Programming in Objective-C, Sixth Edition

Argument and Return Types with Dynamic Typing

If you use dynamic typing to invoke a method, note the following rule: If a method with the same name is implemented in more than one of your classes, each method must agree on the type of each argument and the type of value it returns so that the compiler can generate the correct code for your message expressions.

The compiler performs a consistency check among each class declaration it has seen. If one or more methods conflict in either argument or return type, the compiler issues a warning message. For example, both the Fraction and Complex classes contain add: methods. However, the Fraction class takes as its argument and returns a Fraction object, whereas the Complex class takes and returns a ...