切片基礎概念:
切片是圍繞動態數組的概念構建的,可以按需自動增長和縮小。(注意:切片傳遞的是指針的拷貝值,所以可以在函數裏面修改指針指向的值,對外有影響)
切片的自動增長是通過append()函數來實現的
切片的底層內存也是在連續塊中分配的,所以切片還能獲得索引,迭代以及爲垃圾回收優化的好處。
源碼分析:
結構體定義
type slice struct {
array unsafe.Pointer
len int
cap int
}
slice的結構體包含三個參數,指針,長度,容量
所以傳遞一個切片切片需要24字節的內存:指針字段需要8字節,長度和容量字段分別需要8字節。
切片作爲函數傳遞需要注意的地方:
1.切片作爲函數的參數在函數內改變值
切片作爲參數可以節省空間,但是需要注意的是切片傳遞過去的值爲指針,所以在函數中改變切片指向的值,在函數外也會有影響,這是由切片結構體內存儲的指針特性決定的
func changevalue(arr []int){
arr[0] = 2
fmt.Println(&arr[0])
}
func main() {
arr:=make([]int,0,2)
arr = append(arr,1)
fmt.Println(arr)
changevalue(arr)
fmt.Println(arr)
fmt.Println(&arr[0])
}
輸出
[1]
0xc0000601b0
[2]
0xc0000601b0
2.切片在函數中的append操作
append未在源碼內找到實現方式,但是append的作用如下:
append函數將元素追加到slice的結尾,若切片還有生效的容量,則值直接追加到連續內存的後面。
若追加的值得數目大於切片的原容量,則申請一個新的底層數組,append返回更新的slice。
要注意,切片作爲參數在函數中傳遞本質是是值傳遞,特殊點是它的值中有一個指針
不改變值得方式,append追加不改變函數外面的值,PS:哪怕追加至更改內存
func appendarrNoChange(arr []int){
arr = append(arr,2)
fmt.Println(&arr[0])
}
func main() {
arr:=make([]int,0,2)
arr = append(arr,1)
fmt.Println(arr)
appendarrNoChange(arr)
fmt.Println(arr)
fmt.Println(&arr[0])
}
[1]
0xc0000601b0
[1]
0xc0000601b0
func appendarrNoChange(arr []int){
arr = append(arr,2)
arr = append(arr,3)
fmt.Println(&arr[0])
}
func main() {
arr:=make([]int,0,2)
arr = append(arr,1)
fmt.Println(arr)
appendarrNoChange(arr)
fmt.Println(arr)
fmt.Println(&arr[0])
}
[1]
0xc00005e2a0
[1]
0xc0000601b0
若需要append在函數生效:
func appendarrChange(arr []int)[]int{
arr = append(arr,2)
fmt.Println(&arr[0])
arr = append(arr,3)
fmt.Println(&arr[0])
return arr
}
func main() {
arr:=make([]int,0,2)
arr = append(arr,1)
fmt.Println(arr)
arr = appendarrChange(arr)
fmt.Println(arr)
fmt.Println(&arr[0])
}
可以用append()完畢後獲取到的新切片替代舊切片在函數外賦值
[1]
0xc00000a210
0xc00000e500
[1 2 3]
0xc00000e500
切片的初始化:
func makeslice(et *_type, len, cap int) unsafe.Pointer {
//返回類型佔的字節數目*容量值=要開闢的內存字節數
mem, overflow := math.MulUintptr(et.size, uintptr(cap))
//比對乘法是否溢出,要申請的內存是否少於能提供的最大內存
if overflow || mem > maxAlloc || len < 0 || len > cap {
// 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()
}
//申請內存
return mallocgc(mem, et, true)
}
關於malloc分配的策略(因爲本篇以slice爲主,關於內存分配的部分放到後面詳解,在此大致介紹):
若對象很小(<32kb),則從per-P緩存的空閒列表中分配空間(爲單個goroutine分配的線程空間,不存在併發,所以使用的時候不用加鎖,小對象分配在這上面運行效率會很高)
若對象大於32kb,則直接從堆中獲取內存。
append增加內存的規律:
func growslice(et *_type, old slice, cap int) slice {
if raceenabled {
callerpc := getcallerpc()
racereadrangepc(old.array, uintptr(old.len*int(et.size)), callerpc, funcPC(growslice))
}
if msanenabled {
msanread(old.array, uintptr(old.len*int(et.size)))
}
//禁止縮小容量
if cap < old.cap {
panic(errorString("growslice: cap out of range"))
}
//類型爲空指針則默認不保存原切片所有內容,但是保留長度,並以新的cap獲取新內存(不知道什麼時候會出現若有知曉的還望指教)
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
//若新申請的cap大於舊的二倍,則取新的,否則取舊的
if cap > doublecap {
newcap = cap
} else {
//若舊的長度小於1024,則乘以兩倍,否則增加1.25倍
if old.len < 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
}
}
}
var overflow bool
var lenmem, newlenmem, capmem uintptr
// 根據不同單位值採取不同的計算方式
switch {
case et.size == 1:
//若是1,則不需要做乘法
lenmem = uintptr(old.len)
newlenmem = uintptr(cap)
capmem = roundupsize(uintptr(newcap))
overflow = uintptr(newcap) > maxAlloc
newcap = int(capmem)
case et.size == sys.PtrSize:
lenmem = uintptr(old.len) * sys.PtrSize
newlenmem = uintptr(cap) * sys.PtrSize
capmem = roundupsize(uintptr(newcap) * sys.PtrSize)
overflow = uintptr(newcap) > maxAlloc/sys.PtrSize
newcap = int(capmem / sys.PtrSize)
case isPowerOfTwo(et.size):
var shift uintptr
if 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) << shift
newlenmem = uintptr(cap) << shift
capmem = roundupsize(uintptr(newcap) << shift)
overflow = uintptr(newcap) > (maxAlloc >> shift)
newcap = int(capmem >> shift)
default:
lenmem = uintptr(old.len) * et.size
newlenmem = uintptr(cap) * et.size
capmem, overflow = math.MulUintptr(et.size, uintptr(newcap))
capmem = roundupsize(capmem)
newcap = int(capmem / et.size)
}
// The check of overflow in addition to capmem > maxAlloc is needed
// to prevent an overflow which can be used to trigger a segfault
// on 32bit architectures with this example program:
//
// type T [1<<27 + 1]int64
//
// var d T
// var s []T
//
// func main() {
// s = append(s, d, d, d, d)
// print(len(s), "\n")
// }
if overflow || capmem > maxAlloc {
panic(errorString("growslice: cap out of range"))
}
var p unsafe.Pointer
if et.kind&kindNoPointers != 0 {
p = mallocgc(capmem, nil, false)
// 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.
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 {
// Only shade the pointers in old.array since we know the destination slice p
// only contains nil pointers because it has been cleared during alloc.
bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(old.array), lenmem)
}
}
memmove(p, old.array, lenmem)
return slice{p, old.len, newcap}
}
函數append會智能地處理底層數組的容量增長。在切片的容量小於1024個元素時,總是會成倍地增加容量,一旦元素個數超過1000,容量的增長因子會設爲1.25,每次增長25%,實驗如下。
func main() {
arr:=make([]int,0,2)
s:=cap(arr)
for i:=0;i<2000;i++{
arr = append(arr,i)
t:=cap(arr)
if t>s{
s = t
fmt.Println(t)
}
}
}
內容:
4 8 16 32 64 128 256 512 1024 1280 1696 2304