IOS Dev (48) initializer and convenience constructor

IOS Dev (48) initializer and convenience constructor

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• Initializer

  You should be familiar with this.

  - (id)initWithSomething;

  Convenience constructor

  However, in practice, we often use the following statements:

  [[Foo alloc] init];

  If you define this product as a method, as follows:

  + (id)buildInstance;

  In addition to simple writing, what is the difference?

  Initializer is automatically written by the compiler as follows:

  - (instancetype)initWithSomething;

  Convenience constructor will not be optimized by the compiler. Therefore, problems may occur, such as the following:

  Foo. h

  #import 
      
       @interface Foo : NSObject+ (id)buildInstance;- (id)init;@end
      

  Foo. m

  #import Foo.h@implementation Foo+ (id)buildInstance{    return [[self alloc] init];}- (id)init{    return [super init];}@end

  At this time, you call:

  [[Foo buildInstance] doSomethingElse];[[[Foo alloc] init] doSomethingElse];

  The first sentence does not report an error, and the second sentence will report an error. If you change it:

      + (instancetype)buildInstance

  Both of these two statements will report an error. Why?

  Because in the first sentence, when the returned value of buildInstance is id, the compiler does not know who to consider the returned value, nor can it find the doSomethingElse method.

  Init is treated as an instancetype returned by the compiler. convenience constructor does not

  Of course, there are many other benefits, so let's get started later.

  Reprinted please indicate the blog from laruence: http://prevention.iteye.com