tmp/tmpjdej4kbu/{from.md → to.md}
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| 1 |
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#### In general <a id="format.string.general">[[format.string.general]]</a>
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A *format string* for arguments `args` is a (possibly empty) sequence of
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*replacement fields*, *escape sequences*, and characters other than `{`
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and `}`. Let `charT` be the character type of the format string. Each
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character that is not part of a replacement field or an escape sequence
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is copied unchanged to the output. An escape sequence is one of `{{` or
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`}}`. It is replaced with `{` or `}`, respectively, in the output. The
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syntax of replacement fields is as follows:
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``` bnf
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replacement-field
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'{' arg-idₒₚₜ format-specifierₒₚₜ '}'
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```
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``` bnf
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arg-id
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'0'
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positive-integer
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```
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``` bnf
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positive-integer
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nonzero-digit
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positive-integer digit
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```
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``` bnf
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nonnegative-integer
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digit
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nonnegative-integer digit
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```
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``` bnf
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nonzero-digit one of
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'1 2 3 4 5 6 7 8 9'
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```
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``` bnf
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digit one of
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'0 1 2 3 4 5 6 7 8 9'
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```
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``` bnf
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format-specifier
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':' format-spec
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```
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``` bnf
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format-spec
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as specified by the formatter specialization for the argument type
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```
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The *arg-id* field specifies the index of the argument in `args` whose
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value is to be formatted and inserted into the output instead of the
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replacement field. If there is no argument with the index *arg-id* in
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`args`, the string is not a format string for `args`. The optional
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*format-specifier* field explicitly specifies a format for the
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replacement value.
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[*Example 1*:
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``` cpp
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string s = format("{0}-{{", 8); // value of s is "8-{"
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```
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— *end example*]
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If all *arg-id*s in a format string are omitted (including those in the
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*format-spec*, as interpreted by the corresponding `formatter`
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specialization), argument indices 0, 1, 2, … will automatically be used
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in that order. If some *arg-id*s are omitted and some are present, the
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string is not a format string.
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[*Note 1*: A format string cannot contain a mixture of automatic and
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manual indexing. — *end note*]
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[*Example 2*:
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``` cpp
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string s0 = format("{} to {}", "a", "b"); // OK, automatic indexing
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string s1 = format("{1} to {0}", "a", "b"); // OK, manual indexing
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string s2 = format("{0} to {}", "a", "b"); // not a format string (mixing automatic and manual indexing),
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// throws format_error
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string s3 = format("{} to {1}", "a", "b"); // not a format string (mixing automatic and manual indexing),
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// throws format_error
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```
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— *end example*]
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The *format-spec* field contains *format specifications* that define how
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the value should be presented. Each type can define its own
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interpretation of the *format-spec* field. If *format-spec* does not
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conform to the format specifications for the argument type referred to
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by *arg-id*, the string is not a format string for `args`.
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[*Example 3*:
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- For arithmetic, pointer, and string types the *format-spec* is
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interpreted as a *std-format-spec* as described in
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[[format.string.std]].
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- For chrono types the *format-spec* is interpreted as a
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*chrono-format-spec* as described in [[time.format]].
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- For user-defined `formatter` specializations, the behavior of the
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`parse` member function determines how the *format-spec* is
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interpreted.
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— *end example*]
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