< cpp‎ | utility‎ | format
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

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).


[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 }.

(since C++23)
(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

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

[edit] Example

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


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
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]