如何在 swift 中使用 UnsafeMutablePointer<T?> 3

Question

我是 Swift 的新手，我不明白如何使用 UnsafeMutablePointer<T?>。谁能帮帮我？

struct TestStruct {
    var a = 0
}

func foo(ptr: UnsafeMutablePointer<TestStruct?>?) {
    ptr?.pointee?.a = 123 // pointee is nil, but why?
}

func bar(ptr: TestStruct?) {
    print("hello: \(ptr?.a)") // hello: nil
}

var ref = UnsafeMutablePointer<TestStruct?>.allocate(capacity: 1)

foo(ptr: ref)
bar(ptr: ref.pointee)

ref.deallocate(capacity: 1)

我正在为 TestStruct 分配内存，但是当我将 ref 传递给 foo 时，pointee 指向 nil。如果我将 TestStruct ( UnsafeMutablePointer<TestStruct> ) 设为非可选 - 一切都会很好 - 条形打印 hello: 123.

更新：

感谢@MartinR 和@Hamish！工作代码：

struct TestStruct {
    var a = 0
}

func foo(ptr: UnsafeMutablePointer<TestStruct?>?) {
    ptr?.pointee?.a = 123
}

func bar(ptr: TestStruct?) {
    print("hello: \(ptr?.a)")
}

var ref = UnsafeMutablePointer<TestStruct?>.allocate(capacity: 1)

ref.initialize(to: TestStruct())

foo(ptr: ref)
bar(ptr: ref.pointee)


ref.deinitialize()
ref.deallocate(capacity: 1)

Answer 1

您的程序的行为未定义。

allocate() returns 未初始化内存：

/// Allocates and points at uninitialized aligned memory for `count`
/// instances of `Pointee`.
///
/// - Postcondition: The pointee is allocated, but not initialized.
public static func allocate(capacity count: Int) -> UnsafeMutablePointer<Pointee>

使用前必须先初始化（并在释放内存之前取消初始化）：

let ref = UnsafeMutablePointer<TestStruct?>.allocate(capacity: 1)
ref.initialize(to: TestStruct())

foo(ptr: ref)
bar(ptr: ref.pointee)

ref.deinitialize()
ref.deallocate(capacity: 1)

这是另一个实际没有初始化内存的例子导致崩溃：

class TestClass {
    init() { print("init") }
    deinit { print("deinit") }
}

let ptr = UnsafeMutablePointer<TestClass>.allocate(capacity: 1)
ptr.pointee = TestClass()

赋值语句期望ptr.pointee处于初始化状态并指向一个 TestClass 实例。由于 ARC（自动引用计数），之前指向的对象 已释放 分配新值时。如果 ptr.pointee 是未初始化。

如何在 swift 中使用 UnsafeMutablePointer<T?> 3

How to use UnsafeMutablePointer<T?> in swift 3

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