[temp.constr.op] (original) (raw)

13 Templates [temp]

13.5 Template constraints [temp.constr]

13.5.2 Constraints [temp.constr.constr]

13.5.2.2 Logical operations [temp.constr.op]

There are two binary logical operations on constraints: conjunction and disjunction.

[Note 1:

These logical operations have no corresponding C++ syntax.

For the purpose of exposition, conjunction is spelled using the symbol ∧ and disjunction is spelled using the symbol ∨.

The operands of these operations are called the left and right operands.

In the constraint ,A is the left operand, and B is the right operand.

— _end note_]

A conjunction is a constraint taking two operands.

To determine if a conjunction issatisfied, the satisfaction of the first operand is checked.

If that is not satisfied, the conjunction is not satisfied.

Otherwise, the conjunction is satisfied if and only if the second operand is satisfied.

A disjunction is a constraint taking two operands.

To determine if a disjunction issatisfied, the satisfaction of the first operand is checked.

If that is satisfied, the disjunction is satisfied.

Otherwise, the disjunction is satisfied if and only if the second operand is satisfied.

[Example 1: template<typename T> constexpr bool get_value() { return T::value; } template<typename T> requires (sizeof(T) > 1) && (get_value<T>()) void f(T); void f(int); f('a');

In the satisfaction of the associated constraintsof f, the constraint sizeof(char) > 1 is not satisfied; the second operand is not checked for satisfaction.

— _end example_]

[Note 2:

A logical negation expression ([expr.unary.op]) is an atomic constraint; the negation operator is not treated as a logical operation on constraints.

Furthermore, if substitution to determine whether an atomic constraint is satisfied ([temp.constr.atomic]) encounters a substitution failure, the constraint is not satisfied, regardless of the presence of a negation operator.

[Example 2: template <class T> concept sad = false;template <class T> int f1(T) requires (!sad<T>);template <class T> int f1(T) requires (!sad<T>) && true;int i1 = f1(42); template <class T> concept not_sad = !sad<T>;template <class T> int f2(T) requires not_sad<T>;template <class T> int f2(T) requires not_sad<T> && true;int i2 = f2(42); template <class T> int f3(T) requires (!sad<typename T::type>);int i3 = f3(42); template <class T> concept sad_nested_type = sad<typename T::type>;template <class T> int f4(T) requires (!sad_nested_type<T>);int i4 = f4(42);

Here,requires (!sad<typename T​::​type>) requires that there is a nested type that is not sad, whereasrequires (!sad_­nested_­type<T>) requires that there is no sad nested type.

— _end example_]

— _end note_]