Swift 引用同一个变量时数组内存地址发生变化

Question

我正在学习 Python 并想确认 Objective-C 和 Swift 中的特定行为。

测试如下：

Python

def replace(list):
    list[0] = 3
    print(list)

aList = [1, 2, 3]
print(aList)
replace(aList)
print(aList)

Objective-C

- (void)replace:(NSMutableArray *)array {
    array[0] = @1;
    NSLog(@"array: %@, address: %p\n%lx", array, array, (long)&array);
}

NSMutableArray *array = [@[@1, @2, @3] mutableCopy];
NSLog(@"original: %@, address: %p \n%lx", array, array, (long)&array);
[self replace:array];
NSLog(@"modified: %@, address: %p \n%lx", array, array, (long)&array);

Swift

var numbers = [1, 2, 3]
let replace = { (var array:[Int]) -> Void in
    array[0] = 2
    print("array: \(array) address:\(unsafeAddressOf(array as! AnyObject))")
}

print("original: \(numbers) address:\(unsafeAddressOf(numbers as! AnyObject))")
replace(numbers)
print("modified: \(numbers) address:\(unsafeAddressOf(numbers as! AnyObject))")

除Swift中的地址部分外，所有结果都符合预期。在Objective-C中，array的地址在original和modified中保持不变，但Swift的打印结果是：

original: [1, 2, 3] address:0x00007f8ce1e092c0
array: [2, 2, 3] address:0x00007f8ce1f0c5d0
modified: [1, 2, 3] address:0x00007f8ce4800a10

我是否遗漏了什么？

Answer 1

Swift 中的数组具有值语义，而不是 Python 和 Objective-C 中数组的引用语义。您看到不同地址（和所有地址）的原因是每次执行 as! AnyObject 转换时，您实际上是在告诉 Swift 将您的 Array<Int> 结构桥接到一个实例NSArray 个。因为你桥接了三次，所以你得到了三个不同的地址。

您不需要考虑 Swift 数组的地址，但如果您想（暂时）获取数组缓冲区的地址，您可以这样做：

func getBufferAddress<T>(array: [T]) -> String {
    return array.withUnsafeBufferPointer { buffer in
        return String(buffer.baseAddress)
    }
}

这让您可以看到缓冲区的写时复制正在运行：

var numbers = [1, 2, 3]
let numbersCopy = numbers

// the two arrays share a buffer here
getBufferAddress(numbers)                 // "0x00007fba6ad16770"
getBufferAddress(numbersCopy)             // "0x00007fba6ad16770"

// mutating `numbers` causes a copy of its contents to a new buffer
numbers[0] = 4

// now `numbers` has a new buffer address, while `numbersCopy` is unaffected
getBufferAddress(numbers)                 // "0x00007ff23a52cc30"
getBufferAddress(numbersCopy)             // "0x00007fba6ad16770"

Swift 引用同一个变量时数组内存地址发生变化

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