# std::enable_nonlocking_formatter_optimization

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< cpp‎ | utility‎ | format

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)

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