Cocoa Core Competencies 的Object creation 和IOS面試題示例:寫一個NSString類的實現
object creation:
An object comes into runtime existence through a two-step process that allocates memory for the object and sets its state to reasonable initial values. To allocate an Objective-C object, send an alloc or
allocWithZone: message to the object’s class. The runtime allocates memory for the object and returns a “raw” (uninitialized) instance of the class. It also sets a pointer (known as the isa
After you allocate an object, you must initialize it. Initialization sets the instance variables of an object to reasonable initial values. It can also allocate and prepare other global resources needed by the object. You initialize an object by invoking an init
init. These initializer methods often have one or more parameters that enable you to specify beginning values of an object’s instance variables. If these
methods succeed in initializing an object, they return it; otherwise, they return nil. If an object’s class does not implement an initializer, the Objective-C runtime invokes the initializer of the
nearest ancestor instead.意思是:建立物件需要兩個步驟1分配記憶體 2初始化
1分配記憶體,要sent alloc orallocWithZone: message
to the object’s class. 也就是常見的[Class alloc].或是不常見的[Class
allocWithZone]
2初始化.要呼叫init的方法進行初始化,各種各樣的要或不要引數的init方法都算.
以上建立物件的方式,返回值要送進 物件自動管理池
IOS面試題示例:寫一個NSString類的實現
+ (id)initWithCString:(const char *)nullTerminatedCString encoding:(NSStringEncoding)encoding;
+ (id) stringWithCString: (const char*)nullTerminatedCString
encoding: (NSStringEncoding)encoding
{
NSString *obj;
obj = [self allocWithZone: NSDefaultMallocZone()];
obj = [obj initWithCString: nullTerminatedCString encoding: encoding];
return AUTORELEASE(obj);
}
The Form of an Object-Creation Expression
A convention in Cocoa programming is to nest the allocation call inside the initialization call.
MyCustomClass *myObject = [[MyCustomClass alloc] init]; |
便利函式:就是工廠方法,不需使用者管理.
+ (id)dataWithContentsOfURL:(NSURL *)url; |