使用不安全的指標從[]字串中獲取值

我正在嘗試學習指標如何作業。為什么以下示例不起作用？

package main

import (    
    "fmt"
    "unsafe"
)

type SliceOfStrings []string

// function that creates an slice of []string
// returns interface{} cause I am interested on learning how pointers work
func Create() interface{} {
    var mySlice1 SliceOfStrings = make([]string, 0)
    mySlice1 = append(mySlice1, "str1")
    mySlice1 = append(mySlice1, "str2")

    // return a slice with as ["str1","str2"]
    return mySlice1
}

func main() {

    x := Create()
    // 0xc000021940
    fmt.Printf("address of x is %p \n", &x)

    // get unsafe pointer to address of x

    // unsafe pointer. Prints 0xc000021940
    p1 := unsafe.Pointer(&x)
    fmt.Println(p1)

    // unsigned pointer. Prints 824633858368
    p2 := uintptr(p1)
    fmt.Println(p2)

    // prints same value as p1 0xc000021940
    p3 := unsafe.Pointer(p2)
    fmt.Println(p3)

    // Make p4 point to same address as 0xc000021940
    p4 := (*SliceOfStrings)(p3)
    //fmt.Println(p4)

    // why this does not print "str1" ??
    fmt.Println((*p4)[0])

    // I get error: runtime error: invalid memory address or nil pointer dereference
}

uj5u.com熱心網友回復：

Create()回傳一個 type 的值interface{}，所以 type of xis interface{}，所以 type of &xis*interface{}和not *SliceOfStrings。所以x指向一個interface{}值而不是一個SliceOfStrings值！

如果您從 的回傳值鍵入 assert ，它會起作用：SliceOfStringsCreate()

x := Create().(SliceOfStrings)

還要runtime.KeepAlive(x)在你的末尾添加main()，因為如果你不再參考x它，它可以隨時被垃圾收集。

通過此更改，它可以作業并輸出str1。在Go Playground上嘗試一下。

unsafe一般來說，盡量遠離包裹。您可以在沒有 package 的情況下學習和使用指標unsafe。只考慮unsafe作為最后的手段！

uj5u.com熱心網友回復：

我能夠理解為什么會發生這種情況：

package main

import (
    "fmt"
    "unsafe"
)

type SliceOfStrings []string

// when passing a slice to a method you are passing this data. Lets prove it
type SliceHeader struct {
    Data uintptr
    Len  int
    Cap  int
}



func main() {

    // On go everything is passed by coping values. When you pass a slice to a function you are passing this:
    // reference: https://stackoverflow.com/a/39993797/637142
    /*
        type SliceHeader struct {
            Data uintptr
            Len  int
            Cap  int
        }
    */

    // create a new slice
    var mySlice1 SliceOfStrings = make([]string, 0)

    // when appending we need to keep the new content that is why we reasig it
    mySlice1 = append(mySlice1, "str1")
    mySlice1 = append(mySlice1, "str2")
    // in other words this will make no sense:
    // _ = append(mySlice1, "str3") // doing this will lose the new header value

    // lets see the content of header mySlice
    var pointerToMySlice1 = unsafe.Pointer(&mySlice1)
    var header *SliceHeader = ((*SliceHeader)(pointerToMySlice1))
    fmt.Println(*header)
    // {824634220576 2 2}

    // lets copy that header to another slice
    var copy SliceOfStrings = mySlice1
    var pointerToCopy = unsafe.Pointer(&copy)
    header = ((*SliceHeader)(pointerToCopy))
    fmt.Println(*header)
    // prints the same thing
    // {824634220576 2 2}

    // now lets do the same thing but with an interface
    var copy2 interface{} = mySlice1
    var pointerToCopy2 = unsafe.Pointer(&copy2)
    header = ((*SliceHeader)(pointerToCopy2))
    fmt.Println(*header)
    // this prints!
    // {4845280 824634375976 0}
    // I dont understand why this happens. But this is the reason why the example does not work
    // I was trying to access an array from memory address 4845280 that is wrong

    // now lets do what izca told us
    var copy3 interface{} = mySlice1
    tmp := (copy3).(SliceOfStrings)
    var pointerToCopy3 = unsafe.Pointer(&tmp)
    header = ((*SliceHeader)(pointerToCopy3))
    fmt.Println(*header)
    // this prints!
    // {824634220576 2 2}
    // that is the correct value

    // lets try to get the array from that memory address (824634220576)
    pointerToActualArray := unsafe.Pointer(header.Data)
    // lets cast that to (*[2]string)
    var pointerFinal *[2]string = ((*[2]string)(pointerToActualArray))
    // now print the first value
    fmt.Println((*pointerFinal)[0]) 
    // prints str1
}

標籤：去 指针 不安全

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