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Victor Zverovich
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# Format String Syntax {#syntax}
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# Format String Syntax
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[Formatting functions](api.md) such as `fmt::format` and `fmt::print` use the same
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format string syntax described in this section.
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@ -38,9 +38,11 @@ Named arguments can be referred to by their names or indices.
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Some simple format string examples:
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```c++
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"First, thou shalt count to {0}" // References the first argument
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"Bring me a {}" // Implicitly references the first argument
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"From {} to {}" // Same as "From {0} to {1}"
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```
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The *format_spec* field contains a specification of how the value should
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be presented, including such details as field width, alignment, padding,
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@ -55,7 +57,7 @@ certain positions within it. These nested replacement fields can contain
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only an argument id; format specifications are not allowed. This allows
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the formatting of a value to be dynamically specified.
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See the [Format Examples](#formatexamples) section for some examples.
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See the [Format Examples](#format-examples) section for some examples.
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## Format Specification Mini-Language
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@ -81,12 +83,11 @@ type ::= "a" | "A" | "b" | "B" | "c" | "d" | "e" | "E" | "f" | "F" |
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"g" | "G" | "o" | "p" | "s" | "x" | "X" | "?"
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</pre>
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The *fill* character can be any Unicode code point other than `'{'` or
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`'}'`. The presence of a fill character is signaled by the character
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following it, which must be one of the alignment options. If the second
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character of *format_spec* is not a valid alignment option, then it is
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assumed that both the fill character and the alignment option are
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absent.
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The *fill* character can be any Unicode code point other than `'{'` or `'}'`.
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The presence of a fill character is signaled by the character following it,
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which must be one of the alignment options. If the second character of
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*format_spec* is not a valid alignment option, then it is assumed that both
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the fill character and the alignment option are absent.
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The meaning of the various alignment options is as follows:
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@ -257,14 +258,14 @@ The available presentation types for pointers are:
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| `'p'` | Pointer format. This is the default type for pointers and may be omitted. |
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| none | The same as `'p'`. |
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## Chrono Format Specifications {#chrono-specs}
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## Chrono Format Specifications
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Format specifications for chrono duration and time point types as well
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as `std::tm` have the following syntax:
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<a id="chrono-format-spec"></a>
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<pre>
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chrono_format_spec ::= [[<a href="#replacement-field">fill</a>]<a href="#replacement-field">align</a>][<a href="#replacement-field">width</a>]["." <a href="#replacement-field">precision</a>][chrono_specs]
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chrono_format_spec ::= [[<a href="#format-spec">fill</a>]<a href="#format-spec">align</a>][<a href="#format-spec">width</a>]["." <a href="#format-spec">precision</a>][chrono_specs]
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chrono_specs ::= [chrono_specs] conversion_spec | chrono_specs literal_char
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conversion_spec ::= "%" [modifier] chrono_type
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literal_char ::= <a character other than '{', '}' or '%'\>
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@ -358,24 +359,26 @@ characters or strings are printed according to the provided specification.
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Examples:
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fmt::format("{}", std::vector{10, 20, 30});
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// Result: [10, 20, 30]
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fmt::format("{::#x}", std::vector{10, 20, 30});
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// Result: [0xa, 0x14, 0x1e]
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fmt::format("{}", vector{'h', 'e', 'l', 'l', 'o'});
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// Result: ['h', 'e', 'l', 'l', 'o']
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fmt::format("{:n}", vector{'h', 'e', 'l', 'l', 'o'});
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// Result: 'h', 'e', 'l', 'l', 'o'
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fmt::format("{:s}", vector{'h', 'e', 'l', 'l', 'o'});
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// Result: "hello"
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fmt::format("{:?s}", vector{'h', 'e', 'l', 'l', 'o', '\n'});
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// Result: "hello\n"
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fmt::format("{::}", vector{'h', 'e', 'l', 'l', 'o'});
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// Result: [h, e, l, l, o]
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fmt::format("{::d}", vector{'h', 'e', 'l', 'l', 'o'});
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// Result: [104, 101, 108, 108, 111]
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```c++
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fmt::print("{}", std::vector{10, 20, 30});
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// Output: [10, 20, 30]
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fmt::print("{::#x}", std::vector{10, 20, 30});
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// Output: [0xa, 0x14, 0x1e]
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fmt::print("{}", std::vector{'h', 'e', 'l', 'l', 'o'});
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// Output: ['h', 'e', 'l', 'l', 'o']
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fmt::print("{:n}", std::vector{'h', 'e', 'l', 'l', 'o'});
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// Output: 'h', 'e', 'l', 'l', 'o'
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fmt::print("{:s}", std::vector{'h', 'e', 'l', 'l', 'o'});
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// Output: "hello"
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fmt::print("{:?s}", std::vector{'h', 'e', 'l', 'l', 'o', '\n'});
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// Output: "hello\n"
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fmt::print("{::}", std::vector{'h', 'e', 'l', 'l', 'o'});
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// Output: [h, e, l, l, o]
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fmt::print("{::d}", std::vector{'h', 'e', 'l', 'l', 'o'});
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// Output: [104, 101, 108, 108, 111]
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```
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## Format Examples {#formatexamples}
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## Format Examples
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This section contains examples of the format syntax and comparison with
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the printf formatting.
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@ -389,6 +392,7 @@ the following examples.
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Accessing arguments by position:
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```c++
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fmt::format("{0}, {1}, {2}", 'a', 'b', 'c');
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// Result: "a, b, c"
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fmt::format("{}, {}, {}", 'a', 'b', 'c');
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@ -397,9 +401,11 @@ Accessing arguments by position:
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// Result: "c, b, a"
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fmt::format("{0}{1}{0}", "abra", "cad"); // arguments' indices can be repeated
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// Result: "abracadabra"
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```
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Aligning the text and specifying a width:
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```c++
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fmt::format("{:<30}", "left aligned");
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// Result: "left aligned "
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fmt::format("{:>30}", "right aligned");
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@ -408,54 +414,70 @@ Aligning the text and specifying a width:
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// Result: " centered "
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fmt::format("{:*^30}", "centered"); // use '*' as a fill char
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// Result: "***********centered***********"
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```
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Dynamic width:
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```c++
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fmt::format("{:<{}}", "left aligned", 30);
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// Result: "left aligned "
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```
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Dynamic precision:
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```c++
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fmt::format("{:.{}f}", 3.14, 1);
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// Result: "3.1"
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```
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Replacing `%+f`, `%-f`, and `% f` and specifying a sign:
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```c++
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fmt::format("{:+f}; {:+f}", 3.14, -3.14); // show it always
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// Result: "+3.140000; -3.140000"
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fmt::format("{: f}; {: f}", 3.14, -3.14); // show a space for positive numbers
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// Result: " 3.140000; -3.140000"
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fmt::format("{:-f}; {:-f}", 3.14, -3.14); // show only the minus -- same as '{:f}; {:f}'
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// Result: "3.140000; -3.140000"
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```
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Replacing `%x` and `%o` and converting the value to different bases:
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```c++
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fmt::format("int: {0:d}; hex: {0:x}; oct: {0:o}; bin: {0:b}", 42);
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// Result: "int: 42; hex: 2a; oct: 52; bin: 101010"
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// with 0x or 0 or 0b as prefix:
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fmt::format("int: {0:d}; hex: {0:#x}; oct: {0:#o}; bin: {0:#b}", 42);
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// Result: "int: 42; hex: 0x2a; oct: 052; bin: 0b101010"
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```
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Padded hex byte with prefix and always prints both hex characters:
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```c++
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fmt::format("{:#04x}", 0);
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// Result: "0x00"
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```
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Box drawing using Unicode fill:
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```c++
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fmt::print(
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"┌{0:─^{2}}┐\n"
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"│{1: ^{2}}│\n"
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"└{0:─^{2}}┘\n", "", "Hello, world!", 20);
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```
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prints:
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```
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┌────────────────────┐
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│ Hello, world! │
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└────────────────────┘
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```
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Using type-specific formatting:
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```c++
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#include <fmt/chrono.h>
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auto t = tm();
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t.tm_sec = 58;
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fmt::print("{:%Y-%m-%d %H:%M:%S}", t);
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// Prints: 2010-08-04 12:15:58
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```
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Using the comma as a thousands separator:
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```c++
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#include <fmt/format.h>
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auto s = fmt::format(std::locale("en_US.UTF-8"), "{:L}", 1234567890);
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// s == "1,234,567,890"
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```
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