golang nil slice & empty slice 笔记

示例代码

package main

import (
    "fmt"
)

func main() {
    a := []string{"A", "B", "C", "D", "E"}
    a = nil
    fmt.Println(a, len(a), cap(a))
}

result: [] 0 0 说明: a 是 一个nil 的slice

package main

import (
    "fmt"
)

func main() {
    a := []string{"A", "B", "C", "D", "E"}
    a = a[:0] //空的slice
    fmt.Println(a, len(a), cap(a))
}

result [] 0 5 说明 a是长度是0的emtpy slice

nil slice vs empty slice

• nil slice 的长度len和容量cap都是0
• empty slice的长度是0, 容量是由指向底层数组决定
• empty slice != nil
• nil slice的pointer 是nil, empty slice的pointer是底层数组的地址

slice的底层表示形式

[pointer] [length] [capacity]

nil slice:   [nil][0][0]
empty slice: [addr][0][0] // pointer是底层数组的地址

创建nil slice和empty slice

package main

import (
    "fmt"
)

func main() {
    var nilSlice []string
    emptySlice0 := make([]int, 0)
    var emptySlice1 = []string{}

    fmt.Printf("\nNil:%v Len:%d Capacity:%d", nilSlice == nil, len(nilSlice), cap(nilSlice))
    fmt.Printf("\nnil:%v Len:%d Capacity:%d", emptySlice0 == nil, len(emptySlice0), cap(emptySlice0))
    fmt.Printf("\nnil:%v Len:%d Capacity:%d", emptySlice1 == nil, len(emptySlice1), cap(emptySlice1))
}

make slice 规则

package main

import "fmt"

func main() {
  s1 := make([]int, 5)
  fmt.Printf("The length of s1: %d\n", len(s1))
  fmt.Printf("The capacity of s1: %d\n", cap(s1))
  fmt.Printf("The value of s1: %d\n", s1)
  s2 := make([]int, 5, 8)
  fmt.Printf("The length of s2: %d\n", len(s2))
  fmt.Printf("The capacity of s2: %d\n", cap(s2))
  fmt.Printf("The value of s2: %d\n", s2)
}

make 初始化slice

• 第一个参数表示长度
• 第二个参数表示容量
• make函数初始化切片时，如果不指明其容量，那么它就会和长度一致

links

• Arrays, slices (and strings): The mechanics of 'append' - The Go Blog
• go - The zero value of a slice is not nil - Stack Overflow
• A Tour of Go
• https://time.geekbang.org/col...

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