golang 数组和切片

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本文详细介绍了Go语言中数组和切片在内存分配上的区别，包括深浅拷贝，以及Go1.18及后续版本的扩容规则变化，如容量增长阈值和比例调整。

区别
1.数组长度固定，切片长度可变
2.数组是深拷贝，切片是浅拷贝，切片是引用类型

扩容规则不同版本不一样
https://www.jb51.net/article/280481.htm#_lab2_2_1

go1.18
1.如果期望容量大于当前容量的两倍就会使用期望容量；
2.如果当前切片的容量小于阈值（默认 256）就会将容量翻倍；
3.如果当前切片的容量大于等于阈值（默认 256），小切片的2倍生长，大切片生长1.25倍；

package main

import "fmt"

func main() {

	arr1 := [3]int{1, 2, 3}
	arr2 := arr1
	fmt.Printf("%p\n", &arr1[0])
	fmt.Printf("%p\n", &arr2[0])
	arr2[0] = 10
	fmt.Println(arr1)
	fmt.Println(arr2)

	sli1 := []int{1, 2}
	sli2 := sli1
	fmt.Printf("%p\n", &sli1[0])
	fmt.Printf("%p\n", &sli2[0])
	sli2[0] = 10
	fmt.Println(sli1)
	fmt.Println(sli2)

	sliceAppend(sli1)
	fmt.Println(sli1)
}

// 不会改变原有slice长度
func sliceAppend(s []int) {
	s = append(s, []int{2, 2, 2}...)
	fmt.Printf("sliceAppend %d %d\n", len(s), len(s))
}

package main

import "fmt"

func main() {
	sli1 := []int{1}
	fmt.Printf("len=%d cap=%d\n", len(sli1), cap(sli1))

	// 期望容量大于当前容量的两倍
	sli1 = append(sli1, []int{2, 3, 4}...)
	fmt.Printf("len=%d cap=%d\n", len(sli1), cap(sli1))

	// 如果当前切片的容量小于阈值（默认 256）就会将容量翻倍
	sli1 = append(sli1, 1)
	fmt.Printf("len=%d cap=%d\n", len(sli1), cap(sli1))

	// 如果当前切片的容量大于等于阈值（默认 256），就会每次增加 25% 的容量
	/*
		//从小切片的2倍生长过渡
		//大切片生长1.25倍
		newcap += (newcap + 3*threshold) / 4
	*/
	sli2 := make([]int, 256, 256)
	fmt.Printf("len=%d cap=%d\n", len(sli2), cap(sli2))
	sli2 = append(sli2, 1)
	fmt.Printf("len=%d cap=%d\n", len(sli2), cap(sli2))

	for i := 0; i < 10; i++ {
		sli2 = append(sli2, sli2...)
		fmt.Printf("len=%d cap=%d\n", len(sli2), cap(sli2))
	}

}

runtime/slice

type slice struct {
	array unsafe.Pointer
	len   int
	cap   int
}

func growslice(et *_type, old slice, cap int) slice {
	if raceenabled {
		callerpc := getcallerpc()
		racereadrangepc(old.array, uintptr(old.len*int(et.size)), callerpc, abi.FuncPCABIInternal(growslice))
	}
	if msanenabled {
		msanread(old.array, uintptr(old.len*int(et.size)))
	}
	if asanenabled {
		asanread(old.array, uintptr(old.len*int(et.size)))
	}

	if cap < old.cap {
		panic(errorString("growslice: cap out of range"))
	}

	if et.size == 0 {
		// append should not create a slice with nil pointer but non-zero len.
		// We assume that append doesn't need to preserve old.array in this case.
		return slice{unsafe.Pointer(&zerobase), old.len, cap}
	}

	newcap := old.cap
	doublecap := newcap + newcap
	if cap > doublecap {
		newcap = cap
	} else {
		const threshold = 256
		if old.cap < threshold {
			newcap = doublecap
		} else {
			// Check 0 < newcap to detect overflow
			// and prevent an infinite loop.
			for 0 < newcap && newcap < cap {
				// Transition from growing 2x for small slices
				// to growing 1.25x for large slices. This formula
				// gives a smooth-ish transition between the two.
				newcap += (newcap + 3*threshold) / 4
			}
			// Set newcap to the requested cap when
			// the newcap calculation overflowed.
			if newcap <= 0 {
				newcap = cap
			}
		}
	}