6.4 KiB
[concept.booleantestable]
18 Concepts library [concepts]
18.5 Comparison concepts [concepts.compare]
18.5.2 Boolean testability [concept.booleantestable]
The exposition-only boolean-testable concept specifies the requirements on expressions that are convertible to bool and for which the logical operators ([expr.log.and], [expr.log.or], [expr.unary.op]) have the conventional semantics.
template<class T> concept [boolean-testable-impl](#concept:boolean-testable-impl "18.5.2 Boolean testability [concept.booleantestable]") = [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_to [concept.convertible]")<T, bool>; // exposition only
Let e be an expression such thatdecltype((e)) is T.
T models boolean-testable-impl only if
either remove_cvref_t is not a class type, or a search for the names operator&& and operator|| in the scope of remove_cvref_t finds nothing; and
argument-dependent lookup ([basic.lookup.argdep]) for the names operator&& and operator|| with T as the only argument type finds no disqualifying declaration (defined below).
A disqualifying parameter is a function parameter whose declared type P
is not dependent on a template parameter, and there exists an implicit conversion sequence ([over.best.ics]) from e to P; or
is dependent on one or more template parameters, and either
P contains no template parameter that participates in template argument deduction ([temp.deduct.type]), or
template argument deduction using the rules for deducing template arguments in a function call ([temp.deduct.call]) ande as the argument succeeds.
A key parameter of a function template D is a function parameter of type cv X or reference thereto, where X names a specialization of a class template that has the same innermost enclosing non-inline namespace as D, andX contains at least one template parameter that participates in template argument deduction.
[Example 1:
Innamespace Z {template struct C {}; templatevoid operator&&(C x, T y); templatevoid operator||(C<type_identity_t> x, T y);} the declaration of Z::operator&& contains one key parameter, C x, and the declaration of Z::operator|| contains no key parameters.
â end example]
A disqualifying declaration is
a (non-template) function declaration that contains at least one disqualifying parameter; or
a function template declaration that contains at least one disqualifying parameter, where
at least one disqualifying parameter is a key parameter; or
the declaration contains no key parameters; or
the declaration declares a function template to which no name is bound ([dcl.meaning]).
[Note 1:
The intention is to ensure that given two types T1 and T2 that each model boolean-testable-impl, the && and || operators within the expressionsdeclval() && declval() anddeclval() || declval() resolve to the corresponding built-in operators.
â end note]
template<class T> concept [boolean-testable](#concept:boolean-testable "18.5.2 Boolean testability [concept.booleantestable]") = // exposition only [boolean-testable-impl](#concept:boolean-testable-impl "18.5.2 Boolean testability [concept.booleantestable]")<T> && requires(T&& t) { { !std::forward<T>(t) } -> [boolean-testable-impl](#concept:boolean-testable-impl "18.5.2 Boolean testability [concept.booleantestable]"); };
Let e be an expression such thatdecltype((e)) is T.
T models boolean-testable only ifbool(e) == !bool(!e).
[Example 2:
The typesbool,true_type ([meta.type.synop]),int*, andbitset::reference ([template.bitset]) model boolean-testable.
â end example]