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

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).


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

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.