This chapter is from the book
This chapter is from the book
This chapter is from the book
7.13 Conditional Operator
The ?: operator is called the conditional operator (or sometimes the ternary operator).
conditional-expression:
null-coalescing-expression
null-coalescing-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 of these requirements 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 (§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 (§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 runtime 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 previous step 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.