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package main
import ("fmt")
func main() {
var a []int //定义一个切片fmt.Printf("len: %d cap: %d pointer: %p\n", len(a), cap(a), a)//此时切片长度和容量都是0，运行结果为len: 0 cap: 0 pointer: 0x0a = append(a, 1) // 追加1个元素fmt.Printf("len: %d cap: %d pointer: %p\n", len(a), cap(a), a)//注意此时a的地址已经发生变化为新的切片了，新切片长度和容量都为1运行结果为：len: 1 cap: 1 pointer: 0xc000072098a = append(a, 2, 3, 4) // 追加多个元素fmt.Printf("len: %d cap: %d pointer: %p\n", len(a), cap(a), a)//注意此时a的地址再次发生变化实际上又生成为新的切片了，新切片长度和容量都为4运行结果为：len: 4 cap: 4 pointer: 0xc000070160a = append(a, 5) // 再追加一个元素fmt.Printf("len: %d cap: %d pointer: %p\n", len(a), cap(a), a)//注意切片扩容策略是倍增方式容量由4变成8，而长度是5运行结果为：len: 4 cap: 4 pointer: 0xc000070160
}

package main

import ("fmt")
func main() {var a = []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10} //使用原始定义法来声明并初始化一个切片fmt.Println(a) //运行结果为[1 2 3 4 5 6 7 8 9 10]a = a[1:] // 删除第1个元素fmt.Println(a) //删头第1个元素后，运行结果为[2 3 4 5 6 7 8 9 10]a = a[2:] // 删除前2个元素fmt.Println(a) //删头前2个元素后，运行结果为[4 5 6 7 8 9 10]
}

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

func makeslice(et *_type, len, cap int) slice {
 mem, overflow := math.MulUintptr(et.size, uintptr(cap))//此函数计算et.size也就是每个元素所占空间的大小，并与容量cap相乘，其中mem既为所需要最大内存，overflow代表是否会造成溢出if overflow || mem > maxAlloc || len < 0 || len > cap {//判断是否有溢出，长度为负数或者长度比容量大的情况，如存在 直接panic// NOTE: Produce a 'len out of range' error instead of a// 'cap out of range' error when someone does make([]T, bignumber).// 'cap out of range' is true too, but since the cap is only being// supplied implicitly, saying len is clearer.// See golang.org/issue/4085.mem, overflow := math.MulUintptr(et.size, uintptr(len))if overflow || mem > maxAlloc || len < 0 {panicmakeslicelen()}panicmakeslicecap()}return mallocgc(mem, et, true)// 如果错误检查成功，则分配内存，注意slice对象会被GC所自动清除。
}

func growslice(et *_type, old slice, cap int) slice {

// 单纯地扩容，不写数据 if et.size == 0 { if cap < old.cap { panic(errorString("growslice: cap out of range")) } // 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} }// 扩容规则 1.新的容量大于旧的2倍，直接扩容至新的容量// 2.新的容量不大于旧的2倍，当旧的长度小于1024时，扩容至旧的2倍，否则扩容至旧的1.25倍 newcap := old.cap doublecap := newcap + newcap if cap > doublecap { newcap = cap } else { if old.len < 1024 { newcap = doublecap } else { for newcap < cap { newcap += newcap / 4 } } }
// 跟据切片类型和容量计算要分配内存的大小
var overflow boolvar lenmem, newlenmem, capmem uintptr switch {case et.size == 1:lenmem = uintptr(old.len)newlenmem = uintptr(cap)capmem = roundupsize(uintptr(newcap))overflow = uintptr(newcap) > maxAllocnewcap = int(capmem)case et.size == sys.PtrSize:lenmem = uintptr(old.len) * sys.PtrSizenewlenmem = uintptr(cap) * sys.PtrSizecapmem = roundupsize(uintptr(newcap) * sys.PtrSize)overflow = uintptr(newcap) > maxAlloc/sys.PtrSizenewcap = int(capmem / sys.PtrSize)case isPowerOfTwo(et.size):var shift uintptrif sys.PtrSize == 8 {// Mask shift for better code generation.shift = uintptr(sys.Ctz64(uint64(et.size))) & 63} else {shift = uintptr(sys.Ctz32(uint32(et.size))) & 31}lenmem = uintptr(old.len) << shiftnewlenmem = uintptr(cap) << shiftcapmem = roundupsize(uintptr(newcap) << shift)overflow = uintptr(newcap) > (maxAlloc >> shift)newcap = int(capmem >> shift)default:lenmem = uintptr(old.len) * et.sizenewlenmem = uintptr(cap) * et.sizecapmem, overflow = math.MulUintptr(et.size, uintptr(newcap))capmem = roundupsize(capmem)newcap = int(capmem / et.size)}// 异常情况，旧的容量比新的容量还大或者新的容量超过限制了 if cap < old.cap || uintptr(newcap) > maxSliceCap(et.size) { panic(errorString("growslice: cap out of range")) }
 var p unsafe.Pointer if et.kind&kindNoPointers != 0 {
// 为新的切片开辟容量为capmem的地址空间 p = mallocgc(capmem, nil, false)// 将旧切片的数据搬到新切片开辟的地址中 memmove(p, old.array, lenmem) // The append() that calls growslice is going to overwrite from old.len to cap (which will be the new length). // Only clear the part that will not be overwritten.// 清理下新切片中剩余地址，不能存放堆栈指针
// memclrNoHeapPointers clears n bytes starting at ptr.//// Usually you should use typedmemclr. memclrNoHeapPointers should be// used only when the caller knows that *ptr contains no heap pointers// because either://// 1. *ptr is initialized memory and its type is pointer-free.//// 2. *ptr is uninitialized memory (e.g., memory that's being reused// for a new allocation) and hence contains only "junk". memclrNoHeapPointers(add(p, newlenmem), capmem-newlenmem) } else { // Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory. p = mallocgc(capmem, et, true) if !writeBarrier.enabled { memmove(p, old.array, lenmem) } else { for i := uintptr(0); i < lenmem; i += et.size { typedmemmove(et, add(p, i), add(old.array, i)) } } }
 return slice{p, old.len, newcap}}