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 = convertible_to<T, bool>;  // exposition only

Let e be an expression such that decltype((e)) is T.

T models boolean-testable-impl only if:

(2.1)
either remove_cvref_t<T> is not a class type, or a search for the names operator&& and operator|| in the scope of remove_cvref_t<T> finds nothing; and
(2.2)
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

(3.1)
is not dependent on a template parameter, and there exists an implicit conversion sequence ([over.best.ics]) from e to P; or
(3.2)
is dependent on one or more template parameters, and either
- (3.2.1)
  P contains no template parameter that participates in template argument deduction ([temp.deduct.type]), or
- (3.2.2)
  template argument deduction using the rules for deducing template arguments in a function call ([temp.deduct.call]) and e 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, and X contains at least one template parameter that participates in template argument deduction.

[Example 1:

In namespace Z { template<class> struct C {}; template<class T> void operator&&(C<T> x, T y); template<class T> void operator||(C<type_identity_t<T>> x, T y); } the declaration of Z::operator&& contains one key parameter, C<T> x, and the declaration of Z::operator|| contains no key parameters.

— end example]

A disqualifying declaration is

(5.1)
a (non-template) function declaration that contains at least one disqualifying parameter; or
(5.2)
a function template declaration that contains at least one disqualifying parameter, where
- (5.2.1)
  at least one disqualifying parameter is a key parameter; or
- (5.2.2)
  the declaration contains no key parameters; or
- (5.2.3)
  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 expressions declval<T1>() && declval<T2>() and declval<T1>() || declval<T2>() resolve to the corresponding built-in operators.

— end note]

🔗

template<class T>
  concept boolean-testable =                // exposition only
    boolean-testable-impl<T> && requires(T&& t) {
      { !std::forward<T>(t) } -> boolean-testable-impl;
    };

Let e be an expression such that decltype((e)) is T.

T models boolean-testable only if bool(e) == !bool(!e).

[Example 2:

The types bool, true_type ([meta.type.synop]), int*, and bitset<N>::reference ([template.bitset]) model boolean-testable.

— end example]