什么是assign，copy，retain之间的区别？

• assign： 简单赋值，不更改索引计数（Reference Counting）。
• copy： 建立一个索引计数为1的对象，然后释放旧对象
• retain：释放旧的对象，将旧对象的值赋予输入对象，再提高输入对象的索引计数为1

retain是指针拷贝，copy是内容拷贝

比如一个Car对象，地址为0×1111

Copy到另外一个NSString之后，地址为0×2222，内容相同，新的对象retain为1，旧有对象没有变化

retain到另外一个NSString之后，地址相同（建立一个指针，指针拷贝），内容当然相同，这个对象的retain值+1

- (void)setName:(NSString *)newName {

    if (name != newName) {

       [name release];

       name = [newName retain];

       // name’s retain count has been bumped up by 1

    }

}

对于NSString、NSDictianary等数据型是很特殊的,见下示例

NSLog(@”COPY retain 差异测试”);

NSString * t3 = [[NSString alloc] initWithFormat:@”%@”,@”t3″];
NSLog(@”变量t3=%@ count=%d 地址=%lx “,t3,[t3 retainCount],&t3);
NSLog(@”t4 COPY t3″);
NSString * t4 = [t3 copy];
NSString * t5 = t3;
NSLog(@”变量t3=%@ count=%d 地址=%lx “,t3,[t3 retainCount],&t3);
NSLog(@”变量t4=%@ count=%d 地址=%lx “,t4,[t4 retainCount],&t4);
NSLog(@”变量t5=%@ count=%d 地址=%lx “,t5,[t5 retainCount],&t5);
NSLog(@”t3 retain”);
[t3 retain];

NSLog(@”变量t3=%@ count=%d 地址=%lx “,t3,[t3 retainCount],&t3);
NSLog(@”变量t4=%@ count=%d 地址=%lx “,t4,[t4 retainCount],&t4);
NSLog(@”变量t5=%@ count=%d 地址=%lx “,t5,[t5 retainCount],&t5);

NSLog(@”t3 release”);
[t3 release];
//[t4 release];
NSLog(@”变量t3=%@ count=%d 地址=%lx “,t3,[t3 retainCount],&t3);
NSLog(@”变量t4=%@ count=%d 地址=%lx “,t4,[t4 retainCount],&t4);
NSLog(@”变量t5=%@ count=%d 地址=%lx “,t5,[t5 retainCount],&t5);

NSLog(@”t4 release”);
[t4 release];
//[t4 release];
NSLog(@”变量t3=%@ count=%d 地址=%lx “,t3,[t3 retainCount],&t3);
NSLog(@”变量t4=%@ count=%d 地址=%lx “,t4,[t4 retainCount],&t4);
NSLog(@”变量t5=%@ count=%d 地址=%lx “,t5,[t5 retainCount],&t5);

2010-06-04 18:29:19.177 memmanage[2926:a0f] COPY retain 差异测试
2010-06-04 18:29:19.177 memmanage[2926:a0f] 变量t3=t3 count=1 地址=7fff5fbff750
2010-06-04 18:29:19.178 memmanage[2926:a0f] t4 COPY t3
2010-06-04 18:29:19.178 memmanage[2926:a0f] 变量t3=t3 count=2 地址=7fff5fbff750
2010-06-04 18:29:19.178 memmanage[2926:a0f] 变量t4=t3 count=2 地址=7fff5fbff748
2010-06-04 18:29:19.179 memmanage[2926:a0f] 变量t5=t3 count=2  地址=7fff5fbff740
2010-06-04 18:29:19.179 memmanage[2926:a0f] t3 retain
2010-06-04 18:29:19.180 memmanage[2926:a0f] 变量t3=t3 count=3 地址=7fff5fbff750
2010-06-04 18:29:19.180 memmanage[2926:a0f] 变量t4=t3 count=3 地址=7fff5fbff748
2010-06-04 18:29:19.180 memmanage[2926:a0f] 变量t5=t3 count=3 地址=7fff5fbff740
2010-06-04 18:29:19.181 memmanage[2926:a0f] t3 release
2010-06-04 18:29:19.182 memmanage[2926:a0f] 变量t3=t3 count=2 地址=7fff5fbff750
2010-06-04 18:29:19.182 memmanage[2926:a0f] 变量t4=t3 count=2 地址=7fff5fbff748
2010-06-04 18:29:19.183 memmanage[2926:a0f] 变量t5=t3 count=2 地址=7fff5fbff740
2010-06-04 18:29:19.183 memmanage[2926:a0f] t4 release
2010-06-04 18:29:19.184 memmanage[2926:a0f] 变量t3=t3 count=1 地址=7fff5fbff750
2010-06-04 18:29:19.184 memmanage[2926:a0f] 变量t4=t3 count=1 地址=7fff5fbff748
2010-06-04 18:29:19.185 memmanage[2926:a0f] 变量t5=t3 count=1 地址=7fff5fbff740