1 配对原则
alloc – release
new – release
retain - release
copy – release
2 new和alloc-init的区别
（1）先看看实现源码
+new
{
id newObject =(*_alloc)((Class)self, 0);
Class metaClass = self->isa;
if (class_getVersion(metaClass)> 1)
{
return [newObject init];
}
else
{
return newObject;
}
}
+alloc
{
return(*_zoneAlloc)((Class)self, 0, malloc_default_zone());
}
-init
{
return self;
}
通过源码我们发现，[className new]基本等同于[[classNamealloc] init]；
区别只在于alloc分配内存的时候使用了zone.
这个zone是个什么呢？
它是给对象分配内存的时候，把关联的对象分配到一个相邻的内存区域内，以便于调用时消耗很少的代价，提升了程序处理速度。
（2）为什么不推荐使用new
因为若用了new，则初始化方法只能是init。这样，假如你想调用initWithFrame,initWithVideoId, initWithPlayItem是无法做到的。另一个原因是习惯和风格的问题。
3 NSString 对象为何用copy
（1）NSString在Objective-C中是一种非常特殊的对象，其引用系数不受引用计数规则的控制。NSString对象不管是alloc、retain还是release，其引用计数都是-1。看下面一段代码：
NSString *aStr = [[NSString alloc] initWithString:@"abc"];
NSLog(@"aStr address = %p", aStr);
NSLog(@"aStr retainCount after alloc = %d", [aStr retainCount]);
NSString *bStr = [aStr copy];
NSLog(@"aStr retainCount after copy = %d", [aStr retainCount]);
NSLog(@"bStr address = %p", bStr);
NSLog(@"bStr retainCount = %d", [bStr retainCount]);
NSString *cStr = [aStr retain];
NSLog(@"aStr retainCount after retain = %d", [aStr retainCount]);
NSLog(@"cStr address = %p", cStr);
NSLog(@"cStr retainCount = %d", [cStr retainCount]);
[aStr release];
NSLog(@"aStrretainCount after release = %d", [aStr retainCount]);
输出结果：
2013-10-15 21:51:11.916Test[5575:a0b] aStr address = 0x3548
2013-10-15 21:51:11.918Test[5575:a0b] aStr retainCount after alloc = -1
2013-10-15 21:51:11.919Test[5575:a0b] aStr retainCount after copy = -1
2013-10-15 21:51:11.919Test[5575:a0b] bStr address = 0x3548
2013-10-15 21:51:11.919Test[5575:a0b] bStr retainCount = -1
2013-10-15 21:51:11.920Test[5575:a0b] aStr retainCount after retain = -1
2013-10-15 21:51:11.920Test[5575:a0b] cStr address = 0x3548
2013-10-15 21:51:11.921Test[5575:a0b] cStr retainCount = -1
2013-10-15 21:51:11.921 Test[5575:a0b] aStrretainCount after release = -1
（2）对于NSMutableString，看下面一段代码
NSMutableString *str1 = [[NSMutableString alloc] initWithString:@"abc"];
NSLog(@"str1 address = %p", str1);
NSLog(@"str1 retaincount after alloc = %d", [str1 retainCount]);
NSString *str2 = [str1 copy];
NSLog(@"str1retaincount after copy = %d", [str1 retainCount]);
NSLog(@"str2 address = %p", str2);
NSLog(@"str2 retaincount = %d", [str2 retainCount]);
NSString *str3 = [str1 retain];
NSLog(@"str1 retaincount after retain = %d", [str1 retainCount]);
NSLog(@"str3 address = %p", str3);
NSLog(@"str3 retaincount = %d", [str3 retainCount]);
[str1 setString:@"edf"];
NSLog(@"str1 address = %p", str1);
NSLog(@"after str1 changed, str2 = %@", str2);
NSLog(@"afterstr1 changed, str3 = %@", str3);
运行结果：
2013-10-15 22:14:51.735Test[5794:a0b] str1 address = 0x8c46510
2013-10-15 22:14:51.736Test[5794:a0b] str1 retaincount after alloc = 1
2013-10-15 22:14:51.737Test[5794:a0b] str1 retaincount after copy = 1
2013-10-15 22:14:51.737Test[5794:a0b] str2 address = 0x8c47680
2013-10-15 22:14:51.738Test[5794:a0b] str2 retaincount = 1
2013-10-15 22:14:51.738Test[5794:a0b] str1 retaincount after retain = 2
2013-10-15 22:14:51.739Test[5794:a0b] str3 address = 0x8c46510
2013-10-15 22:14:51.739Test[5794:a0b] str3 retaincount = 2
2013-10-15 22:14:51.739Test[5794:a0b] str1 address = 0x8c46510
2013-10-15 22:14:51.740Test[5794:a0b] after str1 changed, str2 = abc
2013-10-15 22:14:51.740 Test[5794:a0b] after str1changed, str3 = edf
从（1）中的结果可以看出，对于NSString来说，用alloc和用retain都是指向同一块内存，区别不大。
但从（2）中来看，对于NSMutableString来说，alloc实际上就是开辟了一块新内存，再把内容复制进来，而r
补充：移动开发 , IOS ,
