go - slice : Out of bounds error in Go

Question

package main

import "fmt"

func main() {
    a := make([]int, 5)
    printSlice("a", a)
    b := make([]int, 0, 5)
    printSlice("b", b)
    c := b[1:]
    printSlice("c", c)
}


func printSlice(s string, x []int) {
    fmt.Printf("%s len=%d cap=%d %v\n",
        s, len(x), cap(x), x)
}

上面给了我一个越界错误:

a len=5 cap=5 [0 0 0 0 0]
b len=0 cap=5 []
panic: runtime error: slice bounds out of range

goroutine 1 [running]:
main.main()
   /private/var/folders/q_/53gv6r4s0y5f50v9p26qhs3h00911v/T/compile117.go:10 +0x150

为什么创建 c slice 的 slice 表达式会导致错误？

Answer 1

简而言之:问题不在于可以等于或大于len() 的下限。 (在 slice 的情况下，上限由 cap() 决定)。问题在于上限:它必须大于或等于下限。由于您没有指定上限，因此默认为 len() (而不是 cap() !)这是 0 .和 1不小于或等于 0 .

Spec: Slice expressions:

For arrays or strings, the indices are in range if 0 <= low <= high <= len(a), otherwise they are out of range. For slices, the upper index bound is the slice capacity cap(a) rather than the length.

由于您正在 slice ，索引在范围内如果:

0 <= low <= high <= cap(a)

所以这一行:

c := b[1:]

无效，因为:

A missing low index defaults to zero; a missing high index defaults to the length of the sliced operand.

所以在你的情况下low = 1和 high = 0 (隐式)，不满足:

0 <= low <= high <= cap(a)

例如，以下表达式是有效的:

c := b[1:1]        // c len=0 cap=4 []
c := b[1:2]        // c len=1 cap=4 [0]
c := b[1:cap(b)]   // c len=4 cap=4 [0 0 0 0]