Objective-C-无法与下划线Ivar合成的只读属性? [英] Objective-C - readonly properties not synthesized with underscore ivar?
问题描述
如果我正确理解,在Objective-C中,属性将自动与getter和setter进行合成,实例变量声明为带有下划线(_ivar
)的属性名称.
If I understand correctly, in Objective-C, properties are automatically synthesized with getters and setters, with the instance variable declared as the property name with an underscore prepended (_ivar
).
因此,此代码:
#import <Foundation/Foundation.h>
#import "hello.h"
int main(int argc, char *argv[])
{
@autoreleasepool {
Hello *hello = [[Hello alloc] init];
NSLog(@"%@", hello.myString);
return 0;
}
}
hello.h
#import <Foundation/Foundation.h>
@interface Hello : NSObject
@property (copy, nonatomic) NSString *myString;
@end
hello.m
#import "hello.h"
@implementation Hello
-(Hello *)init
{
if (self = [super init]) {
_myString = @"Hello";
}
return self;
}
-(NSString *)myString
{
return [NSString stringWithFormat:@"%@ %@", _myString, @"World"];
}
@end
可以像这样编译和运行:
Can be compiled and run like this:
bash-3.2$ clang -framework Foundation main.m hello.m -o hello
bash-3.2$ ./hello
2013-05-27 13:20:39.738 hello[23320:707] Hello World
现在,当我将myString属性更改为只读时,如下所示:
Now when I change the myString property to readonly like this:
@property (readonly, copy, nonatomic) NSString *myString;
然后当我尝试编译时出现错误:
Then when I try to compile I get an error:
hello.m:11:9: error: unknown type name '_myString'; did you mean 'NSString'?
_myString = @"Hello";
^~~~~~~~~
NSString
因此未定义_myString
.编译器是否没有使用实例变量_myString来合成属性?让我们看看当我自己合成它时是否起作用:
So _myString
is not defined. Did the compiler not synthesize the property with instance variable _myString? Let's see if it works when I synthesize it myself:
在hello.m实现中:
In hello.m implementation:
@synthesize myString = _myString;
现在它又可以工作了:
bash-3.2$ clang -framework Foundation main.m hello.m -o hello
bash-3.2$ ./hello
2013-05-27 13:36:59.916 hello[24219:707] Hello World
所以,我的问题是,为什么在属性上使用readonly
时,它不会自动与下划线ivar合成?还是我完全错误地理解了Objective-C的工作原理?
So, my question is, why is it not automatically synthesized with an underscore ivar when you use readonly
on properties? Or am I totally on the wrong path of understanding how this Objective-C stuff works?
我非常感谢您提供解释性的答案,因为我真的想了解发生了什么以及为什么.
I would very much appreciate an explaining answer, as I really want to understand what's exactly happening and why.
谢谢.
推荐答案
如果您实现所有必需的访问器方法(获取读取的内容,唯一的属性,一个读写属性的getter + setter).
A property is not automatically synthesized if you implement all required accessor methods (getter for a read-only property, getter + setter for a read-write property).
因为您为只读属性实现了getter方法-(NSString *)myString
,
它不是自动合成的.如果您的getter需要实例变量来备份属性值,则必须添加synthesize语句(如您所做的那样)或实例变量.
Because you implement the getter method -(NSString *)myString
for the read-only property,
it is not auto-synthesized. If your getter needs an instance variable to backup the property value, you have to add the synthesize statement (as you did) or an instance variable.
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