[Swift5.1] 21-指针

指针

Swift中也有专门的指针类型，这些都被定性为“Unsafe”（不安全的），常见的有以下4种类型:

• UnsafePointer<Pointee> 类似于 const Pointee *
• UnsafeMutablePointer<Pointee> 类似于 Pointee *
• UnsafeRawPointer 类似于 const void *
• UnsafeMutableRawPointer 类似于 void *

UnsafePointer和UnsafeMutablePointer指针可以通过pointee属性获取指针指向内存地址的值, 并访问或者修改该值.

var age = 10
func test1(_ ptr: UnsafeMutablePointer<Int>) {  // int *
    ptr.pointee += 10
}
func test2(_ ptr: UnsafePointer<Int>) {  // const int *
    print(ptr.pointee)
}
test1(&age) 
test2(&age) // 20
print(age) // 20

• UnsafeMutableRawPointer指针通过storeBytes(of: as:) 存储数据.
• UnsafeRawPointer指针通过load(as:)访问数据.

var age = 10
func test3(_ ptr: UnsafeMutableRawPointer) { // void *
    ptr.storeBytes(of: 20, as: Int.self)
}
func test4(_ ptr: UnsafeRawPointer) { // const void *
    print(ptr.load(as: Int.self))
}
test3(&age)  
test4(&age) // 20
print(age) // 20

指针的应用示例

var arr = NSArray(objects: 11, 22, 33, 44)
arr.enumerateObjects { (obj, idx, stop) in
    print(idx, obj)
    if idx == 2 { // 下标为2就停止遍历
        stop.pointee = true
    }
}

swift推荐遍历使用enumerated, 从(idx, obj) 元组中获取索引和值

var arr = NSArray(objects: 11, 22, 33, 44)
for (idx, obj) in arr.enumerated() {
    print(idx, obj)  
    if idx == 2 {
        break
    }
}

获得指向某个变量的指针

• withUnsafeMutablePointer(to:) { $0 }
• withUnsafePointer(to:) { $0 }

var age = 11
var ptr1 = withUnsafeMutablePointer(to: &age) { $0 }
var ptr2 = withUnsafePointer(to: &age) { $0 }
ptr1.pointee = 22
print(ptr2.pointee) // 22
print(age) // 22

var ptr3 = withUnsafeMutablePointer(to: &age) { UnsafeMutableRawPointer($0) }
var ptr4 = withUnsafePointer(to: &age) { UnsafeRawPointer($0) }
ptr3.storeBytes(of: 33, as: Int.self)
print(ptr4.load(as: Int.self)) // 33
print(age) // 33

获得指向堆空间实例的指针

class Person {}
var person = Person()
var ptr = withUnsafePointer(to: &person) { UnsafeRawPointer($0) }
var heapPtr = UnsafeRawPointer(bitPattern: ptr.load(as: UInt.self))  
//ptr.load(as: UInt.self) 堆空间person对象地址值
print(heapPtr!)  // 0x000000010055b5d0

• ptr指向person指针变量,等价于person本身,存储person地址值
• bitPattern: 传入内存地址, heapPtr存储传入的地址

var age = 11  //值类型
var ptr2 = withUnsafePointer(to: &age) { $0 }
//ptr2.pointee == age
print(ptr2)  // 0x00000001000031b0

创建指针

1)指定内存创建指针

var ptr = UnsafeRawPointer(bitPattern: 0x100001234)

2)malloc方式创建指针

// 创建
var ptr = malloc(16)  // 堆空间分配16个字节
// 存
ptr?.storeBytes(of: 11, as: Int.self)
ptr?.storeBytes(of: 22, toByteOffset: 8, as: Int.self) 
// toByteOffset 偏移量, 从ptr开始偏移8个字节,后8个字节存储Int类型 22 

// 取
print((ptr?.load(as: Int.self))!) // 11
print((ptr?.load(fromByteOffset: 8, as: Int.self))!) // 22
// 销毁
free(ptr)

3)allocate方式创建指针

• 注意:只有UnsafeMutableRawPointer 和 UnsafeMutablePointer 才可以调用 allocate分配内存.
• UnsafeMutableRawPointer调用allocate

var ptr = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1)
// 存储
ptr.storeBytes(of: 11, as: Int.self) 
// 指针偏移8个字节,存储Int类型 22 
ptr.advanced(by: 8).storeBytes(of: 22, as: Int.self)
// 取
print(ptr.load(as: Int.self)) // 11
print(ptr.advanced(by: 8).load(as: Int.self)) // 22
//释放
ptr.deallocate()

• UnsafeMutablePointer 调用allocate
  泛型指针,建议使用initialize初始化指针
  注意:该方式一定要调用deinitialize(count:)和deallocate ()销毁对象,否则产生内存泄漏. 反初始化deinitialize(count:)和allocate(capacity:)对应.

//创建
//有泛型Int , 只需要写容量即可. capacity 表示申请 3* 8 = 24个字节
var ptr = UnsafeMutablePointer<Int>.allocate(capacity: 3)
//存储
ptr.initialize(to: 11)    //初始化前8个字节
ptr.successor().initialize(to: 22) //下一个Int 初始化22
ptr.successor().successor().initialize(to: 33)

//取值方式一:
print(ptr.pointee) // 11
print((ptr + 1).pointee) //22
print((ptr + 2).pointee) //33
//取值方式二:
print(ptr[0]) //11
print(ptr[1]) //22
print(ptr[2]) //33
//取值方式三 等价于 方式一和二
print(ptr.pointee)
print(ptr.successor().pointee)
print(ptr.successor().successor().pointee)

//销毁
ptr.deinitialize(count: 3) 
//反初始化, deinitialize(count:)和allocate(capacity:) 对应
ptr.deallocate()

• 示例: 创建3个指针指向的Person对象

class Person {
    var age: Int
    var name: String
    init(age: Int, name: String) {
        self.age = age
        self.name = name
    }
    deinit { print(name, "deinit") }
}

var ptr = UnsafeMutablePointer<Person>.allocate(capacity: 3)
ptr.initialize(to: Person(age: 10, name: "Jack"))
(ptr + 1).initialize(to: Person(age: 11, name: "Rose"))
(ptr + 2).initialize(to: Person(age: 12, name: "Kate"))
// Jack deinit
// Rose deinit
// Kate deinit
ptr.deinitialize(count: 3)
ptr.deallocate()

指针之间的转换

• assumingMemoryBound(to:) 可以将RawPointer 非泛型指针转为 泛型指针

//创建非泛型指针
var ptr = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1)

ptr.assumingMemoryBound(to: Int.self).pointee = 11
//非泛型指针 + 8 , 就是偏移8个字节
(ptr + 8).assumingMemoryBound(to: Double.self).pointee = 22.0

print(unsafeBitCast(ptr, to: UnsafePointer<Int>.self).pointee) // 11
print(unsafeBitCast(ptr + 8, to: UnsafePointer<Double>.self).pointee) // 22.0

ptr.deallocate()

• unsafeBitCast原理: unsafeBitCast是忽略数据类型的强制转换，不会因为数据类型的变化而改变原来的内存数据.(简单说: 直接将二进制数据搬过去, 只是类型发生改变)
• 类似于C++中的reinterpret_cast

class Person {}
var person = Person()
var ptr = unsafeBitCast(person, to: UnsafeRawPointer.self)
print(ptr)