golang 数组与切片

通过下面几个问题来更好理解golang 的数组和切片

1. 类型

   数组是值类型，将一个数组赋值给另一个数组时，传递的是一份拷贝。
   切片是引用类型，切片包装的数组称为该切片的底层数组。
   我们来看一段代码

   //a是一个数组，注意数组是一个固定长度的，初始化时候必须要指定长度，不指定长度的话就是切片了
   a := [3]int{1, 2, 3}
   //b是数组，是a的一份拷贝
   b := a
   //c是切片，是引用类型，底层数组是a
   c := a[:]
   for i := 0; i < len(a); i++ {
     a[i] = a[i] + 1
   }
   //改变a的值后，b是a的拷贝，b不变，c是引用，c的值改变
   fmt.Println(a) //[2,3,4]
   fmt.Println(b) //[1 2 3]
   fmt.Println(c) //[2,3,4]

2. make
   make 只能用于slice, map 和 channel, 所以下面一段代码生成了一个slice，是引用类型

   s1 := make([]int, 0, 3)
   
   for i := 0; i < cap(s1); i++ {
       s1 = append(s1, i)
   }
   s2 := s1
   for i := 0; i < len(a); i++ {
       s1[i] = s1[i] + 1
   }
   
   fmt.Println(s1)  //[1 2 3]
   fmt.Println(s2)  //[1 2 3]

3. 当对slice append 超出底层数组的界限时

   //n1是n2的底层数组
   n1 := [3]int{1, 2, 3}
   n2 := n1[0:3]
   fmt.Println("address of items in n1: ")
   for i := 0; i < len(n1); i++ {
       fmt.Printf("%p\n", &n1[i])
   }
   //address of items in n1:
   //0xc20801e160
   //0xc20801e168
   //0xc20801e170
   fmt.Println("address of items in n2: ")
   for i := 0; i < len(n2); i++ {
       fmt.Printf("%p\n", &n2[i])
   }
   //address of items in n2:
   //0xc20801e160
   //0xc20801e168
   //0xc20801e170
   
   //对n2执行append操作后，n2超出了底层数组n1的j
   n2 = append(n2, 1)
   fmt.Println("address of items in n1: ")
   for i := 0; i < len(n1); i++ {
       fmt.Printf("%p\n", &n1[i])
   }
   //address of items in n1:
   //0xc20801e160
   //0xc20801e168
   //0xc20801e170
   
   fmt.Println("address of items in n2: ")
   for i := 0; i < len(n2); i++ {
       fmt.Printf("%p\n", &n2[i])
   }
   //address of items in n2:
   //0xc20803a2d0
   //0xc20803a2d8
   //0xc20803a2e0
   //0xc20803a2e8

4. 引用“失效”
   实现了删除slice最后一个item的函数

   func rmLast(a []int) {
       fmt.Printf("[rmlast] the address of a is %p", a)
       a = a[:len(a)-1]
       fmt.Printf("[rmlast] after remove, the address of a is %p", a)
   }

   调用此函数后，发现原来的slice并没有改变

   func main() {
       xyz := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
       fmt.Printf("[main] the address of xyz is %p\n", xyz)
       rmLast(xyz)
       fmt.Printf("[main] after remove, the address of xyz is %p\n", xyz)
       fmt.Printf("%v", xyz) //[1 2 3 4 5 6 7 8 9]
   }

   打印出来的结果如下：

   [main] the address of xyz is 0xc2080365f0
   [rmlast] the address of a is 0xc2080365f0
   [rmlast] after remove, the address of a is 0xc2080365f0
   [main] after remove, the address of xyz is 0xc2080365f0
   [1 2 3 4 5 6 7 8 9]

   这里直接打印了slice的指针值，因为slice是引用类型，所以指针值都是相同的，我们换成打印slice的地址看下

   func rmLast(a []int) {
       fmt.Printf("[rmlast] the address of a is %p", &a)
       a = a[:len(a)-1]
       fmt.Printf("[rmlast] after remove, the address of a is %p", &a)
   }
   func main() {
       xyz := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
       fmt.Printf("[main] the address of xyz is %p\n", &xyz)
       rmLast(xyz)
       fmt.Printf("[main] after remove, the address of xyz is %p\n", &xyz)
       fmt.Printf("%v", xyz) //[1 2 3 4 5 6 7 8 9]
   }

   结果：

   [main] the address of xyz is 0xc20801e1e0
   [rmlast] the address of a is 0xc20801e200
   [rmlast] after remove, the address of a is 0xc20801e200
   [main] after remove, the address of xyz is 0xc20801e1e0
   [1 2 3 4 5 6 7 8 9]

   这次可以看到slice作为函数参数传入函数时，实际上也是拷贝了一份slice，因为slice本身是个指针，所以从现象来看，slice是引用类型