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Difference between revisions of "cpp/utility/format/format to n"

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< cpp‎ | utility‎ | format
(Created page with "{{cpp/title|format_to_n}} {{cpp/utility/format/navbar}} {{dcl begin}} {{dcl header | format }} {{dcl | num=1 | since=c++20 |1= template<class OutputIt, class... Args> std::fo...")
 
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{{tt|OutputIt}} satisfies the concept {{tt|std::output_iterator<const CharT&>}}.
 
{{tt|OutputIt}} satisfies the concept {{tt|std::output_iterator<const CharT&>}}.
 
}}
 
}}
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The behavior is undefined if {{tt|OutputIt}} does not model (meet the semantic requirements of) the the concept {{tt|std::output_iterator<const CharT&>}}, or if {{c|std::formatter<Ti, CharT>}} does not meet the {{named req|Formatter}} requirements for each {{tt|Ti}} in {{tt|Args}}.  
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The behavior is undefined if {{tt|OutputIt}} does not model (meet the semantic requirements of) the the concept {{tt|std::output_iterator<const CharT&>}}, or if {{c|std::formatter<Ti, CharT>}} does not meet the {{named req|Formatter}} requirements for any {{tt|Ti}} in {{tt|Args}}.  
  
 
===Parameters===
 
===Parameters===
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{{par | n | maximum number of characters to be written to the buffer}}
 
{{par | n | maximum number of characters to be written to the buffer}}
 
{{par | fmt | string view representing the format string.
 
{{par | fmt | string view representing the format string.
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{{cpp/utility/format/format string}}
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{{include|cpp/utility/format/format string}}
 
}}
 
}}
 
{{par | args... | arguments to be formatted}}
 
{{par | args... | arguments to be formatted}}
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===Exceptions===
 
===Exceptions===
 
Throws {{lc|std::format_error}} if {{tt|fmt}} is not a valid format string. Also propagates exception thrown by any formatter.
 
Throws {{lc|std::format_error}} if {{tt|fmt}} is not a valid format string. Also propagates exception thrown by any formatter.
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{{todo|the second sentence is not backed by the draft standard}}
 
  
 
===Example===
 
===Example===

Revision as of 22:02, 14 November 2019

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Formatting library
Standard format specification
Formatting functions
(C++20)
(C++20)
format_to_n
(C++20)
(C++20)
(C++20)
(C++20)
Format strings
(C++20)(C++20)(C++20)
(C++26)
Formatting concepts
(C++23)
Formatter
(C++20)
(C++23)
(C++23)
(C++23)
(C++23)
(C++20)(C++20)(C++20)
(C++20)(C++20)(C++20)
(C++23)
(C++23)
Formatting arguments
(C++20)
(C++20)
(C++20)(C++20)(C++20)
(C++20) (deprecated in C++26)
(C++20)(C++20)
Format error
(C++20)
 
Defined in header <format>
template<class OutputIt, class... Args>

std::format_to_n_result<OutputIt>
    format_to_n(OutputIt out, std::iter_difference_t<OutputIt> n,

                std::string_view fmt, const Args&... args);
 (1) (since C++20)
template<class OutputIt, class... Args>

std::format_to_n_result<OutputIt>
    format_to_n(OutputIt out, std::iter_difference_t<OutputIt> n,

                std::wstring_view fmt, const Args&... args);
 (2) (since C++20)
template<class OutputIt, class... Args>

std::format_to_n_result<OutputIt>
    format_to_n(OutputIt out, std::iter_difference_t<OutputIt> n,

                const std::locale& loc, std::string_view fmt, const Args&... args);
 (3) (since C++20)
template<class OutputIt, class... Args>

std::format_to_n_result<OutputIt>
    format_to_n(OutputIt out, std::iter_difference_t<OutputIt> n,

                const std::locale& loc, std::wstring_view fmt, const Args&... args);
 (4) (since C++20)
template<class OutputIt>

struct format_to_n_result {
    OutputIt out;
    std::iter_difference_t<OutputIt> size;

};
 (5) (since C++20)

Format args according to the format string fmt, and write the result to the output iterator out. At most n characters are written. If present, loc is used for locale-specific formatting.

Let CharT be decltype(fmt)::char_type (char for overloads (1,3), wchar_t for overloads (2,4)).

These overloads participate in overload resolution only if OutputIt satisfies the concept std::output_iterator<const CharT&>.

The behavior is undefined if OutputIt does not model (meet the semantic requirements of) the the concept std::output_iterator<const CharT&>, or if std::formatter<Ti, CharT> does not meet the Formatter requirements for any Ti in Args.

Contents

Parameters

out - iterator to the output buffer
n - maximum number of characters to be written to the buffer
fmt - string view representing the format string.
[edit]
an object that represents the format string. The format string consists of
  • ordinary characters (except { and }), which are copied unchanged to the output,
  • escape sequences {{ and }}, which are replaced with { and } respectively in the output, and
  • replacement fields.

Each replacement field has the following format:

{ arg-id (optional) } (1)
{ arg-id (optional) : format-spec } (2)
1) replacement field without a format specification
2) replacement field with a format specification
arg-id - specifies the index of the argument in args whose value is to be used for formatting; if it is omitted, the arguments are used in order.

The arg-id s in a format string must all be present or all be omitted. Mixing manual and automatic indexing is an error.

format-spec - the format specification defined by the std::formatter specialization for the corresponding argument. Cannot start with }.

(since C++23)
(since C++26)
  • For other formattable types, the format specification is determined by user-defined formatter specializations.


args... - arguments to be formatted
loc - std::locale used for locale-specific formatting

Return value

A format_to_n_result such that the out member is an iterator past the end of the output range, and the size member is the total (not truncated) output size.

Exceptions

Throws std::format_error if fmt is not a valid format string. Also propagates exception thrown by any formatter.

Example

This section is incomplete
Reason: no example

See also

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