objective C 内存管理及属性方法具体解释

oc为每一个对象提供一个内部计数器。这个计数器跟踪对象的引用计数，当对象被创建或拷贝时。引用计数为1。每次保持对象时，调用retain接口。引用计数加1。假设不需要这个对象时调用release，引用计数减1，当对像的引用计数为0时，系统就会释放掉这块内存,释放对象调用dealloc

当对象包括其它对象时，就得在dealloc中自己释放他们

有两个基本函数。alloc和dealloc

alloc相似于C++的new。dealloc相似于delete

当对象的retaincount为0时。自己主动调用dealloc函数

release仅仅是使retaincount-1。不是调用dealloc函数

内存管理的原则：

假设使用alloc。copy创建的对象。一定要release

假设你retain一个对象，那么必需要release

Song 类的实现

#import <Foundation/Foundation.h>

@interface Song : NSObject
{
??? NSString *_title;
??? NSString *_artist;
??? long int _duration;
}
@property (nonatomic,retain) NSString *title;
@property (nonatomic,retain) NSString *artist;
@property (nonatomic,assign) long int duration;
-(Song*)initwithTitle:(NSString *)t AndArtist:(NSString *)art AndDuration:(long int)d;
@end

#import "Song.h"

@implementation Song
@synthesize title=_title;
@synthesize artist=_artist;
@synthesize duration=_duration;

-(Song*)initwithTitle:(NSString *)t AndArtist:(NSString *)art AndDuration:(long)d
{
??? self=[super init];
??? if(self)
??? {
??????? self.title=t;
??????? self.artist=art;
??????? self.duration=d;
??? }
??? return self;
}
@end
main函数code

int main(int argc, const char * argv[])
{

??? Song *Song1=[[Song alloc] initwithTitle:@"what" AndArtist:@"hello" AndDuration:3];
??? Song *Song2=[[Song alloc] initwithTitle:@"aaa" AndArtist:@"bbb" AndDuration:4];
??? NSLog(@"Song1 retain count is %ld",[Song1 retainCount]);
??? NSLog(@"Song2 retain count is %ld",[Song2 retainCount]);
??? [Song1 retain];
??? [Song2 retain];
??? NSLog(@"Song1 retain count is %ld",[Song1 retainCount]);
??? NSLog(@"Song2 retain count is %ld",[Song2 retainCount]);
??? [Song1 release];
??? [Song2 release];
??? NSLog(@"Song1 retain count is %ld",[Song1 retainCount]);
??? NSLog(@"Song2 retain count is %ld",[Song2 retainCount]);
??? [Song1 release];
??? [Song2 release];
??? return 0;
}

the result：

2013-05-07 14:44:55.170 Access[2891:303] Song1 retain count is 1
2013-05-07 14:44:55.173 Access[2891:303] Song2 retain count is 1
2013-05-07 14:44:55.173 Access[2891:303] Song1 retain count is 2
2013-05-07 14:44:55.173 Access[2891:303] Song2 retain count is 2
2013-05-07 14:44:55.174 Access[2891:303] Song1 retain count is 1
2013-05-07 14:44:55.174 Access[2891:303] Song2 retain count is 1

内存管理释放池提供了一个对象容器。每次对象发送autorelease时。对象的引用计数并不真正变化。而是内存释放池记录一条记录，记下该对象的要求，直到内存释放池发送retain或release时，当池在销毁之前通知池内全部元素，发送release消息减1。这部分代码必须放在:

NSAutoreleasePool *pool=[[NSAutoreleasePool alloc]init];

和 ??? [pool release]; 之间

int main(int argc, const char * argv[])
{
?
??? NSAutoreleasePool *pool=[[NSAutoreleasePool alloc]init];
??? NSArray *weeks1=[NSArray arrayWithObjects:@"monday",@"tuesday",@"thursday", nil];
??? NSArray *weeks2=[[NSArray alloc ]initWithObjects:@"monday",@"tuesday",@"thursday", nil];
??? //[weeks1 autorelease];
??? [weeks1 release];
??? [weeks2 release];
??? //[weeks2 autorelease];
?? ?
??? NSLog(@"retain count is %ld",[weeks1 retainCount]);
??? NSLog(@"retain count is %ld",[weeks2 retainCount]);
??? [pool release];
????? return 0;
}

属性简单介绍

@property 和@synthesize 能够自己主动生成某个类成员变量的存取方法，

语法 @property(參数) 类型 名字

这里的參数分为三大类：

读写属性：（readwrite/readonly）?readwrite：这个属性是默认的，readonly：仅仅生成getter 不会有setter

原子性（nonatomic）atomic ；是为了保证程序能够并发，避免同步问题

代码例如以下：

@property (nonatomic,retain) NSString *title;

{

? ?//首先推断是否与旧对象一致。假设不一致进行赋值。

?

//对于nil对象运行release，不会抛异常，假设不会nil，release正好释放旧的对象，这样就能够保证不会出现内存泄露

//由于假设是一个对象的话。进行if内的代码会造成一个极端的情况：当此name的retain为1时，使此次的set操作让实例name提前释放。而达不到赋值目的。?

if(newTitle!=title)

{

}

}

在对属性进行赋值时，会调用属性的set方法，使引用计数加一从而保证内存的正确引用。

自己定义的类是不能用COPY的，由于自己定义的类没有实现<NSCopy>协议，该协议里面有各种copy方法，所以，copy别乱用，尽量仅仅在设置字符串时使用。