go语言中切片 slice

参考文章：

​​​ https://studygolang.com/articles/18194?fr=sidebar​​​ https://github.com/cch123/golang-notes/blob/master/slice.md

https://blog.haohtml.com/archives/18094

slice:

先看一下slcie结构：

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

slice三要素： type、len、cap

slice 的底层结构定义非常直观，指向底层数组的指针，当前长度 len 和当前 slice 的 cap。

[]int{1,3,4,5}                                              
                                                               
   struct {                                                    
     array unsafe.Pointer --------------+                      
     len int                            |                      
     cap int                            |                      
   }                                    |                      
                                        |                      
                                        v                      
                                                               
                           +------|-------|------|------+-----+
                           |      |  1    |  3   | 4    |  5  |
                           |      |       |      |      |     |
                           +------|-------|------|------+-----+
                                             [5]int                  

func makeslice(et *_type, len, cap int) slice      

slice扩容机制：

扩容时会判断 slice 的 cap 是不是已经大于 1024，如果在 1024 之内，会按二倍扩容。超过的话就是 1.25 倍扩容了。

slice 扩容必然会导致内存拷贝，如果是性能敏感的系统中，尽可能地提前分配好 slice 是较好的选择。

var arr = make([]int, 0, 10)      

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

    if et.size == 0 {
        if cap < old.cap {
            panic(errorString("growslice: cap out of range"))
        }
        return slice{unsafe.Pointer(&zerobase), old.len, cap}
    }

    newcap := old.cap
    doublecap := newcap + newcap
    if cap > doublecap {
        newcap = cap
    } else {
        // 注意这里的 1024 阈值
        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
    const ptrSize = unsafe.Sizeof((*byte)(nil))
    switch et.size {
    case 1:
        lenmem = uintptr(old.len)
        newlenmem = uintptr(cap)
        capmem = roundupsize(uintptr(newcap))
        overflow = uintptr(newcap) > _MaxMem
        newcap = int(capmem)
    case ptrSize:
        lenmem = uintptr(old.len) * ptrSize
        newlenmem = uintptr(cap) * ptrSize
        capmem = roundupsize(uintptr(newcap) * ptrSize)
        overflow = uintptr(newcap) > _MaxMem/ptrSize
        newcap = int(capmem / ptrSize)
    default:
        lenmem = uintptr(old.len) * et.size
        newlenmem = uintptr(cap) * et.size
        capmem = roundupsize(uintptr(newcap) * et.size)
        overflow = uintptr(newcap) > maxSliceCap(et.size)
        newcap = int(capmem / et.size)
    }

    if cap < old.cap || overflow || capmem > _MaxMem {
        panic(errorString("growslice: cap out of range"))
    }

    var p unsafe.Pointer
    if et.kind&kindNoPointers != 0 {
        p = mallocgc(capmem, nil, false)
        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.
        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}
}      

slice的append()函数

func append(slice []Type, elems ...Type) []Type      

函数说明：内建函数append追加一个或多个elems到一个slice依赖的array的末尾，如果这个slice有足够的capacity，则reslice以容纳新增元素；如果capacity空间不够，则进行扩容，重新分配内存保存新的slice依赖的array，函数返回更新后的slice.

slice是做函数参数是值传递：

func main() {
    var a = make([]int, 10)
    fmt.Println(a)
}

func doSomeHappyThings(sl []int) {
    if len(sl) > 0 {
        sl[0] = 1
    }
}