Format String Syntax — fmt 8.0.0 documentation (2024)

Formatting functions such as fmt::format() andfmt::print() use the same format string syntax described in thissection.

Format strings contain “replacement fields” surrounded by curly braces {}.Anything that is not contained in braces is considered literal text, which iscopied unchanged to the output. If you need to include a brace character in theliteral text, it can be escaped by doubling: {{ and }}.

The grammar for a replacement field is as follows:

replacement_field ::= "{" [arg_id] [":" (format_spec | chrono_format_spec)] "}"arg_id  ::= integer | identifierinteger  ::= 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_idthat specifies the argument whose value is to be formatted and inserted intothe output instead of the replacement field.The arg_id is optionally followed by a format_spec, which is preceded by acolon ':'. These specify a non-default format for the replacement value.

See also the Format Specification Mini-Language 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 beautomatically 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 bepresented, including such details as field width, alignment, padding, decimalprecision and so on. Each value type can define its own “formattingmini-language” or interpretation of the format_spec.

Most built-in types support a common formatting mini-language, which isdescribed in the next section.

A format_spec field can also include nested replacement fields in certainpositions within it. These nested replacement fields can contain only anargument id; format specifications are not allowed. This allows the formattingof a value to be dynamically specified.

See the Format Examples section for some examples.

Format Specification Mini-Language

“Format specifications” are used within replacement fields contained within aformat string to define how individual values are presented (seeFormat String Syntax). Each formattable type may define how the formatspecification 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:

format_spec ::= [[fill]align][sign]["#"]["0"][width]["." precision]["L"][type]fill  ::= <a character other than '{' or '}'>align  ::= "<" | ">" | "^"sign  ::= "+" | "-" | " "width  ::= integer | "{" [arg_id] "}"precision  ::= integer | "{" [arg_id] "}"type  ::= "a" | "A" | "b" | "B" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "o" | "p" | "s" | "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 followingit, which must be one of the alignment options. If the second character offormat_spec is not a valid alignment option, then it is assumed that both thefill 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 availablespace (this is the default for most objects).

'>'

Forces the field to be right-aligned within theavailable space (this is the default for numbers).

'^'

Forces the field to be centered within the availablespace.

Note that unless a minimum field width is defined, the field width will alwaysbe the same size as the data to fill it, so that the alignment option has nomeaning in this case.

The sign option is only valid for number types, and can be one of thefollowing:

Option

Meaning

'+'

indicates that a sign should be used for bothpositive as well as negative numbers.

'-'

indicates that a sign should be used only for negativenumbers (this is the default behavior).

space

indicates that a leading space should be used onpositive numbers, and a minus sign on negative numbers.

The '#' option causes the “alternate form” to be used for theconversion. The alternate form is defined differently for differenttypes. This option is only valid for integer and floating-point types.For integers, when binary, octal, or hexadecimal output is used, thisoption adds the prefix respective "0b" ("0B"), "0", or"0x" ("0X") to the output value. Whether the prefix islower-case or upper-case is determined by the case of the typespecifier, for example, the prefix "0x" is used for the type 'x'and "0X" is used for 'X'. For floating-point numbers thealternate form causes the result of the conversion to always contain adecimal-point character, even if no digits follow it. Normally, adecimal-point character appears in the result of these conversionsonly if a digit follows it. In addition, for 'g' and 'G'conversions, trailing zeros are not removed from the result.

width is a decimal integer defining the minimum field width. If notspecified, then the field width will be determined by the content.

Preceding the width field by a zero ('0') character enables sign-awarezero-padding for numeric types. It forces the padding to be placed after thesign or base (if any) but before the digits. This is used for printing fields inthe form ‘+000000120’. This option is only valid for numeric types and it has noeffect on formatting of infinity and NaN.

The precision is a decimal number indicating how many digits should bedisplayed after the decimal point for a floating-point value formatted with'f' and 'F', or before and after the decimal point for a floating-pointvalue formatted with 'g' or 'G'. For non-number types the fieldindicates the maximum field size - in other words, how many characters will beused from the field content. The precision is not allowed for integer,character, Boolean, and pointer values.

The 'L' option uses the current locale setting to insert the appropriatenumber separator characters. This option is only valid for numeric types.

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 andmay be omitted.

none

The same as 's'.

The available character presentation types are:

Type

Meaning

'c'

Character format. This is the default type forcharacters 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.

'c'

Character format. Outputs the number as a character.

'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, usinglower-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, usingupper-case letters for the digits above 9. Using the'#' option with this type adds the prefix "0X"to the output value.

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 orfalse, 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 inbase 16 with prefix "0x" and lower-case letters fordigits above 9. Uses 'p' to indicate the exponent.

'A'

Same as 'a' except it uses upper-case letters forthe prefix, digits above 9 and to indicate the exponent.

'e'

Exponent notation. Prints the number in scientificnotation using the letter ‘e’ to indicate the exponent.

'E'

Exponent notation. Same as 'e' except it uses anupper-case 'E' as the separator character.

'f'

Fixed point. Displays the number as a fixed-pointnumber.

'F'

Fixed point. Same as 'f', but converts nan toNAN and inf to INF.

'g'

General format. For a given precision p >= 1,this rounds the number to p significant digits andthen formats the result in either fixed-point formator in scientific notation, depending on its magnitude.

A precision of 0 is treated as equivalent to aprecision of 1.

'G'

General format. Same as 'g' except switches to'E' if the number gets too large. Therepresentations of infinity and NaN are uppercased, too.

none

Similar to 'g', except that the default precision isas high as needed to represent the particular value.

The available presentation types for pointers are:

Type

Meaning

'p'

Pointer format. This is the default type forpointers and may be omitted.

none

The same as 'p'.

Chrono Format Specifications

Format specifications for chrono types have the following syntax:

chrono_format_spec ::= [[fill]align][width]["." precision][chrono_specs]chrono_specs  ::= [chrono_specs] conversion_spec | chrono_specs literal_charconversion_spec  ::= "%" [modifier] chrono_typeliteral_char  ::= <a character other than '{', '}' or '%'>modifier  ::= "E" | "O"chrono_type  ::= "a" | "A" | "b" | "B" | "c" | "C" | "d" | "D" | "e" | "F" | "g" | "G" | "h" | "H" | "I" | "j" | "m" | "M" | "n" | "p" | "q" | "Q" | "r" | "R" | "S" | "t" | "T" | "u" | "U" | "V" | "w" | "W" | "x" | "X" | "y" | "Y" | "z" | "Z" | "%"

Literal chars are copied unchanged to the output. Precision is valid only forstd::chrono::duration types with a floating-point representation type.

The available presentation types (chrono_type) for chrono durations and timepoints are:

Type

Meaning

'H'

The hour (24-hour clock) as a decimal number. If the result is asingle digit, it is prefixed with 0. The modified command %OHproduces the locale’s alternative representation.

'M'

The minute as a decimal number. If the result is a single digit,it is prefixed with 0. The modified command %OM produces thelocale’s alternative representation.

'S'

Seconds as a decimal number. If the number of seconds is less than10, the result is prefixed with 0. If the precision of the inputcannot be exactly represented with seconds, then the format is adecimal floating-point number with a fixed format and a precisionmatching that of the precision of the input (or to a microsecondsprecision if the conversion to floating-point decimal secondscannot be made within 18 fractional digits). The character for thedecimal point is localized according to the locale. The modifiedcommand %OS produces the locale’s alternative representation.

Specifiers that have a calendaric component such as 'd' (the day of month)are valid only for std::tm and not durations or time points.

std::tm uses the system’s strftime so refer to itsdocumentation for details on supported conversion specifiers.

Format Examples

This section contains examples of the format syntax and comparison withthe printf formatting.

In most of the cases the syntax is similar to the printf formatting, with theaddition 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 thefollowing 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 = 7;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"
Format String Syntax — fmt 8.0.0 documentation (2024)

FAQs

What is the correct syntax to format the string in Python? ›

Formatting Strings Using the str.

format() was introduced in Python 2.6 as an improvement on the %s syntax. We use {} as placeholders in our string literal, then call the format() method passing in expressions.

What is the best practice to format string in Python? ›

While working on Python 3.6+, use f-strings as this method is faster and better than both %-formatting and str. format(). You have the privilege to embed expressions in the f-string method and these f-strings expressions inside the braces are evaluated at runtime and returned as the final string.

What is documentation string in Python? ›

A Python docstring is a string used to document a Python module, class, function or method, so programmers can understand what it does without having to read the details of the implementation. Also, it is a common practice to generate online (html) documentation automatically from docstrings.

How to use fmt in Go? ›

The Go fmt package supports two closely-related functions for formatting a string to be displayed on the terminal. . Print() accepts strings as arguments and concatenates them without any spacing. . Println() , on the other hand, adds a space between strings and appends a new line to the concatenated output string.

What is the difference between FMT and log in Golang? ›

While the fmt package is great for quick and simple logging, the log package offers more robust and configurable logging capabilities. It provides features like automatic timestamping, severity levels, and the ability to write logs to different output streams.

What is the function of FMT print? ›

The fmt. print() function in Go language formats using the default formats for its operands and writes to standard output. Here Spaces are added between operands when any string is not used as a parameter.

Why do we use string format? ›

The Java string format() method is used to format strings, integers, decimal values, and so on, by using different format specifiers. This method returns the formatted string using the given locale, specified formatter, and arguments. If no locale is provided then it uses the default locale for formatting the strings.

What is the difference between system out Println and system out printf? ›

println adds a new line at the end of the output, so you cant continue on that line and printf supports format strings so you can pass arguments that are substituted into the string e.g.

What is the number format of a string? ›

A standard numeric format string takes the form Axx, where A is an alphabetic character called the format specifier, and xx is an optional integer called the precision specifier. The precision specifier ranges from 0 to 99 and affects the number of digits in the result.

What is the syntax for string in Python? ›

Python has a built-in string class named "str" with many handy features (there is an older module named "string" which you should not use). String literals can be enclosed by either double or single quotes, although single quotes are more commonly used.

What is format() in Python? ›

The python format() method will return a formatted value as specified by the format passed as a parameter. The python format() method is an effective way of converting and organizing values. Also, formatting makes the code look more presentable and readable.

What is the correct syntax to declare a string? ›

Below is the basic syntax for declaring a string. char string_name[size]; In the above syntax string_name is any name given to the string variable and size is used to define the length of the string, i.e the number of characters strings will store.

What is the .2f syntax in Python? ›

2f format specifier is used to format floating-point numbers as strings with two decimal places. This format specifier is part of the Python strings format syntax in Python. When using the . 2f format specifier, the number will be rounded to two decimal places and displayed with two digits after the decimal point.

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