NAME¶

std::format_to_n,std::format_to_n_result - std::format_to_n,std::format_to_n_result

Synopsis¶

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, (1) (since C++20)

std::format_string<Args...> fmt, Args&&... args
);
template< class OutputIt, class... Args >

std::format_to_n_result<OutputIt>
format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n, (2) (since C++20)

std::wformat_string<Args...> fmt, Args&&... args
);
template< class OutputIt, class... Args >

std::format_to_n_result<OutputIt>
format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n, (3) (since C++20)
const std::locale& loc,

std::format_string<Args...> fmt, Args&&... args
);
template< class OutputIt, class... Args >

std::format_to_n_result<OutputIt>
format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n, (4) (since C++20)
const std::locale& loc,

std::wformat_string<Args...> fmt, Args&&... args
);

Helper types¶

template< class OutputIt >

struct format_to_n_result {
OutputIt out; (5) (since C++20)
std::iter_difference_t<OutputIt> size;

};

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<std::remove_cvref_t<Ti>, CharT> does not meet the BasicFormatter
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.

Parameters¶

out - iterator to the output buffer
n - maximum number of characters to be written to the buffer

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

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.
arg-id -
The arg-id s in a format string must all be present or all
be omitted. Mixing manual and automatic indexing is an
error.
fmt - the format specification defined by the std::formatter
format-spec - specialization for the corresponding argument. Cannot start
with }.

* 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 range types, the format specification is interpreted
as range format specification.
* For std::pair and std::tuple, the format specification
is interpreted as tuple format specification.
* For std::thread::id and std::stacktrace_entry, see (since C++23)
thread id format specification and stacktrace entry
format specification.
* For std::basic_stacktrace, no format specifier is
allowed.

* For std::filesystem::path, see path format (since C++26)
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

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¶

Propagates any exception thrown by formatter or iterator operations.

Example¶

At Godbolt's Compiler Explorer: clang (trunk) + libc++, gcc (trunk) + libstdc++.

// Run this code

#include <format>
#include <iostream>
#include <string_view>

int main()
{
char buffer[64];

for (std::size_t max_chars_to_write : {std::size(buffer) - 1, 23uz})
{
const auto result =
std::format_to_n(
buffer, max_chars_to_write,
"Hubble's H{2} {3} {0}{4}{1} km/sec/Mpc.", // 24 bytes w/o formatters
71, // {0}, occupies 2 bytes
8, // {1}, occupies 1 byte
"\u2080", // {2}, occupies 3 bytes, '₀' (SUBSCRIPT ZERO)
"\u2245", // {3}, occupies 3 bytes, '≅' (APPROXIMATELY EQUAL TO)
"\u00B1" // {4}, occupies 2 bytes, '±' (PLUS-MINUS SIGN)
); // 24 + 2 + 1 + 3 + 3 + 2 == 35, no trailing '\0'

*result.out = '\0'; // adds terminator to buffer

const std::string_view str{buffer, result.out}; // uses C++20 constructor

std::cout << "Buffer until '\\0': \"" << str << "\"\n"
<< "Max chars to write: " << max_chars_to_write << '\n'
<< "result.out offset: " << result.out - buffer << '\n'
<< "Untruncated output size: " << result.size << "\n\n";
}
}

Output:¶

Buffer until '\0': "Hubble's H₀ ≅ 71±8 km/sec/Mpc."
Max chars to write: 63
result.out offset: 35
Untruncated output size: 35

Buffer until '\0': "Hubble's H₀ ≅ 71±8"
Max chars to write: 23
result.out offset: 23
Untruncated output size: 35

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 results in
invalid format string compile-time error
objects that are neither
P2418R2 C++20 const-usable nor copyable allow formatting these objects
(such as generator-like objects)
are not formattable
P2508R1 C++20 there's no user-visible name for the name basic_format_string is
this facility exposed

See also¶

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