Go capacity of slice when append

最新推荐文章于 2025-08-05 10:09:48 发布

原创最新推荐文章于 2025-08-05 10:09:48 发布 · 209 阅读

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本文通过一个简单的 Go 语言示例展示了切片的使用及 append 函数的工作原理。探讨了切片在不同情况下的长度与容量变化，并深入分析了容量计算公式：capacity of append(l1,l2) = max(2*cap(l1), len(l1)+len(l2))。

In go-tour the offcial guide, the append make the capacity

package main

import "fmt"

func main() {
	a := make([]int,0)
	printSlice("a", a)

	// append works on nil slices.
	a = append(a, 0)
	printSlice("a", a)

	// the slice grows as needed.
	a = append(a, 1)
	printSlice("a", a)

	// we can add more than one element at a time.
	a = append(a, 2, 3, 4)
	printSlice("a", a)
}

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

finally came out the capacity can be 6 and 8 some times, so check what happened behind the append.

below implement the append.

https://github.com/golang/go/blob/5476967b1a3d29fef4061999c00cadbec19ac0e3/src/cmd/compile/internal/gc/walk.go#L2901

below implements the capcity

https://github.com/golang/go/blob/5476967b1a3d29fef4061999c00cadbec19ac0e3/src/runtime/slice.go#L82

Then drew the conclusion, the capcity can be the

capacity of append(l1,l2) will be max(2*cap(l1), len(l1)+len(l2))