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Use copy property for NSString

 2 years ago
source link: https://fann.im/blog/2012/09/10/use-copy-property-for-nsstring/
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Use copy property for NSString

Sep 10, 2012

一个简短例子来说明一下为什么 NSString @property 最好用 copy 而不是 retain:

#import <Foundation/Foundation.h>

@interface Person : NSObject

@property (nonatomic, retain) NSString *name;
@property (nonatomic, copy) NSString *school;

@end

@implementation Person

@synthesize name, school;

@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        Person *p = [[Person alloc] init];

        NSMutableString *s1 = [NSMutableString stringWithString:@"fannheyward"];
        NSMutableString *s2 = [NSMutableString stringWithString:@"hfut"];
        p.name = s1;
        p.school = s2;
        NSLog(@"%@, %@", p.name, p.school); // fannheyward, hfut
        [s1 appendString:@"---Heybot"];
        [s2 appendString:@"---Heybot"];
        NSLog(@"%@, %@", p.name, p.school); // fannheyward---Heybot, hfut
    }
}

简单来说就是 NSString 可以通过 NSMutableString (isa NSString) 来进行修改，如果 @property 是 retain 的话就可以绕过 Person 实例来修改 name 值（因为 name 指向 s1），大部分时候这种情况都是不应该发生的，用 copy 就没有这个问题。

这样来说象 NSArray/NSDictionary 等可修改类型都应该用 copy。

For attributes whose type is an immutable value class that conforms to the NSCopying protocol, you almost always should specify copy in your @property declaration.

参考 NSString property: copy or retain?

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