Go-深入理解interface

type iface struct {
	tab  *itab
	data unsafe.Pointer  // 保存数据
}

type eface struct {
	_type *_type
	data  unsafe.Pointer
}

type itab struct {
	inter *interfacetype    // 接口的类型
	_type *_type
	hash  uint32 // copy of _type.hash. Used for type switches.
    _     [4]byte
    // 放置和接口方法对应的具体数据类型和方法地址
	fun   [1]uintptr // variable sized. fun[0]==0 means _type does not implement inter.
}

type interfacetype struct {
	typ     _type   
	pkgpath name   // package import名
	mhdr    []imethod  // 接口所定义的函数列表
}

type _type struct {
	size       uintptr // 类型大小
	ptrdata    uintptr // size of memory prefix holding all pointers
	hash       uint32  // 类型hash值
	tflag      tflag   // 类型tags
	align      uint8   // 内存对齐
	fieldalign uint8   // 字段对齐
	kind       uint8  // 类型
	alg        *typeAlg
	// gcdata stores the GC type data for the garbage collector.
	// If the KindGCProg bit is set in kind, gcdata is a GC program.
	// Otherwise it is a ptrmask bitmap. See mbitmap.go for details.
	gcdata    *byte
	str       nameOff
	ptrToThis typeOff
}

	var j = 45
	var i interface{} = j
	var x interface{} = &j

	fmt.Printf("%v\n", &j)   // 0xc000016088
	fmt.Printf("%v\n", &i)   // 0xc0000101e0
	fmt.Printf("%v\n", x)    // 0xc000016088

func convT2I(tab *itab, elem unsafe.Pointer) (i iface) {
	t := tab._type
	if raceenabled {
		raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2I))
	}
	if msanenabled {
		msanread(elem, t.size)
	}
	// 从堆中分配内存
	x := mallocgc(t.size, t, true)
	typedmemmove(t, x, elem)
	i.tab = tab
	i.data = x
	return
}

func convI2I(inter *interfacetype, i iface) (r iface) {
	tab := i.tab
	if tab == nil {
		return
	}
	if tab.inter == inter {
		r.tab = tab
		r.data = i.data
		return
	}
	r.tab = getitab(inter, tab._type, false)
	r.data = i.data
	return
}

func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
	if len(inter.mhdr) == 0 {
		throw("internal error - misuse of itab")
	}

	// easy case
	if typ.tflag&tflagUncommon == 0 {
		if canfail {
			return nil
		}
		name := inter.typ.nameOff(inter.mhdr[0].name)
		panic(&TypeAssertionError{nil, typ, &inter.typ, name.name()})
	}

	var m *itab

	// 先从存在的table中寻找itab
	// First, look in the existing table to see if we can find the itab we need.
	// This is by far the most common case, so do it without locks.
	// Use atomic to ensure we see any previous writes done by the thread
	// that updates the itabTable field (with atomic.Storep in itabAdd).
	t := (*itabTableType)(atomic.Loadp(unsafe.Pointer(&itabTable)))
	if m = t.find(inter, typ); m != nil {
		goto finish
	}

	// Not found.  Grab the lock and try again.
	// 没找到，加锁再次找一下
	lock(&itabLock)
	if m = itabTable.find(inter, typ); m != nil {
		unlock(&itabLock)
		goto finish
	}

	// Entry doesn't exist yet. Make a new entry & add it.
	// 不存在，新建一个，然后加入到table中
	m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr)-1)*sys.PtrSize, 0, &memstats.other_sys))
	m.inter = inter
	m._type = typ
	m.init()
	itabAdd(m)
	unlock(&itabLock)
finish:
	if m.fun[0] != 0 {
		return m
	}
	if canfail {
		return nil
	}
	// this can only happen if the conversion
	// was already done once using the , ok form
	// and we have a cached negative result.
	// The cached result doesn't record which
	// interface function was missing, so initialize
	// the itab again to get the missing function name.
	panic(&TypeAssertionError{concrete: typ, asserted: &inter.typ, missingMethod: m.init()})
}

// find finds the given interface/type pair in t.
// Returns nil if the given interface/type pair isn't present.
func (t *itabTableType) find(inter *interfacetype, typ *_type) *itab {
	// Implemented using quadratic probing.
	// Probe sequence is h(i) = h0 + i*(i+1)/2 mod 2^k.
	// We're guaranteed to hit all table entries using this probe sequence.
	mask := t.size - 1
	h := itabHashFunc(inter, typ) & mask
	for i := uintptr(1); ; i++ {
		p := (**itab)(add(unsafe.Pointer(&t.entries), h*sys.PtrSize))
		// Use atomic read here so if we see m != nil, we also see
		// the initializations of the fields of m.
		// m := *p
		m := (*itab)(atomic.Loadp(unsafe.Pointer(p)))
		if m == nil {
			return nil
		}
		if m.inter == inter && m._type == typ {
			return m
		}
		h += i
		h &= mask
	}
}

func itabAdd(m *itab) {
	// Bugs can lead to calling this while mallocing is set,
	// typically because this is called while panicing.
	// Crash reliably, rather than only when we need to grow
	// the hash table.
	if getg().m.mallocing != 0 {
		throw("malloc deadlock")
	}

	t := itabTable
	// 超过75%，需要扩容
	if t.count >= 3*(t.size/4) { // 75% load factor
		// Grow hash table.
		// t2 = new(itabTableType) + some additional entries
		// We lie and tell malloc we want pointer-free memory because
		// all the pointed-to values are not in the heap.
		// 两倍扩容
		t2 := (*itabTableType)(mallocgc((2+2*t.size)*sys.PtrSize, nil, true))
		t2.size = t.size * 2

		// Copy over entries.
		// Note: while copying, other threads may look for an itab and
		// fail to find it. That's ok, they will then try to get the itab lock
		// and as a consequence wait until this copying is complete.
		iterate_itabs(t2.add)
		if t2.count != t.count {
			throw("mismatched count during itab table copy")
		}
		// Publish new hash table. Use an atomic write: see comment in getitab.
		atomicstorep(unsafe.Pointer(&itabTable), unsafe.Pointer(t2))
		// Adopt the new table as our own.
		t = itabTable
		// Note: the old table can be GC'ed here.
	}
	t.add(m)
}

func (t *itabTableType) add(m *itab) {
	// See comment in find about the probe sequence.
	// Insert new itab in the first empty spot in the probe sequence.
	mask := t.size - 1
	// 计算偏移量
	h := itabHashFunc(m.inter, m._type) & mask
	for i := uintptr(1); ; i++ {
		p := (**itab)(add(unsafe.Pointer(&t.entries), h*sys.PtrSize))
		m2 := *p
		if m2 == m {
			// A given itab may be used in more than one module
			// and thanks to the way global symbol resolution works, the
			// pointed-to itab may already have been inserted into the
			// global 'hash'.
			// 已经存在过了，不用加了
			return
		}
		if m2 == nil {
			// Use atomic write here so if a reader sees m, it also
			// sees the correctly initialized fields of m.
			// NoWB is ok because m is not in heap memory.
			// *p = m
			// 存储m，类似*p = m, 不用写屏障
			atomic.StorepNoWB(unsafe.Pointer(p), unsafe.Pointer(m))
			t.count++
			return
		}
		h += i
		h &= mask
	}
}

	x := i.(int64)
	y, ok := i.(int64)

func assertI2I(inter *interfacetype, i iface) (r iface) {
	tab := i.tab
	if tab == nil {
		// explicit conversions require non-nil interface value.
		panic(&TypeAssertionError{nil, nil, &inter.typ, ""})
	}
	if tab.inter == inter {
		r.tab = tab
		r.data = i.data
		return
	}
	r.tab = getitab(inter, tab._type, false)
	r.data = i.data
	return
}

func assertI2I2(inter *interfacetype, i iface) (r iface, b bool) {
	tab := i.tab
	if tab == nil {
		return
	}
	if tab.inter != inter {
		tab = getitab(inter, tab._type, true)
		if tab == nil {
			return
		}
	}
	r.tab = tab
	r.data = i.data
	b = true
	return
}