The ?:
operator is called the conditional operator. It is at times also called the ternary operator.
A conditional expression of the form b?
x:
y
first evaluates the condition b
. Then, if b
is true
, x
is evaluated and becomes the result of the operation. Otherwise, y
is evaluated and becomes the result of the operation. A conditional expression never evaluates both x
and y
.
The conditional operator is right-associative, meaning that operations are grouped from right to left. For example, an expression of the form a?
b:
c?
d:
e
is evaluated as a?
b:
(c?
d:
e)
.
The first operand of the ?:
operator must be an expression of a type that can be implicitly converted to bool
, or an expression of a type that implements operator
true
. If neither of these requirements are satisfied, a compile-time error occurs.
The second and third operands of the ?:
operator control the type of the conditional expression. Let X
and Y
be the types of the second and third operands. Then,
X
and Y
are the same type, then this is the type of the conditional expression.X
to Y
, but not from Y
to X
, then Y
is the type of the conditional expression.Y
to X
, but not from X
to Y
, then X
is the type of the conditional expression.The run-time processing of a conditional expression of the form b?
x:
y
consists of the following steps:
b
is evaluated, and the bool
value of b
is determined:
b
to bool
exists, then this implicit conversion is performed to produce a bool
value.operator
true
defined by the type of b
is invoked to produce a bool
value.bool
value produced by the step above is true
, then x
is evaluated and converted to the type of the conditional expression, and this becomes the result of the conditional expression.y
is evaluated and converted to the type of the conditional expression, and this becomes the result of the conditional expression.