1、 Objective-c语言中的MRC（MannulReference Counting）
在MRC的内存管理模式下，对变量的管理相关的方法有：retain,release和autorelease。retain和release方法操作的是引用记数器，当引用记数为零时，便自动释放内存。并且可以用NSAutoreleasePool对象，对加入自动释放池（autorelease调用）的变量进行管理，当drain时回收内存。
（1）retain，该方法的作用是将内存数据的所有权附给另一指针变量，引用数加1，即retainCount+= 1;
（2）release，该方法是释放指针变量对内存数据的所有权，引用数减1，即retainCount-= 1;
（3）autorelease，该方法是将该对象内存的管理放到autoreleasepool中。
2.、Objective-c语言中的ARC（AutomaticReference Counting）
在ARC中与内存管理有关的标识符，可以分为变量标识符和属性标识符，对于变量有__strong,__weak,默认是strong；而对于属性表识符有：
@property(nonatomic/atomic,assign/retain/strong/weak/unsafe_unretained/copy,readonly/readwrite) Number* num;//默认为unsafe_unretained。
也存在autoreleasepool。
***对于Core Foundation与objective-cObject进行交换时，需要用到的ARC管理机制有：
(1) (__bridge_transfer<NSType>) op oralternatively CFBridgingRelease(op) isused to consume a retain-count of a CFTypeRef whiletransferring it over to ARC. This could also be represented by id someObj =(__bridge <NSType>) op; CFRelease(op);
(2) (__bridge_retained<CFType>) op oralternatively CFBridgingRetain(op) isused to hand an NSObject overto CF-land while giving it a +1 retain count. You should handle a CFTypeRefyoucreate this way the same as you would handle a result of CFStringCreateCopy().This could also be represented by CFRetain((__bridge CFType)op); CFTypeRef someTypeRef =(__bridge CFType)op;
(3) __bridge justcasts between pointer-land and Objective-C object-land. If you have noinclination to use the conversions above, use this one.
OC的MRC和ARC内存管理机制注意：Xcode4以前没有ARC。
原文：http://www.cnblogs.com/linxiu-0925/p/5030798.html