objective-c - 将 Object 成员变量分配给另一个双强指针

Question

我们可以在 Objective-C 中使用像这样的 __strong 双点。

NSString *__strong * tmp_pointer;
NSString * target = @"first data";
tmp_pointer = target;
*tmp_pointer = @"second data";
NSLog(@"%@", target);

Output : first data

但是对象变量呢？

示例：DataObject 有一个 NSString* 类型变量“item1”。

DataObject *dataObject = [[DataObject alloc] init];
NSString *__strong * tmp_pointer;
tmp_pointer = &dataObject.item1; <- Address of property expression requested error occurred.

我尝试了几种表达方式，但一切都失败了。

tmp_pointer = &(dataObject.item1);
tmp_pointer = &(NSString *)dataObject.item1;
tmp_pointer = &((NSString *)dataObject.item1);

有谁知道我该如何解决这个问题？

谢谢

Answer 1

通过点符号访问属性是方法调用的语法糖，所以你真正在做的是

&[dataObject item1]

这是语法不允许的。

的参数&是必须是左值或函数指示符 ^[1]的表达式，而方法调用两者都不是。

If want the address of the value returned by the getter, you have to turn it into a lvalue first. The most straightforward way is to assign it to a local variable:

NSString * item1 = dataObject.item1;        // now item1 is a lvalue
NSString * __strong * tmp_pointer = &item1; // so it can be the argument of &

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_{[1] A function designator as an expression that has function type.
An object is a region of storage that can be examined and stored into and an lvalue is an expression that refers to such an object.
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