ios - How to subclass NSMutableData -

i trying subclass nsmutabledata add ability subdata without copying. here code

@interface mymutabledata : nsmutabledata  - (nsdata *)subdatawithnocopyingatrange:(nsrange)range;  @end  @interface mymutabledata()  @property (nonatomic, strong) nsdata *parent;  @end  @implementation mymutabledata  - (nsdata *)subdatawithnocopyingatrange:(nsrange)range {     unsigned char *dataptr = (unsigned char *)[self bytes] + range.location;      mymutabledata *data = [[mymutabledata alloc]     initwithbytesnocopy:dataptr length:range.length freewhendone:no];      data.parent = self;      return data; }  @end 

but problem when try instantiate mymutabledata, got error

"-initwithcapacity: defined abstract class.  define -[mymutabledata initwithcapacity:]!'" 

why? inheritance not work?

this calls category. however, category cannot default have properties , instance variables. hence need #import <objc/runtime.h> , use associated objects , set value of parent.

@interface nsmutabledata(mymutabledata)  - (nsdata *)subdatawithnocopyingatrange:(nsrange)range;  @property (nonatomic, strong) nsdata *parent;  @end  @implementation nsmutabledata(mymutabledata)  - (nsdata *)subdatawithnocopyingatrange:(nsrange)range {     unsigned char *dataptr = (unsigned char *)[self bytes] + range.location;      nsmutabledata *data = [[nsmutabledata alloc]     initwithbytesnocopy:dataptr length:range.length freewhendone:no];     data.parent = self;     return data; }  -(nsdata*)parent {     return objc_getassociatedobject(self, @selector(parent)); }  -(void)setparent:(nsdata *)parent {     objc_setassociatedobject(self, @selector(parent), parent, objc_association_retain_nonatomic); }  @end