alloc 、init、new详解

NSObject *obj = [NSObject alloc];
NSLog(@"%p",obj);
obj = [obj init];
NSLog(@"%p",obj);

NSString *name = [NSString alloc];
NSLog(@"%p",name);
name = [name init];
NSLog(@"%p",name);

- (id)init {
    if(self = [super init])  {// 重新赋值
        //…
    }
}

NSObject *obj = [NSObject alloc];
NSLog(@"%p",obj);
obj = [obj init];
NSLog(@"%p",obj);

返回这个接受消息的类的一个实例.
The isa instance variable of the new instance is initialized to a data structure that describes the class; memory for all other instance variables is set to 0.
这个实例初始化后可以用来表示这个类的数据相关的结构;所有其他的实例变量的值都被设置成 0.
You must use an init... method to complete the initialization process. For example:
你必须使用 init... 方法来最终完成这个初始化的步骤,如下:
TheClass *newObject = [[TheClass alloc] init];
Do not override alloc to include initialization code. Instead, implement class-specific versions of init... methods.
不要重写 alloc 来包含初始化的代码.你可以使用指定版本的 init... 方法来达到你的目的.
For historical reasons, alloc invokes allocWithZone:.
由于历史原因,allc 方法调用了 allocWithZone: 方法.

Implemented by subclasses to initialize a new object (the receiver) immediately after memory for it has been allocated.
子类实现初始化一个刚刚获取到内存空间的对象.
An init message is coupled with an alloc (or allocWithZone:) message in the same line of code:
init 消息与 alloc (或者是 allocWithZone:) 消息在一行内执行:
TheClass *newObject = [[TheClass alloc] init];
An object isn’t ready to be used until it has been initialized. The init method defined in the NSObject class does no initialization; it simply returns self.
对象只有执行了 init 方法后才能够被使用. NSObject 类定义了这个初始化方法,但是 NSObject 并没有初始化什么,它直接将指针返回了.
In a custom implementation of this method, you must invoke super’s designated initializer then initialize and return the new object. If the new object can’t be initialized, the method should return nil. For example, a hypothetical BuiltInCamera class might return nil from its init method if run on a device that has no camera.
如果要自定义这个实现方法.你必须调用 super 来先初始化父类的对象.如果这个新对象不能被初始化,这个方法就应该返回 nil.例如,假设有一个照相机相关的类,如果在一个没有照相机的设备上调用了这个类,那么,在父类的 init 方法中就会返回 nil.

Allocates a new instance of the receiving class, sends it an init message, and returns the initialized object.
分配一个类的新实例内存地址，并执行init方法，返回这个初始化得对象。
This method is a combination of alloc and init. Like alloc, it initializes the isa instance variable of the new object so it points to the class data structure. It then invokes the init method to complete the initialization process.
这个方法可以当做alloc和init的组合。首先alloc初始化了一个class结构体对象的指针，然后调用init方法完成初始化过程。

Do not override allocWithZone: to include any initialization code. Instead, class-specific versions of init… methods.

This method exists for historical reasons; memory zones are no longer used by Objective-C.