C++ named requirements: BooleanTestable

 

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C++ named requirements

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Specifies that an expression of such a type and value category is convertible to bool, and for which the logical operators for the type or two different BooleanTestable types have the usual behavior (including short-circuiting).

Requirements

Let e be an expression of a certain type and value category. The type and value category meet the BooleanTestable requirements if:

• !e is well-formed and does not modify the object denoted by e, if any, except that if e is an xvalue, the denoted object can be left in a valid but unspecified state(since C++11).
• Both e and !e are convertible to bool by both implicit conversion and static_cast.
• Conversions in both manners produce equal results and do not modify the source object, if any, except that if e or !e is an xvalue, the source object can be left in a valid but unspecified state(since C++11).
• bool(!e) == !bool(e) holds.
• No viable non-member operator&& and operator|| is visible by argument-dependent lookup for the type of e or !e.
• If e or !e is of a class type, the class does not define any member operator&& or operator||.

Notes

The standard does not define a named requirement with this name. It was originally proposed in an early resolution of LWG2114, but was superseded by the exposition-only concept boolean-testable in the final resolution P2167R3. Because implementations generally expect the provided types to model boolean-testable even in pre-C++20 modes, we intentedly treat P2167R3 as a defect report and transform the boolean-testable to legacy named requirements.

When the && and || operators are used with operands whose type and value category are BooleanTestable, built-in versions are selected and short-circuit evaluation is performed.

The type and value category of an expression e meet the BooleanTestable requirements if and only if decltype((e)) models boolean-testable.

(since C++20)

Examples of BooleanTestable types (with any value category) include bool, std::true_type(since C++11), std::bitset<N>::reference, and int*.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2114 (P2167R3)	C++98	convertibility to bool was too weak to reflect the expectation of implementations	requirements strengthened

See also

boolean-testable  (C++20)

specifies that a type can be used in Boolean contexts (exposition-only concept*)