使用 "id" 作为自定义 "init" 方法的 return 类型而不是指向 class 的指针的优缺点？

Question

假设以下 Objective-C class:

@interface Appliance : NSObject
{
    NSString *productName;
    int voltage;
}
@end

实施 init 方法 A 而不是 B 的优缺点是什么？

A) -(id)initWithName:(NSString *)name;
B) -(Appliance *)initWithName:(NSString *)name;

我看到它们都在 XCode 中工作，即它们都会产生一个有效的 Appliance 实例。 "A" 似乎是我读过的书和看过的代码库中的标准，我想知道为什么会这样。

Answer 1

事实上，在相当长一段时间内，来自 class 初始值设定项（在 Objective-C 中）的最佳实践 return 类型是 instancetype 而不是 id.

Answer 2

哦，重新打开。 :-)

确实，您没有问 id 与 instancetype 的区别。对于 -init… 这个非问题的答案很简单：没有区别，因为编译器会默默地将 id 转换为 instancetype。

您要求 id 与 CustomClass*。你从我那里得到一个完全不同的答案：对于 CustomClass*，subclass 必须转换 superclass' 指定初始化程序的结果。让我们举个例子：

@interface BaseClass : NSObject
- (BaseClass*)initWithWhatever; // Typed to class, designated initializer
@end

@implementation BaseClass
- (BaseClass*)initWithWhatever // Typed to class
{
  self = [super init]; // What's the return type of -init (NSObject)?
  …
}
@end

@interface Subclass : BaseClass
// First problem: I like it to announce in the interface, that a class overwrites
// a method of the base class. Doing so I would have to change the return type. Ugly.
// If I do not redeclare -initWithWhatever it is inherited from BaseClass, still
// having BaseClass* as the return type. Is that the truth? Really?

// However, I do not overwrite it here, but have a new initializer.
- (Subclass*)initWithSomethingElse;
@end

@implementation Subclass
- (Subclass*)initWithSomethingElse
{
  // Second Problem:
  // First, I have to execute the superclass' designated initializer
  self = [super initWithWhatever];
  // Wait a minute! 
  // self is a reference to Subclass. The return value of -initWithWhatever has the type 
  // BaseClass*. So I assign a reference of the base class to a reference of the subclass: 
  // Compiler error, false positive. The code is correct.
  // So I would have to cast. Ugly, ugly, ugly.
@end
…
// Third problem:
Subclass *object = [[Subclass alloc] initWithWhatever];
// Typing -initWithWhatever to BaseClass* would lead to a compiler error here again. 
// Compiler error, false positive. The code is correct.

长话短说：如果没有大量的铸件，就不可能将初始值设定项键入具体的 class。

使用 "id" 作为自定义 "init" 方法的 return 类型而不是指向 class 的指针的优缺点？

Pros and cons of using "id" as the return type of a custom "init" method, instead of a pointer to that class?

initialization

objective-c