C++ named requirements: BooleanTestable
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, but was superseded by the exposition-only concept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
|
(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
|
(C++20)
|
specifies that a type can be used in Boolean contexts
( exposition-only concept* ) |