oc——类——MRC ARC

RC

reference counting，引用计数，instance object内存管理机制，表示instance object被多少个指针引用或拥有 RC = 0时，instance object自动调用dealloc，instance object销毁，因此勿手动调用dealloc，否则会导致多次调用dealloc(多次销毁instance object)

MRC

mannul reference counting，手动引用计数，一种RC管理机制，开发者手动管理RC，MRC机制易导致RC混乱，引发内存错误 常见内存错误：

• 没有销毁instance object(内存泄露)
• 提早销毁instance object(访问非法内存)
• 重复销毁instance object(访问非法内存)

RC相关方法：

- (instancetype)retain OBJC_ARC_UNAVAILABLE;
- (oneway void)release OBJC_ARC_UNAVAILABLE;
- (instancetype)autorelease OBJC_ARC_UNAVAILABLE;
- (NSUInteger)retainCount OBJC_ARC_UNAVAILABLE;
           

注1：protocol NSObject接口，NSObject class实现，MRC下才能调用，ARC下禁止调用 注2：MRC编译选项：-fno-objc-arc

@interface FBAnimal : NSObject

@end

@implementation FBAnimal

- (void)dealloc
{
    NSLog(@"dealloc FBAnimal");
    [super dealloc];
}

@end

@interface FBDog : FBAnimal

@end

@implementation FBDog

- (void)dealloc
{
    NSLog(@"dealloc FBDog");
    [super dealloc];
}

@end
           

- (void)use_mrc
{
    FBDog *dog = [[FBDog alloc] init];
    NSLog(@"RC = %lu", dog.retainCount);
    [dog retain];
    NSLog(@"RC = %lu", dog.retainCount);
    [dog release];
    NSLog(@"RC = %lu", dog.retainCount);
    [dog retain];
    NSLog(@"RC = %lu", dog.retainCount);
    [dog release];
    NSLog(@"RC = %lu", dog.retainCount);
    [dog release];
    NSLog(@"method end");
}
           

output：

RC = 1
RC = 2
RC = 1
RC = 2
RC = 1
dealloc FBDog
dealloc FBAnimal
method end
           

注：dealloc调用后，对象已释放，此时获取的RC值无意义

ARC

automatic reference counting，自动引用计数，一种RC管理机制，编译器替开发者插入RC相关方法调用，ARC下禁止调用RC相关方法，否则会导致RC混乱，引发内存错误 ARC下禁止调用RC相关方法并不表示ARC放弃instance object的RC内存管理机制，实际上还是RC机制(要不就不使用ARC这个明显使用RC的名字了)，ARC下编译器替开发者插入RC相关方法调用，目的是为了减轻开发者负担，减少内存出错状况，提高开发效率 注1：ARC为默认RC管理机制 注2：ARC编译选项：-fobjc-arc

@interface FBAnimal : NSObject

@end

@implementation FBAnimal

- (void)dealloc
{
    NSLog(@"dealloc FBAnimal");
    //[super dealloc];
}

@end

@interface FBDog : FBAnimal

@end

@implementation FBDog

- (void)dealloc
{
    NSLog(@"dealloc FBDog");
    //[super dealloc];
}

@end
           

- (void)use_arc
{
    FBDog *dog = [[FBDog alloc] init];
    //NSLog(@"RC = %lu", dog.retainCount);
    //[dog retain];
    //NSLog(@"RC = %lu", dog.retainCount);
    //[dog release];
    NSLog(@"method end");
}
           

output：

method end
dealloc FBDog
dealloc FBAnimal
           

注1：dog指向instance object，此时RC = 1，离开use_arc后，dog失效，RC减1，此时RC = 0，调用FBDog的dealloc 注2：ARC下，禁止调用RC相关方法，禁止调用dealloc，编译器会自动在dealloc方法末插入[super dealloc]调用父类dealloc，如果手动插入，就会导致[super dealloc]重复调用(重复销毁父类)，导致内存错误