# std::format_to

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< cpp‎ | utility‎ | format

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.

Equivalent to:

1) return std::vformat_to (std::move(out), fmt.str, std::make_format_args (args...));
2) return std::vformat_to (std::move(out), fmt.str, std::make_wformat_args (args...));
3) return std::vformat_to (std::move(out), loc, fmt.str, std::make_format_args (args...));
4) 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&>.

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`.

### 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. 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 thread id format specification and stacktrace entry format specification.
- For `std::basic_stacktrace`, no format specifier is allowed.

(since C++23)

- For `std::filesystem::path`, see path format specification.

(since C++26)

- 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 or std::runtime_format(since C++26) can be used in this case.

### Example

Run this code

    #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()!

### 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

### See also

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)

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