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std::format_to

Defined in header `<format>`
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.

1) equivalent to return std::vformat_to(out, fmt.str, std::make_format_args(args...));

2) equivalent to return std::vformat_to(std::move(out), fmt.str, std::make_wformat_args(args...));

3) equivalent to return std::vformat_to(out, loc, fmt.str, std::make_format_args(args...));

4) equivalent to return std::vformat_to(std::move(out), loc, fmt.str, std::make_wformat_args(args...));

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 BasicFormatter requirements for any Ti in Args (as required by std::make_format_args and std::make_wformat_args).

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-ids 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.

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 std::thread::id, the format specification is interpreted as thread id 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

Return value

Iterator past the end of the output range.

Exceptions

Propagates any exception thrown by formatter or iterator operations.

Notes

As of P2216R3, it is an error if the format string is not a constant expression. std::vformat_to can be used in this case.

Example

#include <format>
#include <iostream>
#include <iterator>
#include <string>
 
auto main() -> int
{
    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::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()!

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
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

format (C++20)	stores formatted representation of the arguments in a new string (function template)
format_to_n (C++20)	writes out formatted representation of its arguments through an output iterator, not exceeding specified size (function template)