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std::format_to

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< cpp‎ | utility‎ | format
 
 
Utilities library
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Formatting library
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format_to
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Formatter
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Formatting arguments
Format error
 
Defined in header <format>
template< class OutputIt, class... Args >
OutputIt format_to( OutputIt out, /*format_string<Args...>*/ fmt, const Args&... args );
(1) (since C++20)
template< class OutputIt, class... Args >
OutputIt format_to( OutputIt out, /*wformat_string<Args...>*/ fmt, const Args&... args );
(2) (since C++20)
template< class OutputIt, class... Args >

OutputIt format_to( OutputIt out, const std::locale& loc,

                    /*format_string<Args...>*/ fmt, const Args&... args );
(3) (since C++20)
template< class OutputIt, class... Args >

OutputIt format_to( OutputIt out, const std::locale& loc,

                    /*wformat_string<Args...>*/ fmt, const 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.

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.

Contents

[edit] Parameters

out - iterator to the output 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

Iterator past the end of the output range.

[edit] Exceptions

Propagates any exception thrown by formatter or iterator operations.

[edit] 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.

[edit] 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()!

[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]
writes out formatted representation of its arguments through an output iterator, not exceeding specified size
(function template) [edit]