Formatting library (since C++20)

 

C++

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Freestanding and hosted

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Feature test macros (C++20)

Language support library

Concepts library (C++20)

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Diagnostics library

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Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

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Filesystem library (C++17)

Regular expressions library (C++11)

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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) is_within_lifetime (C++26) 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) to_chars_result (C++17) from_chars_result (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)

Formatting concepts

formattable (C++23)

Formatter

formatter (C++20)

range_formatter (C++23)

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)

range_format (C++23)

format_kind (C++23)

Formatting arguments

basic_format_arg (C++20)

basic_format_arg::handle (C++20)

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

basic_format_stringformat_stringwformat_string (C++20)(C++20)(C++20)

visit_format_arg (C++20) (deprecated in C++26)

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.

Formatting functions

Defined in header <format>
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)

Formatting concepts

Defined in header <format>
formattable (C++23)	specifies that a type is formattable, that is, it specializes std::formatter and provides member functions parse and format (concept)

Extensibility support and implementation detail

Defined in header <format>
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)
make_format_argsmake_wformat_args (C++20)(C++20)	creates a type-erased object referencing all formatting arguments, convertible to format_args (function template)
visit_format_arg (C++20) (deprecated in C++26)	argument visitation interface for user-defined formatters (function template)
formatter (C++20)	class template that defines formatting rules for a given type (class template)
range_formatter (C++23)	class template that helps implementing std::formatter specializations for range types (class template)
range_format (C++23)	specifies how a range should be formatted (enum)
format_kind (C++23)	selects a suited std::range_format for a range (variable template)
basic_format_arg (C++20)	class template that provides access to a formatting argument for user-defined formatters (class template)
basic_format_argsformat_argswformat_args (C++20)(C++20)(C++20)	class that provides access to all formatting arguments (class template)
basic_format_stringformat_stringwformat_string (C++20)(C++20)(C++20)	class template that performs compile-time format string checks at construction time (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)
basic_format_parse_contextformat_parse_contextwformat_parse_context (C++20)(C++20)(C++20)	formatting string parser state (class template)
format_error (C++20)	exception type thrown on formatting errors (class)

Notes

Feature-test macro	Value	Std	Comment
__cpp_lib_format	201907L	(C++20)	Text formatting
	202106L	(C++20) (DR)	Compile-time format string checks; Reducing parameterization of std::vformat_to
	202110L	(C++20) (DR)	Fixing locale handling in chrono formatters; Supporting non-const-formattable types
	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)	Formatting ranges

We intentionally treat the addition of std::basic_format_string (P2508) as a defect report because all known implementations make these components available in C++20 mode, although it is not so categorized officially.

Example

Run this code

#include <cassert>
#include <format>
 
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

See also

print (C++23)	prints to stdout or a file stream using formatted representation of the arguments (function template)
println (C++23)	same as std::print except that each print is terminated by additional new line (function template)
print(std::ostream) (C++23)	outputs formatted representation of the arguments (function template)