Swift中的withUnsafePointer和copy on write验证

在Swift编程中，我们经常需要使用withUnsafePointer函数来获取指向值的指针。下面是一个示例代码：

        withUnsafeBufferPointer(to: a) { point in
            let address = UnsafeRawPointer(point)
            let addressInt = Int(bitPattern: address)
            print("\(addressInt)")
        }

对于值类型的赋值，Swift中的容器类型（如Array、Set）采用了copy on write（写时复制）的机制进行优化。这意味着在赋值操作时，并不会立即进行拷贝，只有在修改值时才会发生拷贝。这种优化可以避免不必要的性能损失。

对于Array、Dictionary、Set类型，当它们赋值的时候不会发生拷贝，只有在修改的之后才会发生拷贝。

通过代码验证，我们可以发现在使用withUnsafePointer函数时，数组赋值后两个变量的地址会不一样，这是合理的，因为值类型的赋值会使得两个变量在内存上互为独立个体。如果要验证copy on write机制，Array提供了另一个方法，即withUnsafeBufferPointer。使用该方法查看赋值后的数组，会发现输出的地址不变。

实际上，withUnsafeBufferPointer指向的地址是数组存储数据的内存地址，而withUnsafePointer指向的是数组对象本身。

以下是一个代码示例：

        var array1 = [1, 2, 3]
        array1.withUnsafeBufferPointer { point in
            let address = point.baseAddress!
            let addressInt = Int(bitPattern: address)
            print("\(addressInt)")
            // 105553141216992
        }
        withUnsafePointer(to: array1) { point in
            let address = UnsafeRawPointer(point)
            let addressInt = Int(bitPattern: address)
            print("\(addressInt)")
            // 6089438336
        }
        var array2 = array1  // 这里并没有真正复制数据，而是共享内存
        array2.withUnsafeBufferPointer { point in
            let address = point.baseAddress!
            let addressInt = Int(bitPattern: address)
            print("\(addressInt)")
            // 105553141216992
        }
        withUnsafePointer(to: array2) { point in
            let address = UnsafeRawPointer(point)
            let addressInt = Int(bitPattern: address)
            print("\(addressInt)")
            // 6089438320
        }
        array2[2] = 1
        array2.withUnsafeBufferPointer { point in
            let address = point.baseAddress!
            let addressInt = Int(bitPattern: address)
            print("\(addressInt)")
            // 105553141218080
        }
        withUnsafePointer(to: array2) { point in
            let address = UnsafeRawPointer(point)
            let addressInt = Int(bitPattern: address)
            print("\(addressInt)")
            // 6089438280
        }