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 | digitIn 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 |
|---|---|
| String format. This is the default type for strings andmay be omitted. |
none | The same as |
The available character presentation types are:
Type | Meaning |
|---|---|
| Character format. This is the default type forcharacters and may be omitted. |
none | The same as |
The available integer presentation types are:
Type | Meaning |
|---|---|
| Binary format. Outputs the number in base 2. Using the |
| Binary format. Outputs the number in base 2. Using the |
| Character format. Outputs the number as a character. |
| Decimal integer. Outputs the number in base 10. |
| Octal format. Outputs the number in base 8. |
| Hex format. Outputs the number in base 16, usinglower-case letters for the digits above 9. Using the |
| Hex format. Outputs the number in base 16, usingupper-case letters for the digits above 9. Using the |
none | The same as |
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 |
|---|---|
| Hexadecimal floating point format. Prints the number inbase 16 with prefix |
| Same as |
| Exponent notation. Prints the number in scientificnotation using the letter ‘e’ to indicate the exponent. |
| Exponent notation. Same as |
| Fixed point. Displays the number as a fixed-pointnumber. |
| Fixed point. Same as |
| General format. For a given precision A precision of |
| General format. Same as |
none | Similar to |
The available presentation types for pointers are:
Type | Meaning |
|---|---|
| Pointer format. This is the default type forpointers and may be omitted. |
none | The same as |
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 |
|---|---|
| The hour (24-hour clock) as a decimal number. If the result is asingle digit, it is prefixed with 0. The modified command |
| The minute as a decimal number. If the result is a single digit,it is prefixed with 0. The modified command |
| 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 |
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"
