go 为什么切片作为传参的时候，触发扩容机制，形参为什么影响不到实参，传入切片指针又可以

1. 当传入切片时，触发扩容时，形参不能影响实参

func main() {

	a := make([]int, 2, 3)

	// fmt.Printf("主函数对象地址是%p", &a)
	fmt.Println(a)
	test01(a)
	// test02(&a)
	fmt.Println(a)
}
func test02(s *[]int) {
	// fmt.Printf("test处理前对象地址是%p", &s)
	*s = append(*s, 2, 3, 4, 5)
	fmt.Println(*s)
	// fmt.Printf("test处理后对象地址是%p", &s)

}
func test01(s []int) {
	s = append(s, 2, 3, 4, 5)
	fmt.Println(s)

}

原因是看扩容方法最后一行，扩容之后是return一个新的切边对象，扩容之前 s变量和a变量的value值是同一个地址,所谓的引用传递，只是把a变量的value值赋值一份给到s,扩容之后生成一个新的切片对象，所以s变量的value值改变了，指向了新的地址空间，但是main方法a变量的value值没有改变，他还是指向旧的地址空间

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}
}

1. 传切片的指针可以达到效果，执行test02方法

package main

import "fmt"

func main() {

	a := make([]int, 2, 3)

	// fmt.Printf("主函数对象地址是%p", &a)
	fmt.Println(a)
	// test01(a)
	test02(&a)
	fmt.Println(a)
}
func test02(s *[]int) {
	// fmt.Printf("test处理前对象地址是%p", &s)
	*s = append(*s, 2, 3, 4, 5)
	fmt.Println(*s)
	// fmt.Printf("test处理后对象地址是%p", &s)

}
func test01(s []int) {
	s = append(s, 2, 3, 4, 5)
	fmt.Println(s)

}

原因是虽然生成了新的切片对象，但是他是在原来a变量的value值去超作，当生成新的切片对象时，a变量的value值可以随机改变，指向新的地址空间