如何在Objective-C中访问公共实例变量？

Question

I am having following condition:

@interface MyClass:NSObject
    @public NSString *str;
@end

@implementation 
-(id)init{ 

}

@end

Now I want to access str variable outside MyClass in Other Class, (1) Using MyClass Object (2) without using MyClass Object, How can I achieve that?

Answer 1

You can call using this:

MyClass *a;
a.str;

Without the object, you cannot call an instance variable. However, you can call static method with this declaration:

@interface MyClass:NSObject

+ (void)doX;

@end

@implementation 

+ (void)doX {
  // do whatever
}

then in another class you just need to call:

[MyClass doX];

However, let a public instance variable is not a good practice. The reason is that it will let any class, methods change that instance variable without your control. For example, they can set the NSString *str to nil and then nobody can call anything, or they may forget to do memory management when they call.

A better practice for public variable is using @property

For example, your string should be declared like:

@property (nonatomic, retain) NSString * str;

and then in the implementation:

@implementation MyClass

@synthesize str;

The good thing about property is that compiler will generate gettter and setter methods for you and those setters will handle memory correctly for you.

More about properties here

Answer 2

Sigh, i realise this post is LONG dead but I believe The above answer is incorrect. well the first bit.

Please see the link below. http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocObjectsClasses.html#//apple_ref/doc/uid/TP30001163-CH11-SW1

for the above interface to work, you NEED to declare a property for use outside of its class Because the instance variable it is not visible outside its class.

well; You don't NEED to. Doing something like MyClass->str is valid.

Please see this example

@interface Foo : NSObject { 

@public  NSInteger publicMember;
@private NSInteger aproperty;

}
@property (assign) NSInteger aproperty;`

then the calling class

Foo *f = [Foo new];
f.aproperty = 90;
//f.publicMember = 100; property 'publicMember' not found of type Foo *
f->publicMember = 100;

But as the above post said, you should always use @properties because if var public was a string, you are not retaining the string in any way.