在golang中拥有nil切片和空切片有什么意义? [英] What is the point of having nil slice and empty slice in golang?

问题描述

区分零切片的意义是什么，即未初始化的片，以及一个空片，即.初始化但空片?

What is the point of differentiating between a nil slice, ie. uninitialized slice, and an empty slice, ie. initialized but empty slice?

我了解两者之间的区别，但是我想知道，在这两种情况之间存在细微差别的背后动机是什么?出于所有意图和目的，nil切片和空切片在使用它们时的行为相同.

I understand the difference, but I'm wondering what the motivation is behind having the subtle difference between the two cases? For all intents and purposes, a nil slice and an empty slice behave the same when using them.

似乎Go开发人员只有一种情况，例如只允许空片，这将简化心智模型并消除细微错误的来源.

It seems as though if Go developers just had one case, for example only allowed empty slices, it would have simplified the mental model and eliminated sources of subtle bugs.

创建这两个用例是否有原因?

Is there a reason why those two use cases were created?

推荐答案

nil切片值不需要分配.在您要在切片中构建某些内容的情况下，这可能会有所不同，但是通常没有要追加的数据，因此切片可能保持为nil，因此完全不需要分配.

A nil slice value needs no allocation. That might make a difference in cases where you want to build something in a slice, but often there will be no data to be appended, so the slice may remain nil, so no allocation will be required altogether.

空片可能需要分配，即使其容量为零.

An empty slice might require an allocation, even if its capacity is zero.

空片也意味着其长度为0，但其容量可能不为0.因此对于所有意图和目的，nil切片和空切片在使用它们时的行为都是相同的." .您可以分配一个长度为0且容量较大的切片，为进一步的追加进行优化以避免分配(和复制):

Also an empty slice means its length is 0, but its capacity may not be; so it's not true that "For all intents and purposes, a nil slice and an empty slice behave the same when using them.". You can allocate a slice with 0 length and a big capacity, optimizing for further appends to avoid allocations (and copying over):

s := make([]int, 0)
fmt.Println(s, len(s), cap(s))
s = append(s, 1)
fmt.Println(s, len(s), cap(s))

s = make([]int, 0, 10)
fmt.Println(s, len(s), cap(s))
s = append(s, 1)
fmt.Println(s, len(s), cap(s))

上述内容的输出(在游乐场上尝试):

Output of the above (try it on the Go Playground):

[] 0 0
[1] 1 2
[] 0 10
[1] 1 10

我们看到了什么?在第一个示例中，我们创建了一个长度为0且容量为0的空切片.如果我们向其添加元素，则其长度将变为1(显然)，其容量将增加为2.这是因为append()在引擎盖下分配了一个大小为2的新数组(考虑了未来的增长)，现有元素覆盖(在这种情况下为空)，并分配了新元素.

What do we see? In the first example we created an empty slice with 0 length and 0 capacity. If we append an element to it, its length will become 1 (obviously), and its capacity increased to 2. This is because under the hood append() allocated a new array with size of 2 (thinking of future growth), copied the existing elements over (which was none in this case), and assigned the new element.

在第二种情况下，我们从一个空切片开始，但容量为10.这意味着我们可以向其添加10个元素，而不会引起新的分配和复制现有元素.当切片很大且需要多次完成时，这可能是一个很大的优点.

In the second case we started with an empty slice but with a capacity of 10. This means we can append 10 elements to it without causing a new allocation and copying existing elements. This can be a big plus when slices are big and this needs to be done many times.

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