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std::format_to_n

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< cpp‎ | utility‎ | format
 
 
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Formatting library
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format_to_n
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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 only participate in overload resolution 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.

5) std::format_to_n_result has no base classes, or members other than out, size and implicitly declared special member functions.

Contents

[edit] 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. 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:

  • introductory { character;
  • (optional) arg-id, a non-negative number;
  • (optional) a colon (:) followed by a format specification;
  • final } character.

arg-id specifies the index of the argument in args whose value is to be used for formatting; if arg-id is omitted, the arguments are used in order. The arg-ids in a format string must all be present or all be omitted. Mixing manual and automatic indexing is an error.

The format specification is defined by the std::formatter specialization for the corresponding argument.

  • For basic types and standard string types, the format specification is interpreted as standard format specification.
  • For chrono types, the format specification is interpreted as chrono format specification.
  • 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

[edit] 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.

[edit] Exceptions

Throws std::format_error if fmt is not a valid format string for the provided arguments. Also propagates any exception thrown by formatter or iterator operations.

[edit] Example

#include <format>
#include <string_view>
#include <iostream>
 
int main()
{
    char buffer[64];
 
    const auto result =
        std::format_to_n(buffer, std::size(buffer), 
                         "Hubble's H{0} {1} {2} miles/sec/mpc.",
                         "\u2080", "\u2245", 42);
 
    std::cout << "Buffer: \"" << std::string_view{buffer, result.size} << "\"\n"
              << "Buffer size = " << std::size(buffer) << '\n'
              << "Untruncated output size = " << result.size << '\n';
}

Output:

Buffer: "Hubble's H₀ ≅ 42 miles/sec/mpc."
Buffer size = 64
Untruncated output size = 35

[edit] See also

(C++20)
stores formatted representation of the arguments in a new string
(function template) [edit]
(C++20)
writes out formatted representation of its arguments through an output iterator
(function template) [edit]
determines the number of characters necessary to store the formatted representation of its arguments
(function template) [edit]