數組，字符串，切片
字符串
Golang 中的 string 底層數據類型定義在 runtime/string.go 中：
type stringStruct struct {
str unsafe.Pointer
len int
}
在反射包中與之對應的數據結構是
// StringHeader is the runtime representation of a string.
// It cannot be used safely or portably and its representation may
// change in a later release.
// Moreover, the Data field is not sufficient to guarantee the data
// it references will not be garbage collected, so programs must keep
// a separate, correctly typed pointer to the underlying data.
//
// Deprecated: Use unsafe.String or unsafe.StringData instead.
type StringHeader struct {
Data uintptr
Len int
}
不管是 string 本身的底層結構還是其反射的底層結構都是一個 int 類型加一個指針類型。指針指向字符串的底層 Byte 數組，而 int 表示的是字符串的長度（Byte 數組長度）。因此一個 string 類型的變量佔用內存大小永遠爲 16 Byte。
如果希望訪問字符串的底層數據 Byte 可以通過 for+len 的方式
s := "unsafe.SizeOf"
for i := 0; i < len(s); i++ {
// TODO
}
但是如果希望以字符的方式訪問，則只需要通過 for-range 的方式遍歷即可。這時在迭代過程中的迭代變量爲 rune 類型，Golang 會自動將其底層的 Byte 轉環爲 rune（UTF-8 字符），其實現是在 runtime/utf8.go 中的 decoderune 方法和 encoderune 方法。
切片
底層結構
type slice struct {
array unsafe.Pointer
len int
cap int
}
反射包中的 切片
// SliceHeader is the runtime representation of a slice.
// It cannot be used safely or portably and its representation may
// change in a later release.
// Moreover, the Data field is not sufficient to guarantee the data
// it references will not be garbage collected, so programs must keep
// a separate, correctly typed pointer to the underlying data.
type SliceHeader struct {
Data uintptr
Len int
Cap int
}
切片的創建有三種方法：
- 通過數組或其他切片
- 通過實例
- 通過 make 方法
其中通過 make 方法創建的時候調用的是 runtime/makeslice 方法。
func makeslice(et *_type, len, cap int) unsafe.Pointer {
mem, overflow := math.MulUintptr(et.size, uintptr(cap))
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()
}
return mallocgc(mem, et, true)
}
而通過實例創建的時候實際上是創建了一個與實例相同的數組，然後包裝到 slice 當中。
切片中比較特殊的應該是擴容，即當通過 append 向切片末尾添加個元素的時候若超過切片容量，切片會進行擴容操作。而擴容的時候需要滿足如下規律：
- 當期待擴容量爲當前容量 2 倍的時候擴容期待容量（原容量爲 3，添加 6 個元素，期待擴容爲 6，爲原容量的 2 倍，故擴容量爲6）。
- 如果不滿足條件 1，同時容量小於 1024，那麼每次會擴容當前容量的一倍（原容量爲 3，添加 1 個元素，期待擴容爲 1，不是原來的 2 倍，故擴容量爲 3）。
- 如果容量超過 1024 每次只會擴容原本容量的 25%（原容量爲 2000，添加 1 個元素，不是 2 倍，超過 1024，則擴容量爲 500）。
當切片發生擴容的時候實際調用的是 runtime/gorwslice 方法
// growslice allocates new backing store for a slice.
//
// arguments:
//
// oldPtr = pointer to the slice's backing array
// newLen = new length (= oldLen + num)
// oldCap = original slice's capacity.
// num = number of elements being added
// et = element type
//
// return values:
//
// newPtr = pointer to the new backing store
// newLen = same value as the argument
// newCap = capacity of the new backing store
//
// Requires that uint(newLen) > uint(oldCap).
// Assumes the original slice length is newLen - num
//
// A new backing store is allocated with space for at least newLen elements.
// Existing entries [0, oldLen) are copied over to the new backing store.
// Added entries [oldLen, newLen) are not initialized by growslice
// (although for pointer-containing element types, they are zeroed). They
// must be initialized by the caller.
// Trailing entries [newLen, newCap) are zeroed.
//
// growslice's odd calling convention makes the generated code that calls
// this function simpler. In particular, it accepts and returns the
// new length so that the old length is not live (does not need to be
// spilled/restored) and the new length is returned (also does not need
// to be spilled/restored).
func growslice(oldPtr unsafe.Pointer, newLen, oldCap, num int, et *_type) slice {
oldLen := newLen - num
if raceenabled {
callerpc := getcallerpc()
racereadrangepc(oldPtr, uintptr(oldLen*int(et.size)), callerpc, abi.FuncPCABIInternal(growslice))
}
if msanenabled {
msanread(oldPtr, uintptr(oldLen*int(et.size)))
}
if asanenabled {
asanread(oldPtr, uintptr(oldLen*int(et.size)))
}
if newLen < 0 {
panic(errorString("growslice: len 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 oldPtr in this case.
return slice{unsafe.Pointer(&zerobase), newLen, newLen}
}
newcap := oldCap
doublecap := newcap + newcap
if newLen > doublecap {
newcap = newLen
} else {
const threshold = 256
if oldCap < threshold {
newcap = doublecap
} else {
// Check 0 < newcap to detect overflow
// and prevent an infinite loop.
for 0 < newcap && newcap < newLen {
// 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 = newLen
}
}
}
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(oldLen)
newlenmem = uintptr(newLen)
capmem = roundupsize(uintptr(newcap))
overflow = uintptr(newcap) > maxAlloc
newcap = int(capmem)
case et.size == goarch.PtrSize:
lenmem = uintptr(oldLen) * goarch.PtrSize
newlenmem = uintptr(newLen) * 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(oldLen) << shift
newlenmem = uintptr(newLen) << shift
capmem = roundupsize(uintptr(newcap) << shift)
overflow = uintptr(newcap) > (maxAlloc >> shift)
newcap = int(capmem >> shift)
capmem = uintptr(newcap) << shift
default:
lenmem = uintptr(oldLen) * et.size
newlenmem = uintptr(newLen) * et.size
capmem, overflow = math.MulUintptr(et.size, uintptr(newcap))
capmem = roundupsize(capmem)
newcap = int(capmem / et.size)
capmem = uintptr(newcap) * 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: len 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 oldLen to newLen.
// Only clear the part that will not be overwritten.
// The reflect_growslice() that calls growslice will manually clear
// the region not cleared here.
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 oldPtr since we know the destination slice p
// only contains nil pointers because it has been cleared during alloc.
bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(oldPtr), lenmem-et.size+et.ptrdata)
}
}
memmove(p, oldPtr, lenmem)
return slice{p, newLen, newcap}
}