std::enable_nonlocking_formatter_optimization

 

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Defined in header <format>
template< class T > constexpr bool enable_nonlocking_formatter_optimization = false;		(since C++23)

This template can be used by implementations to enable efficient implementations of std::print and std::println.

If std::enable_nonlocking_formatter_optimization<T> is true, printing an argument of type T can be performed in a more efficient way (see std::print for details). std::enable_nonlocking_formatter_optimization specializations can be true in the following cases:

• T is one of the types where std::formatter<T, CharT> is a basic standard specialization or standard specialization for a library type (see below).
• A program may specialize this template for any cv-unqualified program-defined type T. Such specializations must be usable in constant expressions and have type const bool.

Basic standard specializations

In the following list, CharT is either char or wchar_t, ArithmeticT is any cv-unqualified arithmetic type other than char, wchar_t, char8_t, char16_t, or char32_t:

Nonlocking flag for character formatters
template<> constexpr bool enable_nonlocking_formatter_optimization<CharT> = true;	(1)
Nonlocking flag for string formatters
template<> constexpr bool enable_nonlocking_formatter_optimization<CharT*> = true;	(2)
template<> constexpr bool enable_nonlocking_formatter_optimization<const CharT*> = true;	(3)
template< std::size_t N > constexpr bool enable_nonlocking_formatter_optimization<CharT[N]> = true;	(4)
template< class Traits, class Alloc > constexpr bool enable_nonlocking_formatter_optimization     <std::basic_string<CharT, Traits, Alloc>> = true;	(5)
template< class Traits > constexpr bool enable_nonlocking_formatter_optimization     <std::basic_string_view<CharT, Traits>> = true;	(6)
Nonlocking flag for arithmetic formatters
template<> constexpr bool enable_nonlocking_formatter_optimization<ArithmeticT> = true;	(7)
Nonlocking flag for pointer formatters
template<> constexpr bool enable_nonlocking_formatter_optimization<std::nullptr_t> = true;	(8)
template<> constexpr bool enable_nonlocking_formatter_optimization<void*> = true;	(9)
template<> constexpr bool enable_nonlocking_formatter_optimization<const void*> = true;	(10)

Standard specializations for library types

Specializations of enable_nonlocking_formatter_optimization for all specializations of the following standard templates are defined as true:

• std::chrono::zoned_time when its template parameter type TimeZonePtr is const std::chrono::time_zone*

Specializations of enable_nonlocking_formatter_optimization for all specializations of the following standard templates are defined as conditionally true:

• std::pair
• std::tuple
• std::chrono::duration

Specialization of enable_nonlocking_formatter_optimization for all formattable range types are always defined as false for which the range format kind is not std::range_format::disabled.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_print	202403L	(C++26) (DR23)	Formatted output with stream locking
	202406L	(C++26) (DR23)	Enabling nonlocking formatter optimization for more formattable types

See also

formatter (C++20)	defines formatting rules for a given type (class template)
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)