Objective-C的 null(nil)安全

Objective-Cにおいてメッセージ送信先のレシーバがnilの場合、Objective-Cランタイムはなにもしない(エラーは発生せずメソッドも呼び出されない)。メソッドが戻り値型を持つ場合、値 0 相当が返却されたかのように振る舞う。

Sending Messages to nil
In Objective-C, it is valid to send a message to nil -- it simply has no effect at runtime. There are several patterns in Cocoa that take advantage of this fact. The value returned from a message to nil may also be valid:

  • If the method returns an object, then a message sent to nil returns 0 (nil). For example:
Person *motherInLaw = [[aPerson spouse] mother];

If the spouse object here is nil, then mother is sent to nil and the method returns nil.

  • If the method returns any pointer type, any integer scalar of size less than or equal to sizeof(void*), a float, a double, a long double, or a long long, then a message sent to nil returns 0.
  • If the method returns a struct, as defined by the OS X ABI Function Call Guide to be returned in registers, then a message sent to nil returns 0.0 for every field in the struct. Other struct data types will not be filled with zeros.
  • If the method returns anything other than the aforementioned value types, the return value of a message sent to nil is undefined.
The Objective-C Programming Language, Objects, Classes, and Messaging