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Formatting library (since C++20)

The text formatting library offers a safe and extensible alternative to the printf family of functions. It is intended to complement the existing C++ I/O streams library.

Formatting functions

Defined in header <format>
(C++20)
 stores formatted representation of the arguments in a new string
(function template)
(C++20)
 writes out formatted representation of its arguments through an output iterator
(function template)
(C++20)
 writes out formatted representation of its arguments through an output iterator, not exceeding specified size
(function template)
(C++20)
 determines the number of characters necessary to store the formatted representation of its arguments
(function template)

Extensibility support and implementation detail

Defined in header <format>
(C++20)
 non-template variant of std::format using type-erased argument representation
(function)
(C++20)
 non-template variant of std::format_to using type-erased argument representation
(function template)
(C++20)(C++20)
 creates a type-erased object referencing all formatting arguments, convertible to format_args
(function template)
(C++20)
 argument visitation interface for user-defined formatters
(function template)
(C++20)
 class template that defines formatting rules for a given type
(class template)
(C++20)
 class template that provides access to a formatting argument for user-defined formatters
(class template)
(C++20)(C++20)(C++20)
 class that provides access to all formatting arguments
(class template)
(C++20)(C++20)(C++20)
 class template that performs compile-time format string checks at construction time
(class template)
(C++20)(C++20)(C++20)
 formatting state, including all formatting arguments and the output iterator
(class template)
(C++20)(C++20)(C++20)
 formatting string parser state
(class template)
(C++20)
 exception type thrown on formatting errors
(class)

Notes

Feature-test macro Value Std Comment
__cpp_lib_format 201907L (C++20)
__cpp_lib_format 202106L (C++20)
(DR)		 Compile-time format string checks;
Reducing parameterization of std::vformat_to
__cpp_lib_format 202110L (C++20)
(DR)		 Fixing locale handling in chrono formatters;
Supporting non-const-formattable types
__cpp_lib_format 202207L (C++23) Exposing std::basic_format_string; clarify handling of encodings in localized formatting of chrono types
__cpp_lib_format_ranges 202207L (C++23)

Example

#include <format>
#include <cassert>
 
int main()
{
    std::string message = std::format("The answer is {}.", 42);
    assert( message == "The answer is 42." );
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P2418R2 C++20 objects that are neither const-formattable nor copyable
(such as generator-like objects) are not formattable 		allow formatting these objects
(relaxed formatter requirements)
P2508R1 C++20 there's no user-visible name for this facility the name basic_format_string is exposed

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