goalng Slice&String
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goalng Slice&String

BeforeIeave
2022-07-27 / 0 评论 / 129 阅读 / 正在检测是否收录...

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。
字符串.png

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)
}

切片.png

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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