[Go] "self = [super init]" explanation and hidden bug

Objective-c's recommended Init method is as follows:

-(ID) init

{

if (self = [super init])

{

To initialize a subclass by adding a property

}

return self;

}

Here are a few questions,

1. The role of [Super Init]:

The object-oriented embodiment, first uses the parent class's Init method to initialize the parent class part property of the subclass instance.

2. Why should self be assigned a value of [Super init]:

Simply to prevent the parent class from initializing the method to release the space that self points to and re-alloc a space. At this point, [super init] may fail alloc, then the statement in if is no longer executed.

3. Super as the essence of the message recipient:

Super is not really a pointer, [Super message] the essence is by Self to accept the parent class's message . It is important to note that in [Super message], the message method appears as self in the [Super message] context, that is, the subclass instance.

Hidden bugs:

Suppose there is a parent class aobj and a subclass bobj.

When Aobj's Init method is as follows:

-(ID) init

{

ID tmp = self;

Self = [Aobj alloc];

[TMP release];

Other staffs

return self;

}

The Init method for Bobj is as follows:

-(ID) init

{

if (self = [super init])

{

Other staffs

}

return self;

}

At this point the compilation can pass, but when the Bobj instance uses the Bobj extended attribute, a run-time error occurs. The reason for the error is that the Aobj init method uses [Aobj alloc] to regain a space that is only suitable for storing aobj instances. And Bobj's Init method thinks this is a space suitable for storing bobj. A run-time error occurs when attempting to read and write an extended property of Bobj.

Therefore, when the Init method needs to re-alloc a piece of space, the correct wording is as follows:

-(ID) init

{

ID tmp = self;

self = [[self class] alloc];

[TMP release];

Other staffs

return self;

}

Note In line 5th ,[self class] obtains the class instance corresponding to the instance that the self points to, in this case bobj. This way, the init method of any subclass of Aobj is guaranteed to be secure.

if (self = [super init]) This is the usual suggested notation, which assigns and measures 0 only to prevent the superclass from changing during initialization, returning different objects

Why must you Super Init ?

As we all know, Objective-c is an object-oriented language, in general, when we define a class in objective-c, we always provide an initialization method, which is generally written by everyone:

1 -(MyClass *) init

2 {

3 Self = [super init];

4 if (self) {

5 //Perform initialization of some resources and variables

6 }

7 return self;

8 }

This is a simple code, but there are a lot of things to think about:

1. Why should I invoke the initialization method of the parent class through [super Init] and what is being done in the initialization method of the parent class?

First of all, we know the concept of object inheritance, a subclass inherits from the parent class, but also to implement all the functions of the parent class, this is the is-a relationship, such as the dog is a mammal, then the dog must have the characteristics and functions of mammals. Therefore, in the initialization method of the subclass, you must first call the parent class's initialization method to implement the initialization of the parent class related resources. For example, when we initialize a dog, we must first initialize the Mammal object and give the result to the dog, so that the dog satisfies the characteristic of the mammal.

Typically, under IOS , all classes inherit from NSObject, and NSObject's Init method is simple, which is return self. When the initialization of the parent class is complete, that is, self is not nil, you can begin the initialization of the subclass.

2, whether must provide initialization method, whether must use Init as initialization method?

We create an object in objective-c typically using the

1 MyClass *newclass = [[MyClass alloc] init];

Or

MyClass *newclass = [MyClass new];

The new method is a static method of the NSObject object, which, according to Apple's documentation, is actually a combination of the Alloc and Init methods, in fact the two are the same, but Apple still recommends that we use the first method, why? Because using the first method, you can use your own Init method to do some initialization (with your own written init***** method), of course, if the subclass does not provide the Init method, the natural call is the parent class Init method. So, from a security point of view, as a developer we must initialize the object before the object is used, so be sure to provide an initialization method when defining the class. But are you sure you want to use Init as the method name? The answer is not necessarily. Using init as the method name is just a rewrite of the NSObject init method, and if you redefine an initialization method yourself, it is perfectly possible, as long as you remember to invoke the new defined initialization method when you use it.

However, this approach is not advisable from a design standpoint. In terms of reusability, if it is necessary to define some initialization methods that accept different parameters, my suggestion is to first define a common method of init, and then call it in other methods, such as:

-(ID) Common methods of init init

In. {

Self = [super init];

if (self) {

to

}

The return self;

(On }

the

Ten -(ID) initwithstring: (NSString *) astring

Each {

[self init];

self.name = astring;

+ }

the

-(ID) initwithimage: (UIImage *) Aimage

+ {

[self init];

self.image = aimage;

( }

Add:

In object-oriented programming, if you write a class without a constructor, the class can compile and work perfectly. If the class does not provide an explicit constructor, the compiler provides you with a default constructor function. In addition to creating the object itself, the only function of the default constructor is to call its superclass's constructor. In many cases, this superclass is part of the language framework, such as the object class in Java and the NSObject class in Objective-c.

In either case, having at least one constructor in a class is a good programming practice, and if there are attributes in the class, it is often good practice to initialize those properties.

[Go] "self = [super init]" explanation and hidden bug