1.. _syntax: 2 3******************** 4Format String Syntax 5******************** 6 7Formatting functions such as :ref:`fmt::format() <format>` and 8:ref:`fmt::print() <print>` use the same format string syntax described in this 9section. 10 11Format strings contain "replacement fields" surrounded by curly braces ``{}``. 12Anything that is not contained in braces is considered literal text, which is 13copied unchanged to the output. If you need to include a brace character in the 14literal text, it can be escaped by doubling: ``{{`` and ``}}``. 15 16The grammar for a replacement field is as follows: 17 18.. productionlist:: sf 19 replacement_field: "{" [`arg_id`] [":" `format_spec`] "}" 20 arg_id: `integer` | `identifier` 21 integer: `digit`+ 22 digit: "0"..."9" 23 identifier: `id_start` `id_continue`* 24 id_start: "a"..."z" | "A"..."Z" | "_" 25 id_continue: `id_start` | `digit` 26 27In less formal terms, the replacement field can start with an *arg_id* 28that specifies the argument whose value is to be formatted and inserted into 29the output instead of the replacement field. 30The *arg_id* is optionally followed by a *format_spec*, which is preceded 31by a colon ``':'``. These specify a non-default format for the replacement value. 32 33See also the :ref:`formatspec` section. 34 35If the numerical arg_ids in a format string are 0, 1, 2, ... in sequence, 36they can all be omitted (not just some) and the numbers 0, 1, 2, ... will be 37automatically inserted in that order. 38 39Named arguments can be referred to by their names or indices. 40 41Some simple format string examples:: 42 43 "First, thou shalt count to {0}" // References the first argument 44 "Bring me a {}" // Implicitly references the first argument 45 "From {} to {}" // Same as "From {0} to {1}" 46 47The *format_spec* field contains a specification of how the value should be 48presented, including such details as field width, alignment, padding, decimal 49precision and so on. Each value type can define its own "formatting 50mini-language" or interpretation of the *format_spec*. 51 52Most built-in types support a common formatting mini-language, which is 53described in the next section. 54 55A *format_spec* field can also include nested replacement fields in certain 56positions within it. These nested replacement fields can contain only an 57argument id; format specifications are not allowed. This allows the formatting 58of a value to be dynamically specified. 59 60See the :ref:`formatexamples` section for some examples. 61 62.. _formatspec: 63 64Format Specification Mini-Language 65================================== 66 67"Format specifications" are used within replacement fields contained within a 68format string to define how individual values are presented (see 69:ref:`syntax`). Each formattable type may define how the format 70specification is to be interpreted. 71 72Most built-in types implement the following options for format specifications, 73although some of the formatting options are only supported by the numeric types. 74 75The general form of a *standard format specifier* is: 76 77.. productionlist:: sf 78 format_spec: [[`fill`]`align`][`sign`]["#"]["0"][`width`]["." `precision`][`type`] 79 fill: <a character other than '{' or '}'> 80 align: "<" | ">" | "^" 81 sign: "+" | "-" | " " 82 width: `integer` | "{" [`arg_id`] "}" 83 precision: `integer` | "{" [`arg_id`] "}" 84 type: `int_type` | "a" | "A" | "c" | "e" | "E" | "f" | "F" | "g" | "G" | "L" | "p" | "s" 85 int_type: "b" | "B" | "d" | "o" | "x" | "X" 86 87The *fill* character can be any Unicode code point other than ``'{'`` or 88``'}'``. The presence of a fill character is signaled by the character following 89it, which must be one of the alignment options. If the second character of 90*format_spec* is not a valid alignment option, then it is assumed that both the 91fill character and the alignment option are absent. 92 93The meaning of the various alignment options is as follows: 94 95+---------+----------------------------------------------------------+ 96| Option | Meaning | 97+=========+==========================================================+ 98| ``'<'`` | Forces the field to be left-aligned within the available | 99| | space (this is the default for most objects). | 100+---------+----------------------------------------------------------+ 101| ``'>'`` | Forces the field to be right-aligned within the | 102| | available space (this is the default for numbers). | 103+---------+----------------------------------------------------------+ 104| ``'^'`` | Forces the field to be centered within the available | 105| | space. | 106+---------+----------------------------------------------------------+ 107 108Note that unless a minimum field width is defined, the field width will always 109be the same size as the data to fill it, so that the alignment option has no 110meaning in this case. 111 112The *sign* option is only valid for number types, and can be one of the 113following: 114 115+---------+----------------------------------------------------------+ 116| Option | Meaning | 117+=========+==========================================================+ 118| ``'+'`` | indicates that a sign should be used for both | 119| | positive as well as negative numbers. | 120+---------+----------------------------------------------------------+ 121| ``'-'`` | indicates that a sign should be used only for negative | 122| | numbers (this is the default behavior). | 123+---------+----------------------------------------------------------+ 124| space | indicates that a leading space should be used on | 125| | positive numbers, and a minus sign on negative numbers. | 126+---------+----------------------------------------------------------+ 127 128The ``'#'`` option causes the "alternate form" to be used for the 129conversion. The alternate form is defined differently for different 130types. This option is only valid for integer and floating-point types. 131For integers, when binary, octal, or hexadecimal output is used, this 132option adds the prefix respective ``"0b"`` (``"0B"``), ``"0"``, or 133``"0x"`` (``"0X"``) to the output value. Whether the prefix is 134lower-case or upper-case is determined by the case of the type 135specifier, for example, the prefix ``"0x"`` is used for the type ``'x'`` 136and ``"0X"`` is used for ``'X'``. For floating-point numbers the 137alternate form causes the result of the conversion to always contain a 138decimal-point character, even if no digits follow it. Normally, a 139decimal-point character appears in the result of these conversions 140only if a digit follows it. In addition, for ``'g'`` and ``'G'`` 141conversions, trailing zeros are not removed from the result. 142 143.. ifconfig:: False 144 145 The ``','`` option signals the use of a comma for a thousands separator. 146 For a locale aware separator, use the ``'L'`` integer presentation type 147 instead. 148 149*width* is a decimal integer defining the minimum field width. If not 150specified, then the field width will be determined by the content. 151 152Preceding the *width* field by a zero (``'0'``) character enables sign-aware 153zero-padding for numeric types. It forces the padding to be placed after the 154sign or base (if any) but before the digits. This is used for printing fields in 155the form '+000000120'. This option is only valid for numeric types and it has no 156effect on formatting of infinity and NaN. 157 158The *precision* is a decimal number indicating how many digits should be 159displayed after the decimal point for a floating-point value formatted with 160``'f'`` and ``'F'``, or before and after the decimal point for a floating-point 161value formatted with ``'g'`` or ``'G'``. For non-number types the field 162indicates the maximum field size - in other words, how many characters will be 163used from the field content. The *precision* is not allowed for integer, 164character, Boolean, and pointer values. 165 166Finally, the *type* determines how the data should be presented. 167 168The available string presentation types are: 169 170+---------+----------------------------------------------------------+ 171| Type | Meaning | 172+=========+==========================================================+ 173| ``'s'`` | String format. This is the default type for strings and | 174| | may be omitted. | 175+---------+----------------------------------------------------------+ 176| none | The same as ``'s'``. | 177+---------+----------------------------------------------------------+ 178 179The available character presentation types are: 180 181+---------+----------------------------------------------------------+ 182| Type | Meaning | 183+=========+==========================================================+ 184| ``'c'`` | Character format. This is the default type for | 185| | characters and may be omitted. | 186+---------+----------------------------------------------------------+ 187| none | The same as ``'c'``. | 188+---------+----------------------------------------------------------+ 189 190The available integer presentation types are: 191 192+---------+----------------------------------------------------------+ 193| Type | Meaning | 194+=========+==========================================================+ 195| ``'b'`` | Binary format. Outputs the number in base 2. Using the | 196| | ``'#'`` option with this type adds the prefix ``"0b"`` | 197| | to the output value. | 198+---------+----------------------------------------------------------+ 199| ``'B'`` | Binary format. Outputs the number in base 2. Using the | 200| | ``'#'`` option with this type adds the prefix ``"0B"`` | 201| | to the output value. | 202+---------+----------------------------------------------------------+ 203| ``'d'`` | Decimal integer. Outputs the number in base 10. | 204+---------+----------------------------------------------------------+ 205| ``'o'`` | Octal format. Outputs the number in base 8. | 206+---------+----------------------------------------------------------+ 207| ``'x'`` | Hex format. Outputs the number in base 16, using | 208| | lower-case letters for the digits above 9. Using the | 209| | ``'#'`` option with this type adds the prefix ``"0x"`` | 210| | to the output value. | 211+---------+----------------------------------------------------------+ 212| ``'X'`` | Hex format. Outputs the number in base 16, using | 213| | upper-case letters for the digits above 9. Using the | 214| | ``'#'`` option with this type adds the prefix ``"0X"`` | 215| | to the output value. | 216+---------+----------------------------------------------------------+ 217| ``'L'`` | Locale-specific format. This is the same as ``'d'``, | 218| | except that it uses the current locale setting to insert | 219| | the appropriate number separator characters. | 220+---------+----------------------------------------------------------+ 221| none | The same as ``'d'``. | 222+---------+----------------------------------------------------------+ 223 224Integer presentation types can also be used with character and Boolean values. 225Boolean values are formatted using textual representation, either ``true`` or 226``false``, if the presentation type is not specified. 227 228The available presentation types for floating-point values are: 229 230+---------+----------------------------------------------------------+ 231| Type | Meaning | 232+=========+==========================================================+ 233| ``'a'`` | Hexadecimal floating point format. Prints the number in | 234| | base 16 with prefix ``"0x"`` and lower-case letters for | 235| | digits above 9. Uses ``'p'`` to indicate the exponent. | 236+---------+----------------------------------------------------------+ 237| ``'A'`` | Same as ``'a'`` except it uses upper-case letters for | 238| | the prefix, digits above 9 and to indicate the exponent. | 239+---------+----------------------------------------------------------+ 240| ``'e'`` | Exponent notation. Prints the number in scientific | 241| | notation using the letter 'e' to indicate the exponent. | 242+---------+----------------------------------------------------------+ 243| ``'E'`` | Exponent notation. Same as ``'e'`` except it uses an | 244| | upper-case ``'E'`` as the separator character. | 245+---------+----------------------------------------------------------+ 246| ``'f'`` | Fixed point. Displays the number as a fixed-point | 247| | number. | 248+---------+----------------------------------------------------------+ 249| ``'F'`` | Fixed point. Same as ``'f'``, but converts ``nan`` to | 250| | ``NAN`` and ``inf`` to ``INF``. | 251+---------+----------------------------------------------------------+ 252| ``'g'`` | General format. For a given precision ``p >= 1``, | 253| | this rounds the number to ``p`` significant digits and | 254| | then formats the result in either fixed-point format | 255| | or in scientific notation, depending on its magnitude. | 256| | | 257| | A precision of ``0`` is treated as equivalent to a | 258| | precision of ``1``. | 259+---------+----------------------------------------------------------+ 260| ``'G'`` | General format. Same as ``'g'`` except switches to | 261| | ``'E'`` if the number gets too large. The | 262| | representations of infinity and NaN are uppercased, too. | 263+---------+----------------------------------------------------------+ 264| ``'L'`` | Locale-specific format. This is the same as ``'g'``, | 265| | except that it uses the current locale setting to insert | 266| | the appropriate number separator characters. | 267+---------+----------------------------------------------------------+ 268| none | Similar to ``'g'``, except that fixed-point notation, | 269| | when used, has at least one digit past the decimal | 270| | point. The default precision is as high as needed to | 271| | represent the particular value. | 272+---------+----------------------------------------------------------+ 273 274.. ifconfig:: False 275 276 +---------+----------------------------------------------------------+ 277 | | The precise rules are as follows: suppose that the | 278 | | result formatted with presentation type ``'e'`` and | 279 | | precision ``p-1`` would have exponent ``exp``. Then | 280 | | if ``-4 <= exp < p``, the number is formatted | 281 | | with presentation type ``'f'`` and precision | 282 | | ``p-1-exp``. Otherwise, the number is formatted | 283 | | with presentation type ``'e'`` and precision ``p-1``. | 284 | | In both cases insignificant trailing zeros are removed | 285 | | from the significand, and the decimal point is also | 286 | | removed if there are no remaining digits following it. | 287 | | | 288 | | Positive and negative infinity, positive and negative | 289 | | zero, and nans, are formatted as ``inf``, ``-inf``, | 290 | | ``0``, ``-0`` and ``nan`` respectively, regardless of | 291 | | the precision. | 292 | | | 293 +---------+----------------------------------------------------------+ 294 295The available presentation types for pointers are: 296 297+---------+----------------------------------------------------------+ 298| Type | Meaning | 299+=========+==========================================================+ 300| ``'p'`` | Pointer format. This is the default type for | 301| | pointers and may be omitted. | 302+---------+----------------------------------------------------------+ 303| none | The same as ``'p'``. | 304+---------+----------------------------------------------------------+ 305 306.. _formatexamples: 307 308Format Examples 309=============== 310 311This section contains examples of the format syntax and comparison with 312the printf formatting. 313 314In most of the cases the syntax is similar to the printf formatting, with the 315addition of the ``{}`` and with ``:`` used instead of ``%``. 316For example, ``"%03.2f"`` can be translated to ``"{:03.2f}"``. 317 318The new format syntax also supports new and different options, shown in the 319following examples. 320 321Accessing arguments by position:: 322 323 fmt::format("{0}, {1}, {2}", 'a', 'b', 'c'); 324 // Result: "a, b, c" 325 fmt::format("{}, {}, {}", 'a', 'b', 'c'); 326 // Result: "a, b, c" 327 fmt::format("{2}, {1}, {0}", 'a', 'b', 'c'); 328 // Result: "c, b, a" 329 fmt::format("{0}{1}{0}", "abra", "cad"); // arguments' indices can be repeated 330 // Result: "abracadabra" 331 332Aligning the text and specifying a width:: 333 334 fmt::format("{:<30}", "left aligned"); 335 // Result: "left aligned " 336 fmt::format("{:>30}", "right aligned"); 337 // Result: " right aligned" 338 fmt::format("{:^30}", "centered"); 339 // Result: " centered " 340 fmt::format("{:*^30}", "centered"); // use '*' as a fill char 341 // Result: "***********centered***********" 342 343Dynamic width:: 344 345 fmt::format("{:<{}}", "left aligned", 30); 346 // Result: "left aligned " 347 348Dynamic precision:: 349 350 fmt::format("{:.{}f}", 3.14, 1); 351 // Result: "3.1" 352 353Replacing ``%+f``, ``%-f``, and ``% f`` and specifying a sign:: 354 355 fmt::format("{:+f}; {:+f}", 3.14, -3.14); // show it always 356 // Result: "+3.140000; -3.140000" 357 fmt::format("{: f}; {: f}", 3.14, -3.14); // show a space for positive numbers 358 // Result: " 3.140000; -3.140000" 359 fmt::format("{:-f}; {:-f}", 3.14, -3.14); // show only the minus -- same as '{:f}; {:f}' 360 // Result: "3.140000; -3.140000" 361 362Replacing ``%x`` and ``%o`` and converting the value to different bases:: 363 364 fmt::format("int: {0:d}; hex: {0:x}; oct: {0:o}; bin: {0:b}", 42); 365 // Result: "int: 42; hex: 2a; oct: 52; bin: 101010" 366 // with 0x or 0 or 0b as prefix: 367 fmt::format("int: {0:d}; hex: {0:#x}; oct: {0:#o}; bin: {0:#b}", 42); 368 // Result: "int: 42; hex: 0x2a; oct: 052; bin: 0b101010" 369 370Padded hex byte with prefix and always prints both hex characters:: 371 372 fmt::format("{:#04x}", 0); 373 // Result: "0x00" 374 375Box drawing using Unicode fill:: 376 377 fmt::print( 378 "┌{0:─^{2}}┐\n" 379 "│{1: ^{2}}│\n" 380 "└{0:─^{2}}┘\n", "", "Hello, world!", 20); 381 382prints:: 383 384 ┌────────────────────┐ 385 │ Hello, world! │ 386 └────────────────────┘ 387 388Using type-specific formatting:: 389 390 #include <fmt/chrono.h> 391 392 auto t = tm(); 393 t.tm_year = 2010 - 1900; 394 t.tm_mon = 6; 395 t.tm_mday = 4; 396 t.tm_hour = 12; 397 t.tm_min = 15; 398 t.tm_sec = 58; 399 fmt::print("{:%Y-%m-%d %H:%M:%S}", t); 400 // Prints: 2010-08-04 12:15:58 401 402Using the comma as a thousands separator:: 403 404 #include <fmt/locale.h> 405 406 auto s = fmt::format(std::locale("en_US.UTF-8"), "{:L}", 1234567890); 407 // s == "1,234,567,890" 408 409.. ifconfig:: False 410 411 Nesting arguments and more complex examples:: 412 413 >>> for align, text in zip('<^>', ['left', 'center', 'right']): 414 ... '{0:{fill}{align}16}") << text, fill=align, align=align) 415 ... 416 'left<<<<<<<<<<<<' 417 '^^^^^center^^^^^' 418 '>>>>>>>>>>>right' 419 >>> 420 >>> octets = [192, 168, 0, 1] 421 Format("{:02X}{:02X}{:02X}{:02X}") << *octets) 422 'C0A80001' 423 >>> int(_, 16) 424 3232235521 425 >>> 426 >>> width = 5 427 >>> for num in range(5,12): 428 ... for base in 'dXob': 429 ... print('{0:{width}{base}}") << num, base=base, width=width), end=' ') 430 ... print() 431 ... 432 5 5 5 101 433 6 6 6 110 434 7 7 7 111 435 8 8 10 1000 436 9 9 11 1001 437 10 A 12 1010 438 11 B 13 1011 439