std:: format_to

From cppreference.com

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 ( 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 ( 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 & > .

OutputIt must model (meet the semantic requirements of) the concept std:: output_iterator < const CharT & > , and std:: formatter < Ti, CharT > must meet the BasicFormatter requirements for any Ti in Args (as required by std::make_format_args and std::make_wformat_args ). Otherwise, the behavior is undefined.

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 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
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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Standard format specification
Formatting functions
format (C++20)
format_to (C++20)
format_to_n (C++20)
formatted_size (C++20)
vformat (C++20)
vformat_to (C++20)
Format strings
basic_format_string format_string wformat_string (C++20) (C++20) (C++20)
runtime_format (C++26)
Formatting concepts
formattable (C++23)
Formatter
formatter (C++20)
formatter < pair-or-tuple > (C++23)
formatter < range > (C++23)
range_formatter (C++23)
enable_nonlocking_formatter_optimization (C++23)
basic_format_parse_context format_parse_context wformat_parse_context (C++20) (C++20) (C++20)
basic_format_context format_context wformat_context (C++20) (C++20) (C++20)
range_format (C++23)
format_kind (C++23)
Formatting arguments
basic_format_arg (C++20)
basic_format_arg::handle (C++20)
basic_format_args format_args wformat_args (C++20) (C++20) (C++20)
visit_format_arg (C++20) (deprecated in C++26)
make_format_args make_wformat_args (C++20) (C++20)
Format error
format_error (C++20)

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)