The operand for the expression.
- iOS 3.0+
- macOS 10.4+
- tvOS 9.0+
- watchOS 2.0+
The operand for an expression is the object on which the expression’s selector or block will be invoked. The object is the result of evaluating a key path or one of the defined functions. Accessing this property raises an exception if it is not applicable to the expression.