swift指標及記憶體管理記憶體系結範例詳解

2022-11-25 14:01:15

swift API 繫結

swift提供了3種不同的API來繫結/重新系結指標

assumingMemoryBound(to:)
bindMemory(to: capacity:)
withMemoryRebound(to: capacity: body:)

繞過編譯器檢查 - assumingMemoryBound

就是假定記憶體系結

func testPointer(_ p: UnsafePointer<Int>) {
    print(p)
}
let tuple = (30, 40)
withUnsafePointer(to: tuple) { (tuplePtr: UnsafePointer<(Int, Int)>) in
    testPointer(UnsafeRawPointer(tuplePtr)
    .assumingMemoryBound(to: Int.self))
}

其實兩者本質沒什麼區別，都是指向記憶體的指標

UnsafePointer 指向1塊Int記憶體

UnsafePointer<Int, Int> 指向一個元組tuple記憶體, 也就是一塊連續的記憶體，包含連個連續的Int

兩者都是首地址

一種方式就是不強轉 UnsafePointer<Int, Int> 為 UnsafePointer

先把元組指標轉換成原始指標 UnsafeRawPointer(tuplePtr)
原始指標呼叫 assumingMemoryBound 繫結成Int 指標 UnsafeRawPointer(tuplePtr).assumingMemoryBound(to: Int.self)

func testPointer(_ p: UnsafePointer<Int>) {
    print(p[0])
    print(p[1])
}
let tuple = (30, 40)
withUnsafePointer(to: tuple) { (tuplePtr: UnsafePointer<(Int, Int)>) in
    testPointer(UnsafeRawPointer(tuplePtr).assumingMemoryBound(to: Int.self))
}

結果

30

40

assumingMemoryBound的意義在於：

有時候不想做指標型別轉換來增加程式碼的複雜度

就可以呼叫此api繞過編譯器檢查，但是並沒有發生實際的指標轉換

記憶體轉換 - bindMemory

實際發生了轉換，改變當前記憶體指標繫結的型別

func testPointer(_ p: UnsafePointer<Int>) {
    print(p[0])
    print(p[1])
}
let tuple = (30, 40)
withUnsafePointer(to: tuple) { (tuplePtr: UnsafePointer<(Int, Int)>) in
    testPointer(UnsafeRawPointer(tuplePtr)
    .bindMemory(to: Int.self, capacity: 1))
}

結果

30

40

bindMemory - 相比於assumingMemoryBound，就是改變記憶體系結型別

臨時改變記憶體系結 - withMemoryRebound

func testPointer(_ p: UnsafePointer<Int8>) {
    print(p)
}
let UInt8Ptr = UnsafePointer<UInt8>.init(bitPattern: 30)
UInt8Ptr?.withMemoryRebound(to: Int8.self, capacity: 1, 
	{ (Int8Ptr: UnsafePointer<Int8>) in
    testPointer(Int8Ptr)
})

結果