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Showing content from https://en.cppreference.com/w/cpp/language/../memory/../named_req/../concepts/boolean-testable.html below:

boolean-testable - cppreference.com

(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).

[edit] Equality preservation

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

[edit] Notes

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

[edit] References

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