boolean-testable

 

C++

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Feature test macros (C++20)

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Technical specifications

Symbols index

External libraries

 

Concepts library

Core language concepts same_as (C++20) derived_from (C++20) convertible_to (C++20) common_reference_with (C++20) common_with (C++20) integral (C++20) signed_integral (C++20) unsigned_integral (C++20) floating_point (C++20) swappableswappable_with (C++20)(C++20) destructible (C++20) constructible_from (C++20) default_initializable (C++20)   move_constructible (C++20) copy_constructible (C++20) assignable_from (C++20)

Comparison concepts equality_comparableequality_comparable_with (C++20)(C++20) totally_orderedtotally_ordered_with (C++20)(C++20) Object concepts movable (C++20) copyable (C++20) semiregular (C++20) regular (C++20) Callable concepts invocableregular_invocable (C++20)(C++20) predicate (C++20) relation (C++20) equivalence_relation (C++20) strict_weak_order (C++20)

Exposition-only concepts boolean-testable  (C++20)

 

template< class B > concept __boolean_testable_impl = std::convertible_to<B, bool>;	(1)	(since C++20) (exposition only*)
template< class B > concept boolean-testable =     __boolean_testable_impl<B> &&     requires (B&& b) {         { !std::forward<B>(b) } -> __boolean_testable_impl;     };	(2)	(since C++20) (exposition only*)

The exposition-only concept boolean-testable specifies the requirements for expressions that are convertible to bool and for which the logical operators have the usual behavior (including short-circuiting), even for two different boolean-testable types.

Formally, to model the exposition-only concept __boolean_testable_impl, the type must not define any member operator&& and operator||, and no viable non-member operator&& and operator|| may be visible by argument-dependent lookup. Additionally, given an expression e such that decltype((e)) is B, boolean-testable is modeled only if bool(e) == !bool(!e).

Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

Notes

Examples of boolean-testable types include bool, std::true_type, std::bitset<N>::reference, and int*.

References

• C++23 standard (ISO/IEC 14882:2024):

• 18.5.2 Boolean testability [concept.booleantestable]

• C++20 standard (ISO/IEC 14882:2020):

• 18.5.2 Boolean testability [concept.booleantestable]

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