Swift5.1 - 指针Pointer

指针分类

使用swift提供指针类型：

• UnsafePointer
• UnsafeMutablePointer
• UnsafeRawPointer
• UnsafeMutableRawPointer

UnsafePointer / UnsafeMutablePointer用法：

使用 pointee 属性可以进行读写操作;

var age = 10
func change1(_ ptr: UnsafeMutablePointer<Int>) {
    ptr.pointee += 20
}

func change2(_ ptr: UnsafePointer<Int>) {
    print(ptr.pointee)
}

change1(&age) 
change2(&age) // 30
print(age)    // 30

UnsafeRawPointer / UnsafeMutableRawPointer

var age = 10
func change3(_ ptr: UnsafeMutableRawPointer) {
    ptr.storeBytes(of: 20, as: Int.self)
}

func change4(_ ptr: UnsafeRawPointer) {
    print(ptr.load(as: Int.self))
}

change3(&age) 
change4(&age) // 20
print(age)    // 20

获取堆空间地址

class Person {}
var person = Person()

//获取person指针的地址值
var ptr = withUnsafePointer(to: &person) { UnsafeRawPointer($0) }  

// 获取person指针的指向的地址，就是为Person的堆空间地址
var headPtr = UnsafeRawPointer(bitPattern: ptr.load(as: UInt.self))

print(ptr)
print(headPtr)
print(person)

0x00000001000031f8
0x00000001006b6340

• withUnsafePointer 的闭包返回类型就是person变量指针的地址值（栈空间）；
• headPtr 是获取 ptr指向的地址值，就是Person实例对象的地址（堆空间）；

创建 UnsafeRawPointer / UnsafeMutableRawPointer 创建指针

1. 方式一：通过地址值创建

var ptr = UnsafeRawPointer(bitPattern: 0x0000000100003008)

2. 方式二：

var ptr = malloc(16) // UnsafeMutableRawPointer
ptr?.storeBytes(of: 10, as: Int.self) // 8字节
ptr?.storeBytes(of: 20, toByteOffset: 8, as: Int.self) // 存放在后8字节中

print( (ptr?.load(as: Int.self))! )   // 10
print( (ptr?.load(fromByteOffset: 8, as: Int.self))! ) // 20
free(ptr)

3. 方式三：

var ptr = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1)
ptr.storeBytes(of: 10, as: Int.self)
//ptr.advanced(by: 8).storeBytes(of: 20, as: Int.self)
(ptr + 8).storeBytes(of: 20, as: Int.self)

print( ptr.load(as: Int.self) )   // 10

print( ptr.load(fromByteOffset: 8, as: Int.self) ) // 20
print( ptr.advanced(by: 8).load(as: Int.self) )  // 20
print( (ptr + 8).load(as: Int.self) ) //20

ptr.deallocate()

fromByteOffset 与 advanced都能让指针进行偏移；创建指针后，记得释放；

UnsafePointer / UnsafeMutablePointer 创建指针

var ptr = UnsafeMutablePointer<Int>.allocate(capacity: 3)
ptr.initialize(to: 10)
ptr.successor().initialize(to: 20)
ptr.successor().successor().initialize(to: 30)

print(ptr.pointee)  // 10
print(ptr.successor().pointee) // 20
print(ptr.successor().successor().pointee) // 30

ptr.deinitialize(count: 3)
ptr.deallocate()

或者：

var ptr = UnsafeMutablePointer<Int>.allocate(capacity: 3)
ptr.initialize(to: 10)
(ptr + 1).initialize(to: 20)
(ptr + 2).initialize(to: 30)

print(ptr.pointee)  // 10
print((ptr + 1).pointee) // 20
print((ptr + 2).pointee) // 30

ptr.deinitialize(count: 3)
ptr.deallocate()

ptr表示创建3个整形的指针；initialize为初始化；successor表示每次偏移8字节（整形）；

指针之间的转换

var ptr = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1)

// assumingMemoryBound 返回 UnsafeMutablePointer<T>类型
ptr.assumingMemoryBound(to: Int.self).pointee = 10
(ptr + 8).assumingMemoryBound(to: Int.self).pointee = 20

print( unsafeBitCast(ptr, to: UnsafePointer<Int>.self).pointee ) // 10
print( unsafeBitCast(ptr + 8, to: UnsafePointer<Int>.self).pointee ) // 20
ptr.deallocate()

• unsafeBitCast 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结构；

总结

• 以上都是使用整形存储在指针中，实际上存储对象，结构体等；
• UnsafeMutableRawPointer指针+1表示一个字节；UnsafeMutablePointer+1表示泛型对象占用内存字节数+1，即下一个对象；

源代码demo