@property 属性 assign retain copy的setter方法的内部实现

    /*
     
     assign retain copy的setter方法的内部实现
     assign:
     
     //@property float price;
    
     - (void)setPrice:(float)price
     {
        _price = price;
     }
    
     - (float)price
     {
        return _price;
     }
     
     retain:
     
     //@property (retain, readwrite, nonatomic) NSString *company;
     - (void)setCompany:(NSString *)company{
        if(_company != company){
            [_company release];
            [company retain];
            _company = company;
        }
     }
     
     copy:
     
     //@property (copy, readwrite, nonatomic) NSString *company;
     - (void) setCompany:(NSString *)company{
        if(_company != company){
            [_company release];
            [company copy];
            _company = company; 
        } 
     }
     
     
     */