Member variables and attributes of classes in Objective-C

Member variables and attributes of classes in Objective-C

In the concepts of Objective-C classes and objects. the differences and relationships between member variables and attributes have not been clarified. it was not until I learned the Objective-C object-oriented experience of MOOC online course that I really felt a little bit. the most important conclusion is:Use the member variable {} in the class, and use the attribute @ property outside the class..

Member variable and Its get method.

First, let's look at the basic class member variables and their usage.

// People.h@interface People : NSObject{    NSString *_peopleName;}@end

Do not do anything in. m, and then call the _ peopleName member variable in main. m,
(It can be seen that when calling a class member variable, an error occurs when using the. Syntax symbol. RequiredUse-> to call):
Call class member variable title =>
The result of calling p1-> _ peopleName is as follows:

That is, the _ peopleName is protected by default, and the external call must be set@ Public. Change: <喎?http: www.bkjia.com kf ware vc " target="_blank" class="keylink"> VcD4NCjxwcmUgY2xhc3M9 "brush: java;"> // People.h@interface People : NSObject{ @public NSString *_peopleName;}@end

The result of calling p1-> _ peopleName is as follows:

2015-05-06 15:58:41.039 memberAndProperty[2851:304100] p1._peopleName : (null)

Class internal use member variables

If you want to initialize _ peopleName in init, in People. m:

// People.m- (instancetype)init{    self = [super init];    if (self) {        _peopleName = @people name 1;    }    return self;}

The result of calling p1-> _ peopleName is as follows:

2015-05-06 16:01:36.974 memberAndProperty[2895:306281] p1._peopleName : people name 1

Other internal methods of the class that use this member variable are similar.

Set Method

The above is a call to the class member Variable _ peopleName. What if I want to add a new value to it?

        // main.m        People *p1 = [[People alloc] init];        NSLog(@p1._peopleName : %@, p1->_peopleName);        p1->_peopleName = @people name 2;        NSLog(@p1._peopleName : %@, p1->_peopleName);

Result:

2015-05-06 16:05:34.915 memberAndProperty[2931:309406] p1._peopleName : people name 12015-05-06 16:05:34.916 memberAndProperty[2931:309406] p1._peopleName : people name 2

As shown above, after _ peopleName is set to @ public, you can perform the get/set operation on it outside the class and directly call or assign values.
HoweverP1-> _ peopleName is not recommendedThis form is not in line with what we call
"Use the member variable {} in the class and use the attribute @ property outside the class."Conclusion. It is not safe to set the member Variable _ peopleName to @ public.

Get/set Method for custom member variables

The member Variable _ peopleName is still @ protected by default. It is a good choice to read or assign values to _ peopleName from the internal method of the class and then indirectly pass it to the external class.
First, declare the getName and setName methods in. h:

// People.h@interface People : NSObject{    NSString *_peopleName;}-(NSString *)getName;-(void)setName:(NSString *)name;@end

In. m, the get/set methods are as follows:Call or assign a value to the member Variable _ peopleName within the class.

// People.m-(NSString *)getName {    return _peopleName;}-(void)setName:(NSString *)name {    _peopleName = name;}

Therefore, the call is very convenient:

        // main.m        People *p1 = [[People alloc] init];        NSLog(@p1.getName : %@, p1.getName);        [p1 setName:@people name 2];        NSLog(@p1.getName : %@, p1.getName);

The result is as follows:

2015-05-06 16:25:36.019 memberAndProperty[3036:320317] p1.getName : people name 12015-05-06 16:25:36.020 memberAndProperty[3036:320317] p1.getName : people name 2

Using custom get/set, Calling member variables within a class is a common method, but you need to manually add these two methods.

Attribute

Is there a simpler way? In other words, can a very common get/set operation be provided to us by default? The answer is yes!
In the new iOS SDK, @ property is used to define the attributes of a class, which is used to call or assign values to it from outside the class.
First declare the peopleName attribute in. h:

// People. h @ interface People: NSObject {NSString * _ peopleName;} @ property (nonatomic, strong) NSString * peopleName; // nonatomic non-atomic access without synchronization mechanism, multithreading is not a high-performance access. // strong is equivalent to a deep copy operation @ end.

In. m, use the @ synthesize command to associate the peopleName attribute with the _ peopleName member variable:

// People.m@implementation People@synthesize peopleName = _peopleName;- (instancetype)init{    self = [super init];    if (self) {        _peopleName = @people name 1;    }    return self;}@end

That is, when the compiler encounters @ synthesize peopleName = _ peopleName;, the get/set Method for peopleName is automatically generated.
The underline _ is essential. Otherwise, the attribute cannot be correctly associated with the member variable.
Then, we can call it directly:

        // main.m        People *p1 = [[People alloc] init];                 NSLog(@p1.peopleName : %@, p1.peopleName);        p1.peopleName = @people name 2;        NSLog(@p1.peopleName : %@, p1.peopleName);

The result is as follows:

2015-05-06 16:32:29.142 memberAndProperty[3094:325295] p1.peopleName : people name 12015-05-06 16:32:29.143 memberAndProperty[3094:325295] p1.peopleName : people name 2

In fact, the compiler is smarter than we think. the member variable {NSString * _ peopleName;} in the H file does not need to be declared. only @ property (nonatomic, strong) NSString * peopleName is declared. xcode will automatically declare a class for us by default.PeopleName member variable and the hidden get/set method. Here, the underline is displayed..