Formatting library (C++20)

 

C++

Compiler support

Freestanding and hosted

Language

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)

Technical specifications

Symbols index

External libraries

 

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 is_constant_evaluated (C++20) launder (C++17) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)    is_eqis_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator> rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

Formatting library

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)

Formatter

formatter (C++20)

basic_format_parse_contextformat_parse_contextwformat_parse_context (C++20)(C++20)(C++20)

basic_format_contextformat_contextwformat_context (C++20)(C++20)(C++20)

Formatting arguments

basic_format_arg (C++20)

basic_format_argsformat_argswformat_args (C++20)(C++20)(C++20)

visit_format_arg (C++20)

make_format_argsmake_wformat_args (C++20)(C++20)

Format error

format_error (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 and reuse some of its infrastructure such as overloaded insertion operators for user-defined types.

Run this code

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

Formatting functions

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

Extensibility support and implementation detail

vformat (C++20)	non-template variant of std::format using type-erased argument representation (function)
vformat_to (C++20)	non-template variant of std::format_to using type-erased argument representation (function template)
basic_format_arg (C++20)	class template that provides access to a formatting argument for user-defined formatters (class template)
formatter (C++20)	class template that defines formatting rules for a given type (class template)
basic_format_parse_contextformat_parse_contextwformat_parse_context (C++20)(C++20)(C++20)	formatting string parser state (class template)
basic_format_contextformat_contextwformat_context (C++20)(C++20)(C++20)	formatting state, including all formatting arguments and the output iterator (class template)
visit_format_arg (C++20)	argument visitation interface for user-defined formatters (function template)
make_format_argsmake_wformat_args (C++20)(C++20)	creates a type-erased object referencing all formatting arguments, convertible to format_args (function template)
basic_format_argsformat_argswformat_args (C++20)(C++20)(C++20)	class that provides access to all formatting arguments (class template)
format_error (C++20)	exception type thrown on formatting errors (class)

Notes

Feature-test macro:

__cpp_lib_format

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)