1 字符串
1.1 字符串的源码
//runtime 包中的定义
type stringStruct struct {
str unsafe.Pointer
len int
}
//reflact 包种的string头部
type StringHeader struct{
Data uintptr
Len int //表示的是字节数组的长度,不是字节的数量(遍历中文的时候会出问题)
}
说明:字符串本质上是一个结构体,str指针指向的是底层的Byte数组。
1.2 字符串的遍历
方法一:直接用len(str)作为限制条件,使用for进行访问。最后记得转换为string
方法二:直接使用for range的方式进行访问,最后转换为string或用fmt.Printf("%c", each)打印。
注意:使用range的时候,会自动解码成rune的形式,由utf8.go文件进行解码。
1.3 字符串的切分
先将其转为rune数组,再切片,最后转为string
s = string([]rune(s)[:3])
1.4 注意事项
字符串的底层是Byte数组,存在多个字符串的底层共用一个字节内存,所以字符串数不允许更改的。
2 切片
2.1 切片源码
type slice struct {
array unsafe.Pointer //指向底层数组的指针
len int //切片引用的长度
cap int //整个切片的大小(容量)
}
2.2 切片的追加
1.不需要扩容的时候:当向切片追加元素时,仅由编译器调整len的大小。
2.需要扩容的时候:编译时转为调用runtime.growslice()函数
2.3 切片的扩容机制
一般情况下,会重新开辟一个两倍长的数组替代原有的数组(因为数组空间必须时连续的),但是如果容量大于目前容量的两倍,则会直接扩容成期望容量的大小。
对于过大的切片,会有新的规则,若切片的长度小于1024,则容量会翻倍增长,大于1024则每次仅增加25%的大小。注意切片扩容的时候并发不安全,一定要加锁。
2.4 扩容源码阅读
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
}
}
}
var overflow bool
var lenmem, newlenmem, capmem uintptr
// Specialize for common values of et.size.
// For 1 we don't need any division/multiplication.
// For goarch.PtrSize, compiler will optimize division/multiplication into a shift by a constant.
// For powers of 2, use a variable shift.
switch {
case et.size == 1:
lenmem = uintptr(old.len)
newlenmem = uintptr(cap)
capmem = roundupsize(uintptr(newcap))
overflow = uintptr(newcap) > maxAlloc
newcap = int(capmem)
case et.size == goarch.PtrSize:
lenmem = uintptr(old.len) * goarch.PtrSize
newlenmem = uintptr(cap) * goarch.PtrSize
capmem = roundupsize(uintptr(newcap) * goarch.PtrSize)
overflow = uintptr(newcap) > maxAlloc/goarch.PtrSize
newcap = int(capmem / goarch.PtrSize)
case isPowerOfTwo(et.size):
var shift uintptr
if goarch.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.ptrdata == 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 lenmem > 0 && 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-et.size+et.ptrdata)
}
}
memmove(p, old.array, lenmem)
return slice{p, old.len, newcap}
}
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