go切片复制 copy() -更像是一种指定开始索引的替换

一、语法

go语言的内置函数 copy() 可以将一个数组切片复制到另一个数组切片中，如果加入的两个数组切片不一样大，就会按照其中较小的那个数组切片的元素个数进行复制。

func copy(dst, src []Type) int
使用copy有几个特征

1. 如果之前是两个不同的地址，copy后也是两个不同的地址，改变其中一个不影响另一个。
2. copy到目标slice，到底复制多少，取决于它的大小以及起始位置。
3. 更像是一种指定开始索引的替换

二、使用示范

1、同样的大小

func TestCopy1(t *testing.T) {
    from := []int{1, 2, 3}
    var to []int = make([]int, len(from), cap(from))
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    t.Logf("to slice: %[1]v, address: %[1]p\n", to)

    copy(to, from)
    t.Log("copy(to, from)后：")
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    t.Logf("to slice: %[1]v, address: %[1]p\n", to)

    to[0] = 11
    t.Log("to[0] = 11后：")
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    t.Logf("to slice: %[1]v, address: %[1]p\n", to)
}

输出：

from slice: [1 2 3], address: 0xc0000cc060
to slice: [0 0 0], address: 0xc0000cc080
copy(to[2:], from)后：
from slice: [1 2 3], address: 0xc0000cc060
to slice: [1 2 3], address: 0xc0000cc080
to[0] = 11后：
from slice: [1 2 3], address: 0xc0000cc060
to slice: [11 2 3], address: 0xc0000cc080

2、目标的slice为长度len为0的时候，复制不成功，还是为空[]

func TestCopy2(t *testing.T) {
    from := []int{1, 2, 3}
    var to []int = make([]int, 0, cap(from))
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    t.Logf("to slice: %[1]v, address: %[1]p\n", to)

    copy(to, from)
    t.Log("copy(to, from)后：")
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    t.Logf("to slice: %[1]v, address: %[1]p\n", to)
}

输出：

from slice: [1 2 3], address: 0xc000144060
to slice: [], address: 0xc000144080
copy(to, from)后：
from slice: [1 2 3], address: 0xc000144060
to slice: [], address: 0xc000144080

3、目标的slice为长度len为1的时候，只能复制第一个

func TestCopy3(t *testing.T) {
    from := []int{1, 2, 3}
    var to []int = make([]int, 1, cap(from))
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    t.Logf("to slice: %[1]v, address: %[1]p\n", to)

    copy(to, from)
    t.Log("copy(to, from)后：")
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    t.Logf("to slice: %[1]v, address: %[1]p\n", to)
}

输出：

from slice: [1 2 3], address: 0xc0000cc060
to slice: [0], address: 0xc0000cc080
copy(to, from)后：
from slice: [1 2 3], address: 0xc0000cc060
to slice: [1], address: 0xc0000cc080

4、目标的slice为长度len远大于的时候，替换开始的一部分

func TestCopy4(t *testing.T) {
    from := []int{1, 2, 3}
    var to []int = make([]int, 10, 10)
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    t.Logf("to slice: %[1]v, address: %[1]p\n", to)

    copy(to, from)
    t.Log("copy(to, from)后：")
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    t.Logf("to slice: %[1]v, address: %[1]p\n", to)
}

输出：

from slice: [1 2 3], address: 0xc000016160
to slice: [0 0 0 0 0 0 0 0 0 0], address: 0xc0000200f0
copy(to, from)后：
from slice: [1 2 3], address: 0xc000016160
to slice: [1 2 3 0 0 0 0 0 0 0], address: 0xc0000200f0

5、指定开始的替换索引 copy(to[3:], from)

func TestCopy5(t *testing.T) {
    from := []int{1, 2, 3}
    var to []int = make([]int, 10, 10)
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    t.Logf("to slice: %[1]v, address: %[1]p\n", to)

    copy(to[3:], from)
    t.Log("copy(to[3:], from)后：")
    t.Logf("from slice: %[1]v, address: %[1]p\n", from)
    

输出：

from slice: [1 2 3], address: 0xc000016160
to slice: [0 0 0 0 0 0 0 0 0 0], address: 0xc0000200f0
copy(to[3:], from)后：
from slice: [1 2 3], address: 0xc000016160
to slice: [0 0 0 1 2 3 0 0 0 0], address: 0xc0000200f0

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