C++ named requirements: BooleanTestable
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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).
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[edit] 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||.
[edit] Notes
The standard does not define a named requirement with this name. It was originally proposed in an early resolution of 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.
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The type and value category of an expression e meet the BooleanTestable requirements if and only if decltype((e)) models |
(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*.
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| P2167R3) | C++98 | convertibility to bool was too weak to reflect the expectation of implementations | requirements strengthened |
[edit] See also
| (C++20) |
specifies that a type can be used in Boolean contexts (exposition-only concept*)[edit] |
