Declaring private variables in header file vs declaring variables in class extension

问题

What's the difference between declaring a @private ivar in the header file and declaring the same ivar in the class extension without @private? As far as I understand it's the same thing.

Also, can you declare a private property in the header?

回答1:

What's the difference between declaring a @private ivar in the header file and declaring the same ivar in the class extension without @private?

There are a few differences. In short, variables declared in the header file are visible to subclasses and class categories. Variables declared in the implementation are not.

1) Instance variables declared in a class's main @interface block are available to external class categories or extensions, even if those variables are declared @private. E.g.:

// YourClass.h
@interface YourClass : NSObject {
    @private
    int _yourPrivateIVar; 
}
@end

// MyExtension.m
@implementation YourClass(MyExtension)
- (void)reset { _yourPrivateIVar = 0; } // This is allowed.
@end

Instance variables declared in the implementation are not available to external class categories.

2) A base class and its subclass cannot both declare the same ivar in their @interface, even if both ivars are @private. E.g., this is not allowed:

@interface Base : NSObject
{
    @private
    int _foo;
}
@end

@interface Subclass : Base
{
    @private
    int _foo; // Error: Duplicate member _foo
}
@end

If both ivars are declared in a class extension or implementation block then not only does it compile but it works as expected: both classes have their own separate _foo ivars that do not conflict with one another. On other words, both variables are truly private and separate:

@implementation Base {
    int _foo;
}
@end

@implementation Subclass {
    int _foo;
}
- (void)reset { _foo = 123; } // Does not affect base class's _foo
@end

Note: If the base class and subclass declare a "private" property or method with the same name it will compile without warning or error, but it will fail spectacularly at runtime as both classes unknowingly interfere with each other's private data.

回答2:

The concept is to declare in the header file only those things (methods, properties, etc) which are public. Declare all private items in the implementation file's class extension.

This provides the class users only information that is available for their use and hides all else. It also make it easier for a user of the class quickly see the functionality available to him. Writing code is all about readability and understandability to the developer.

This way a developer is free to change anything that is not exposed in the header files without making any externally visible changes.

In recent versions of Objective this is finally fully releasable via class extensions.

回答3:

Private property can't be declared in header file. decalring iVar in .m file is same as you declare @private in .h file

来源：https://stackoverflow.com/questions/32630628/declaring-private-variables-in-header-file-vs-declaring-variables-in-class-exten

标签

ios

objective-c

instance-variables