std::Boolean
Defined in header <concepts>


template<class B> concept Boolean = 
(since C++20)  
The concept Boolean<B>
specifies the requirements for a type usable in Boolean contexts. For Boolean
to be satisfied, the logical operators must have the usual behavior (including shortcircuiting). More precisely, given

b1
,b2
, two lvalues of type const std::remove_reference_t<B>,
Boolean<B>
is satisfied only if:
 bool(b1) == !bool(!b1)
 b1 && b2, b1 && bool(b2) and bool(b1) && b2 are all equal to bool(b1) && bool(b2) and have the same shortcircuit evaluation;
 b1  b2, b1  bool(b2) and bool(b1)  b2 are all equal to bool(b1)  bool(b2) and have the same shortcircuit evaluation;
 bool(b1 == b2), bool(b1 == bool(b2)), and bool(bool(b1) == b2) are all equal to (bool(b1) == bool(b2));
 bool(b1 != b2), bool(b1 != bool(b2)), and bool(bool(b1) != b2) are all equal to (bool(b1) != bool(b2)).
[edit] Equality preservation
An expression is equality preserving if it results in equal outputs given equal inputs.
 The inputs to an expression consist of its operands.
 The outputs of an expression consist of its result and all operands modified by the expression (if any).
Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.
Unless noted otherwise, every expression used in a requiresexpression is required to be equality preserving and stable, and the evaluation of the expression may only modify its nonconstant operands. Operands that are constant must not be modified.
[edit] Implicit expression variations
A requiresexpression that uses an expression that is nonmodifying for some constant lvalue operand also implicitly requires additional variations of that expression that accept a nonconstant lvalue or (possibly constant) rvalue for the given operand unless such an expression variation is explicitly required with differing semantics. These implicit expression variations must meet the same semantic requirements of the declared expression. The extent to which an implementation validates the syntax of the variations is unspecified.
[edit] Notes
Examples of Boolean
types include bool, std::true_type, and std::bitset<N>::reference. Pointers are not Boolean
types.