对golang slice cap感到困惑

Question

我有一个关于切片帽的问题，代码：

var slice []int
list := []int{1,2,3,4,5}
for _,item := range list {
    slice = append(slice, item)
}
fmt.Println(len(slice),cap(slice))

if item == 1: len(slice)=1,cap(slice)=1

if item == 2: len(slice)=2,cap(slice)= 1*2

if item ==3: len(slice) = 3,cap(slice) = 2*2

if item == 4:len(slice) = 4,cap(slice) = 4

if item == 5:len(slice) = 5,cap(slice) = 4*2

所以输出：

len(切片) = 5,cap(切片) = 8

没问题，但是当我更改代码时：

var slice []int
slice = append(slice,1,2,3,4,5)
fmt.Println(len(slice),cap(slice))

输出：

len(切片) = 5,cap(切片) = 6

为什么上限（切片）= 6？

Answer 1

在src/runtime/slice.go中追加func growslice(et *_type, old slice, cap int)时可以看到容量计算的算法-第162行

    newcap := old.cap
    doublecap := newcap + newcap
    if cap > doublecap {
        newcap = cap
    } else {
        if old.cap < 1024 {
            newcap = doublecap
        } else {
            // Check 0 < newcap to detect overflow
            // and prevent an infinite loop.
            for 0 < newcap && newcap < cap {
                newcap += newcap / 4
            }
            // Set newcap to the requested cap when
            // the newcap calculation overflowed.
            if newcap <= 0 {
                newcap = cap
            }
        }
    }

First multiply the old slice capacity by 2. If the capacity after multiplying by 2 is still less than the new slice capacity, then take the new slice capacity (append multiple elems)

If the new slice is less than twice the old slice capacity, multiply the old slice capacity by 2

If the old slice capacity is greater than or equal to 1024, the new slice capacity is multiplied by the old slice capacity by 1.25

参考-

对golang slice cap感到困惑

Confused about golang slice cap

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