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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,

                 /*format_string<Args...>*/ 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,

                 /*wformat_string<Args...>*/ 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, /*format_string<Args...>*/ 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, /*wformat_string<Args...>*/ 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 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 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 - parameter of unspecified type, whose initialization is valid only if the argument is convertible to std::string_view (for (1,3)) or std::wstring_view (for (2,4)), and the result of conversion is a constant expression and a valid format string for Args. 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

Propagates any exception thrown by formatter or iterator operations.

[edit] Example

#include <format>
#include <iostream>
#include <string_view>
 
int main()
{
    char buffer[64];
 
    const auto result =
        std::format_to_n(buffer, std::size(buffer) - 1, 
                         "Hubble's H{2} {3} {0}{4}{1} km/sec/Mpc.",
                         71,       // {0}, occupies 2 bytes
                         8,        // {1}, occupies 1 byte
                         "\u2080", // {2}, occupies 3 bytes
                         "\u2245", // {3}, occupies 3 bytes
                         "\u00B1"  // {4}, occupies 2 bytes
                         );
    *result.out = '\0';
 
    const std::string_view str{buffer, result.out}; // uses C++20 ctor
 
    std::cout << "Buffer: \"" << str << "\"\n"
              << "Buffer size = " << std::size(buffer) << '\n'
              << "Untruncated output size = " << result.size << '\n';
}

Output:

Buffer: "Hubble's H₀ ≅ 71±8 km/sec/Mpc."
Buffer size = 64
Untruncated output size = 35

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P2216R3 C++20 throws std::format_error for invalid format string invalid format string results in compile-time error

[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]