例1.
先看个可变字符串与不可变字符串的例子：
NSMutableString *mutableStr = [NSMutableString stringWithString:@"jack"];
NSString *name = mutableStr;
NSLog(@"name = %@", name);
[mutableStr appendString:@" and rose"];
NSLog(@"name = %@", name);
mutableStr = nil;
NSLog(@"name = %@", name);
输出：
2020-08-14 09:52:25.021500+0800 [61167:1662404] name = jack
2020-08-14 09:52:25.021599+0800 [61167:1662404] name = jack and rose
2020-08-14 09:52:25.021677+0800 [61167:1662404] name = jack and rose
将
mutableStr赋值给name，此时name也就是可变字符串，故改变mutableStr的值name也会改变。注意mutableStr = nil是将mutableStr指针中保存的堆内存地址变为nil，并不是把堆内存地址的值变为nil，所以最后一行输出name依旧为jack and rose。
例2.
再看可变类型在block中
NSMutableArray *arr = [NSMutableArray arrayWithObjects:@"1",@"2", nil];
void(^block)(void) = ^{
NSLog(@"block1 - %@",arr);
[arr addObject:@"5"];
NSLog(@"block2 - %@",arr);
};
[arr addObject:@"3"];
[arr addObject:@"4"];
arr = nil;
block();
先想下输出会是什么：
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~
~
~
~
~
~
~
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2020-08-14 10:24:26.403654+0800 [61959:1683035] block1 - (
1,
2,
3,
4
)
2020-08-14 10:24:26.403781+0800 [61959:1683035] block2 - (
1,
2,
3,
4,
5
)
由于是可变类型，在block外更改
arr的内容是可以影响到block内的arr（反之亦然），但是arr = nil是不会将block内的arr置空，理由同例1。
