英文:
Go linked list pointer assignment from non-pointer struct element
问题
你好！以下是你提供的代码的翻译：
我在Go语言中遇到了一个“无法分配给指针”的错误，很困惑。
这是可以工作的代码：http://play.golang.org/p/P9FjK8A-32，与Go的原始container/list代码相同。
type List struct {
root Element
len int
}
type Element struct {
next, prev *Element
list *List
Value interface{}
}
原始代码将root定义为值，并在需要指针类型时引用它，但为什么不一开始就将root定义为指针呢？
type List struct {
root *Element
len int
}
type Element struct {
next, prev *Element
list *List
Value interface{}
}
这样会导致错误：http://play.golang.org/p/1gCAR_rcx1 -> invalid memory address or nil pointer dereference。
为什么会出现这个错误？
为什么Go在定义next和prev时使用指针，而在定义root时不使用指针？
谢谢！
英文:
https://github.com/golang/go/blob/master/src/container/list/list.go#L49
I am having hard time why I am getting cannot assign to pointer error in Go.
Here's the code that works: http://play.golang.org/p/P9FjK8A-32 which is same as Go's original container/list code
type List struct {
root Element
len int
}
type Element struct {
next, prev *Element
list *List
Value interface{}
}
The original code has root as a value and reference it everytime it needs to be in pointer type but why not at first place define root as a pointer?
type List struct {
root *Element
len int
}
type Element struct {
next, prev *Element
list *List
Value interface{}
}
This give me an error: http://play.golang.org/p/1gCAR_rcx1 -> invalid memory address or nil pointer dereference
Why am I getting this error?
Why does Go define root as a non-pointer value when it defines next, and prev as pointers?
Thanks
答案1
得分: 1
指针默认为nil，需要进行初始化。
原始代码：
// Init initializes or clears list l.
func (l *List) Init() *List {
l.root.next = l.root
l.root.prev = l.root
l.len = 0
return l
}
应改为：
// Init initializes or clears list l.
func (l *List) Init() *List {
l.root = new(Element) // 为了避免解引用空指针，需要进行初始化
l.root.next = l.root
l.root.prev = l.root
l.len = 0
return l
}
示例代码可在 http://play.golang.org/p/EYSscTMYnn 查看。
在标准库的情况下，root不需要是指针，但对于prev和next来说是必需的，否则结构体定义将会是递归的，这是不允许的，因为理论上会导致无限大小的结构体...
英文:
A pointer is nil by default and needs to be initialized.
This:
// Init initializes or clears list l.
func (l *List) Init() *List {
l.root.next = l.root
l.root.prev = l.root
l.len = 0
return l
}
should become this:
// Init initializes or clears list l.
func (l *List) Init() *List {
l.root = new(Element) // necessary to avoid dereferencing a nil pointer
l.root.next = l.root
l.root.prev = l.root
l.len = 0
return l
}
Demo at http://play.golang.org/p/EYSscTMYnn
In the case of the standard library, it is not necessary to have root be a pointer, however, for prev and next it is necessary, otherwise the struct definition would be recursive, which is not allowed, because it would in theory cause a struct of infinite size...
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