7.12 Conditional operator

The ?: operator is called the conditional operator. It is at times also called the ternary operator.

conditional-expression:

conditional-or-expression
conditional-or-expression   ?   expression   :   expression

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 requirement is 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,

• If X and Y are the same type, then this is the type of the conditional expression.
• Otherwise, if an implicit conversion (Section 6.1) exists from X to Y, but not from Y to X, then Y is the type of the conditional expression.
• Otherwise, if an implicit conversion (Section 6.1) exists from Y to X, but not from X to Y, then X is the type of the conditional expression.
• Otherwise, no expression type can be determined, and a compile-time error occurs.

The run-time processing of a conditional expression of the form b ? x : y consists of the following steps:

• First, b is evaluated, and the bool value of b is determined:
  • If an implicit conversion from the type of b to bool exists, then this implicit conversion is performed to produce a bool value.
  • Otherwise, the operator true defined by the type of b is invoked to produce a bool value.
• If the 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.
• Otherwise, y is evaluated and converted to the type of the conditional expression, and this becomes the result of the conditional expression.