
| none | The same as ``'p'``. |
|
|
+---------+----------------------------------------------------------+
|
|
|
|
.. _formatexamples:
|
|
|
|
Format Examples
|
|
===============
|
|
|
|
This section contains examples of the format syntax and comparison with
|
|
the printf formatting.
|
|
|
|
In most of the cases the syntax is similar to the printf formatting, with the
|
|
addition of the ``{}`` and with ``:`` used instead of ``%``.
|
|
For example, ``"%03.2f"`` can be translated to ``"{:03.2f}"``.
|
|
|
|
The new format syntax also supports new and different options, shown in the
|
|
following examples.
|
|
|
|
Accessing arguments by position::
|
|
|
|
fmt::format("{0}, {1}, {2}", 'a', 'b', 'c');
|
|
// Result: "a, b, c"
|
|
fmt::format("{}, {}, {}", 'a', 'b', 'c');
|
|
// Result: "a, b, c"
|
|
fmt::format("{2}, {1}, {0}", 'a', 'b', 'c');
|
|
// Result: "c, b, a"
|
|
fmt::format("{0}{1}{0}", "abra", "cad"); // arguments' indices can be repeated
|
|
// Result: "abracadabra"
|
|
|
|
Aligning the text and specifying a width::
|
|
|
|
fmt::format("{:<30}", "left aligned");
|
|
// Result: "left aligned "
|
|
fmt::format("{:>30}", "right aligned");
|
|
// Result: " right aligned"
|
|
fmt::format("{:^30}", "centered");
|
|
// Result: " centered "
|
|
fmt::format("{:*^30}", "centered"); // use '*' as a fill char
|
|
// Result: "***********centered***********"
|
|
|
|
Dynamic width::
|
|
|
|
fmt::format("{:<{}}", "left aligned", 30);
|
|
// Result: "left aligned "
|
|
|
|
Dynamic precision::
|
|
|
|
fmt::format("{:.{}f}", 3.14, 1);
|
|
// Result: "3.1"
|
|
|
|
Replacing ``%+f``, ``%-f``, and ``% f`` and specifying a sign::
|
|
|
|
fmt::format("{:+f}; {:+f}", 3.14, -3.14); // show it always
|
|
// Result: "+3.140000; -3.140000"
|
|
fmt::format("{: f}; {: f}", 3.14, -3.14); // show a space for positive numbers
|
|
// Result: " 3.140000; -3.140000"
|
|
fmt::format("{:-f}; {:-f}", 3.14, -3.14); // show only the minus -- same as '{:f}; {:f}'
|
|
// Result: "3.140000; -3.140000"
|
|
|
|
Replacing ``%x`` and ``%o`` and converting the value to different bases::
|
|
|
|
fmt::format("int: {0:d}; hex: {0:x}; oct: {0:o}; bin: {0:b}", 42);
|
|
// Result: "int: 42; hex: 2a; oct: 52; bin: 101010"
|
|
// with 0x or 0 or 0b as prefix:
|
|
fmt::format("int: {0:d}; hex: {0:#x}; oct: {0:#o}; bin: {0:#b}", 42);
|
|
// Result: "int: 42; hex: 0x2a; oct: 052; bin: 0b101010"
|
|
|
|
Padded hex byte with prefix and always prints both hex characters::
|
|
|
|
fmt::format("{:#04x}", 0);
|
|
// Result: "0x00"
|
|
|
|
Box drawing using Unicode fill::
|
|
|
|
fmt::print(
|
|
"┌{0:─^{2}}┐\n"
|
|
"│{1: ^{2}}│\n"
|
|
"└{0:─^{2}}┘\n", "", "Hello, world!", 20);
|
|
|
|
prints::
|
|
|
|
┌────────────────────┐
|
|
│ Hello, world! │
|
|
└────────────────────┘
|
|
|
|
Using type-specific formatting::
|
|
|
|
#include <fmt/chrono.h>
|
|
|
|
auto t = tm();
|
|
t.tm_year = 2010 - 1900;
|
|
t.tm_mon = 6;
|
|
t.tm_mday = 4;
|
|
t.tm_hour = 12;
|
|
t.tm_min = 15;
|
|
t.tm_sec = 58;
|
|
fmt::print("{:%Y-%m-%d %H:%M:%S}", t);
|
|
// Prints: 2010-08-04 12:15:58
|
|
|
|
Using the comma as a thousands separator::
|
|
|
|
#include <fmt/locale.h>
|
|
|
|
auto s = fmt::format(std::locale("en_US.UTF-8"), "{:L}", 1234567890);
|
|
// s == "1,234,567,890"
|
|
|
|
.. ifconfig:: False
|
|
|
|
Nesting arguments and more complex examples::
|
|
|
|
>>> for align, text in zip('<^>', ['left', 'center', 'right']):
|
|
... '{0:{fill}{align}16}") << text, fill=align, align=align)
|
|
...
|
|
'left<<<<<<<<<<<<'
|
|
'^^^^^center^^^^^'
|
|
'>>>>>>>>>>>right'
|
|
>>>
|
|
>>> octets = [192, 168, 0, 1]
|
|
Format("{:02X}{:02X}{:02X}{:02X}") << *octets)
|
|
'C0A80001'
|
|
>>> int(_, 16)
|
|
3232235521
|
|
>>>
|
|
>>> width = 5
|
|
>>> for num in range(5,12):
|
|
... for base in 'dXob':
|
|
... print('{0:{width}{base}}") << num, base=base, width=width), end=' ')
|
|
... print()
|
|
...
|
|
5 5 5 101
|
|
6 6 6 110
|
|
7 7 7 111
|
|
8 8 10 1000
|
|
9 9 11 1001
|
|
10 A 12 1010
|
|
11 B 13 1011
|