7 Concepts library [concepts.lib]

The Boolean concept specifies the requirements on a type that is usable in Boolean contexts.

template <class B> concept bool Boolean = Movable<decay_t<B>> && // (see [concepts.lib.object.movable]) requires(const remove_reference_t<B>& b1, const remove_reference_t<B>& b2, const bool a) { { b1 } -> ConvertibleTo<bool>&&; { !b1 } -> ConvertibleTo<bool>&&; { b1 && a } -> Same<bool>&&; { b1 || a } -> Same<bool>&&; { b1 && b2 } -> Same<bool>&&; { a && b2 } -> Same<bool>&&; { b1 || b2 } -> Same<bool>&&; { a || b2 } -> Same<bool>&&; { b1 == b2 } -> ConvertibleTo<bool>&&; { b1 == a } -> ConvertibleTo<bool>&&; { a == b2 } -> ConvertibleTo<bool>&&; { b1 != b2 } -> ConvertibleTo<bool>&&; { b1 != a } -> ConvertibleTo<bool>&&; { a != b2 } -> ConvertibleTo<bool>&&; };

Given const lvalues b1 and b2 of type remove_reference_t<B>, then Boolean<B> is satisfied only if

• (2.1)

  bool(b1) == !bool(!b1).

• (2.2)

  (b1 && b2), (b1 && bool(b2)), and (bool(b1) && b2) are all equal to (bool(b1) && bool(b2)), and have the same short-circuit evaluation.

• (2.3)

  (b1 || b2), (b1 || bool(b2)), and (bool(b1) || b2) are all equal to (bool(b1) || bool(b2)), and have the same short-circuit evaluation.

• (2.4)

  bool(b1 == b2), bool(b1 == bool(b2)), and bool(bool(b1) == b2) are all equal to (bool(b1) == bool(b2)).

• (2.5)

  bool(b1 != b2), bool(b1 != bool(b2)), and bool(bool(b1) != b2) are all equal to (bool(b1) != bool(b2)).

[ Example: The types bool, std::true_type, and std::bitset<N>::reference are Boolean types. Pointers, smart pointers, and types with explicit conversions to bool are not Boolean types. — end example ]