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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../utility/format/format_to.html below:

std::format_to - cppreference.com

template< class OutputIt, class... Args > OutputIt format_to( OutputIt out, std::format_string<Args...> fmt, Args&&... args ); (1) (since C++20) template< class OutputIt, class... Args > OutputIt format_to( OutputIt out, std::wformat_string<Args...> fmt, Args&&... args ); (2) (since C++20) template< class OutputIt, class... Args > OutputIt format_to( OutputIt out, const std::locale& loc, std::format_string<Args...> fmt, Args&&... args ); (3) (since C++20) template< class OutputIt, class... Args > OutputIt format_to( OutputIt out, const std::locale& loc, std::wformat_string<Args...> fmt, Args&&... args ); (4) (since C++20)

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

Equivalent to:

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&>.

If any of the following conditions is satisfied, the behavior is undefined:

OutputIt does not model std::output_iterator<const CharT&>.
std::formatter<Ti, CharT> does not meet the BasicFormatter requirements (as required by std::make_format_args and std::make_wformat_args) for some Ti in Args.

[edit] Parameters out - iterator to the output buffer fmt - 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 }.

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

Iterator past the end of the output range.

[edit] Exceptions

Propagates any exception thrown by formatter or iterator operations.

[edit] Notes

It is an error if the format string is not a constant expression unless it is initialized from the result of std::runtime_format(since C++26). std::vformat_to does not have this requirement.

[edit] Example

#include <format>
#include <iostream>
#include <iterator>
#include <string>
 
int main()
{
    std::string buffer;
 
    std::format_to
    (
        std::back_inserter(buffer), // < OutputIt
        "Hello, C++{}!\n",          // < fmt
        "20"                        // < arg
    );
    std::cout << buffer;
    buffer.clear();
 
    std::format_to
    (
        std::back_inserter(buffer), // < OutputIt
        "Hello, {0}::{1}!{2}",      // < fmt
        "std",                      // < arg {0}
        "format_to()",              // < arg {1}
        "\n",                       // < arg {2}
        "extra param(s)..."         // < unused
    );
    std::cout << buffer << std::flush;
 
    std::wstring wbuffer;
    std::format_to
    (
        std::back_inserter(wbuffer),// < OutputIt
        L"Hello, {2}::{1}!{0}",     // < fmt
        L"\n",                      // < arg {0}
        L"format_to()",             // < arg {1}
        L"std",                     // < arg {2}
        L"...is not..."             // < unused
        L"...an error!"             // < unused
    );
    std::wcout << wbuffer;
}

Output:

Hello, C++20!
Hello, std::format_to()!
Hello, std::format_to()!

[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 LWG 3539 C++20 out could not be a move-only iterator it can be P2216R3 C++20 throws std::format_error for invalid format string results in compile-time error instead P2418R2 C++20 objects that are neither const-usable nor copyable
(such as generator-like objects) are not formattable allow formatting these objects P2508R1 C++20 there's no user-visible name for this facility the name basic_format_string is exposed [edit] See also stores formatted representation of the arguments in a new string
(function template) [edit] writes out formatted representation of its arguments through an output iterator, not exceeding specified size
(function template) [edit]

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