Slice type conversion in Go

php Editor Apple introduces slice type conversion in Go language. In the Go language, a slice is a dynamic array that is often used to store and operate a group of elements of the same type. Slice type conversion refers to converting one type of slice to another type of slice, which is very common in actual development. This article will introduce in detail the considerations and practical applications of slice type conversion to help readers better understand and use this feature.

Question content

I am very new to go, coming from a c background, and stumbled upon some strange issues. code show as below:

package main

import (
"fmt"
"unsafe"
)

func main() {
     arr := []string { "one", "two", "three" }
     address := unsafe.pointer(&arr)
     addptr := (*[]string)(unsafe.pointer(*(*uintptr)(address)))
     fmt.println((*addptr)[0])
}

This code fails with:

runtime error: growslice: len out of range

For example, if I change the cast to:

addptr := (*[0]string)(unsafe.pointer(*(*uintptr)(address)))

The above code works fine.

I understand that this is a cast to an array pointer, and that the array must have a constant size, But how do I convert it to a pointer to a slice?

To make things even more confusing, it is possible to get the slice address and assign it to a pointer like this:

func main() {
     arr := []string { "one", "two", "three" }
     var arrPtr *[]string = &arr
     fmt.Println((*arrPtr)[0])
}

Everything will work fine this time, although the pointer is of the same type that I cast the unsafe pointer to in the first example. Can anyone help understand what exactly is going on here?

Workaround

Some background: The slice header contains pointers to the backing array, length and capacity.

The code in the first part of the question converts the slice header into a pointer to the slice header. go vet The command warns that the code in question may abuse unsafe.pointer.

Fixed by removing the extra dereference operation so that the code converts from a pointer to the slice header to a pointer to the slice header.

arr := []string{"one", "two", "three"}
address := unsafe.pointer(&arr)
addptr := (*[]string)(unsafe.pointer((*uintptr)(address)))
fmt.println((*addptr)[0]) // prints one

No need to convert to *uintptr. Simplified to:

arr := []string{"one", "two", "three"}
address := unsafe.pointer(&arr)
addptr := (*[]string)(unsafe.pointer(address))
fmt.println((*addptr)[0]) // prints one

No need for unsafe pranks. Simplified to:

arr := []string{"one", "two", "three"}
addptr := &arr
fmt.println((*addptr)[0]) // prints one

Use the following code to convert the slice's backing array pointer to an array pointer. The code is fragile because it assumes the first word of the slice header is a pointer to the backing array.

arr := []string{"one", "two", "three"}
address := unsafe.pointer(&arr)
addptr := (*[1]string)(unsafe.pointer(*(*uintptr)(address)))
fmt.println((*addptr)[0]) // prints one

The uintptr conversion in the previous code snippet is not required. Simplified to:

arr := []string{"one", "two", "three"}
address := unsafe.Pointer(&arr)
addPtr := (*[]string)(address)
fmt.Println((*addPtr)[0]) // prints one

I hope this helps.

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