引用计数（retainCount）

ClassA.h:

#import <Foundation/Foundation.h>
@interface ClassA:NSObject
{
    NSString *name;
}
-(void)setName:(NSString*)s;
-(NSString*)name;
-(void)dealloc;

@end

ClassA.m:

#import "ClassA.h"
@implementation ClassA
-(void)setName:(NSString *)s
{
    name=s;
    [name retain];
}
-(NSString*)name
{
    return name;
}
-(void)dealloc
{
    NSLog(@"delloc coling!");
    [name release];
    [super dealloc];
}
@end

main.m:

#import <Foundation/Foundation.h>
#import "ClassA.h"
int main(int argc,const char *argv[])
{
    NSAutoreleasePool *pool=[[NSAutoreleasePool alloc]init];
    NSMutableString *str1=[NSMutableString stringWithString:@"Sam"];
    ClassA *classA=[[ClassA alloc]init];
    NSLog(@"Str1 count: %x",[str1 retainCount]);

    [classA setName:str1];
    NSLog(@"str1 count:%x",[str1 retainCount]);

    [str1 release];
    NSLog(@"str1 count:%x",[str1 retainCount]);
    [classA release];
    [pool drain];
    return  ;
}

console log:

2013-09-22 16:35:50.886 demo[1028:303] Str1 count: 1

2013-09-22 16:35:50.890 demo[1028:303] str1 count:2

2013-09-22 16:35:50.891 demo[1028:303] str1 count:1

2013-09-22 16:35:50.892 demo[1028:303] delloc coling!