具体类型转换成接口类型
到此已经知道什么是接口以及接口的底层结构，那么当具体类型赋值给接口类型时，是如何进行转换的？再来看下2.3中的示例
package main
import "fmt"
type Runner interface {
Run()
}
type Person struct {
Name string
}
func (p Person) Run() {
fmt.Printf("%s is running\n", p.Name)
}
func main() {
var r Runner
r = Person{Name: "song_chh"}
r.Run()
}
通过Go提供的工具生成汇编代码
go tool compile -S interface.go
只截取与第19行相关的代码
0x001d 00029 (interface.go:19) PCDATA $2, $0
0x001d 00029 (interface.go:19) PCDATA $0, $1
0x001d 00029 (interface.go:19) XORPS X0, X0
0x0020 00032 (interface.go:19) MOVUPS X0, ""..autotmp_1+32(SP)
0x0025 00037 (interface.go:19) PCDATA $2, $1
0x0025 00037 (interface.go:19) LEAQ go.string."song_chh"(SB), AX
0x002c 00044 (interface.go:19) PCDATA $2, $0
0x002c 00044 (interface.go:19) MOVQ AX, ""..autotmp_1+32(SP)
0x0031 00049 (interface.go:19) MOVQ $8, ""..autotmp_1+40(SP)
0x003a 00058 (interface.go:19) PCDATA $2, $1
0x003a 00058 (interface.go:19) LEAQ go.itab."".Person,"".Runner(SB), AX
0x0041 00065 (interface.go:19) PCDATA $2, $0
0x0041 00065 (interface.go:19) MOVQ AX, (SP)
0x0045 00069 (interface.go:19) PCDATA $2, $1
0x0045 00069 (interface.go:19) PCDATA $0, $0
0x0045 00069 (interface.go:19) LEAQ ""..autotmp_1+32(SP), AX
0x004a 00074 (interface.go:19) PCDATA $2, $0
0x004a 00074 (interface.go:19) MOVQ AX, 8(SP)
0x004f 00079 (interface.go:19) CALL runtime.convT2I(SB)
0x0054 00084 (interface.go:19) MOVQ 16(SP), AX
0x0059 00089 (interface.go:19) PCDATA $2, $2
0x0059 00089 (interface.go:19) MOVQ 24(SP), CX
可以看到，编译器在构造itab后调用runtime.convT2I(SB)转换函数，看下函数的实现
//runtime/iface.go
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)
}
x := mallocgc(t.size, t, true)
typedmemmove(t, x, elem)
i.tab = tab
i.data = x
return
}
首先根据类型大小调用mallocgc申请一块内存空间，将elem指针的内容拷贝到新空间，将tab赋值给iface的tab，将新内存指针赋值给iface的data，这样一个iface就创建完成
将示例代码稍作更改，使用结构体指针类型的变量赋值给接口变量
r = &Person{Name: "song_chh"}
再次通过工具生成汇编代码
go tool compile -S interface.go
0x001d 00029 (interface.go:19) PCDATA $2, $1
0x001d 00029 (interface.go:19) PCDATA $0, $0
0x001d 00029 (interface.go:19) LEAQ type."".Person(SB), AX
0x0024 00036 (interface.go:19) PCDATA $2, $0
0x0024 00036 (interface.go:19) MOVQ AX, (SP)
0x0028 00040 (interface.go:19) CALL runtime.newobject(SB)
0x002d 00045 (interface.go:19) PCDATA $2, $2
0x002d 00045 (interface.go:19) MOVQ 8(SP), DI
0x0032 00050 (interface.go:19) MOVQ $8, 8(DI)
0x003a 00058 (interface.go:19) PCDATA $2, $-2
0x003a 00058 (interface.go:19) PCDATA $0, $-2
0x003a 00058 (interface.go:19) CMPL runtime.writeBarrier(SB), $0
0x0041 00065 (interface.go:19) JNE 105
0x0043 00067 (interface.go:19) LEAQ go.string."song_chh"(SB), AX
0x004a 00074 (interface.go:19) MOVQ AX, (DI)
首先编译器通过type."".Person(SB)获取Person结构体类型，作为参数调用runtime.newobject()函数，同样的在源码中查看函数定义
import "unsafe"
// runtime/malloc.go
// implementation of new builtin
// compiler (both frontend and SSA backend) knows the signature
// of this function
func newobject(typ *_type) unsafe.Pointer {
return mallocgc(typ.size, typ, true)
}
newobject以*Person作为入参，创建新的Person结构体指针，之后由编译器设置值，iface由编译器直接生成
除了convT2I函数外，其实在runtime/runtime.go文件中，还有很多转换函数的定义
// Non-empty-interface to non-empty-interface conversion.
func convI2I(typ *byte, elem any) (ret any)
// Specialized type-to-interface conversion.
// These return only a data pointer.
func convT16(val any) unsafe.Pointer // val must be uint16-like (same size and alignment as a uint16)
func convT32(val any) unsafe.Pointer // val must be uint32-like (same size and alignment as a uint32)
func convT64(val any) unsafe.Pointer // val must be uint64-like (same size and alignment as a uint64 and contains no pointers)
func convTstring(val any) unsafe.Pointer // val must be a string
func convTslice(val any) unsafe.Pointer // val must be a slice
// Type to empty-interface conversion.
func convT2E(typ *byte, elem *any) (ret any)
func convT2Enoptr(typ *byte, elem *any) (ret any)
// Type to non-empty-interface conversion.
func convT2I(tab *byte, elem *any) (ret any) //for the general case
func convT2Inoptr(tab *byte, elem *any) (ret any) //for structs that do not contain pointers
convI2I用于接口转换成另一个接口时调用，在3.4会进行讲解
convT2Inoptr用于变量内部不含指针的转换，noptr可以理解为no pointer，转换过程与convT2I类似
convT16、convT32、convT64、convTstring 和 convTslice是针对简单类型转接口的特例优化，有兴趣的可以看下函数实现的源码，因为这几个函数内容相似，这里就简单介绍下convT64
//runtime/iface.go
func convT64(val uint64) (x unsafe.Pointer) {
if val == 0 {
x = unsafe.Pointer(&zeroVal[0])
} else {
x = mallocgc(8, uint64Type, false)
*(*uint64)(x) = val
}
return
}
相较于convT2系列函数，缺少typedmemmove和memmove函数的调用，减少内存拷贝。另外如果变量值为该类型的零值，则不会调用 mallocgc 去申请一块新内存，而是直接返回指向zeroVal[0]的指针
再来看下，空接口转换函数convT2E
func convT2E(t *_type, elem unsafe.Pointer) (e eface) {
if raceenabled {
raceReadObjectPC(t, elem, getcallerpc(), funcPC(convT2E))
}
if msanenabled {
msanread(elem, t.size)
}
x := mallocgc(t.size, t, true)
// TODO: We allocate a zeroed object only to overwrite it with actual data.
// Figure out how to avoid zeroing. Also below in convT2Eslice, convT2I, convT2Islice.
typedmemmove(t, x, elem)
e._type = t
e.data = x
return
}
接口与接口的转换
如果某个类型实现多个接口，那接口直接是如何进行转换的，还是先看一段示例：
package main
import "fmt"
type Runner interface {
Run()
Say()
}
type Sayer interface {
Say()
}
type Person struct {
Name string
}
func (p Person) Run() {
fmt.Printf("%s is running\n", p.Name)
}
func (p Person) Say() {
fmt.Printf("hello, %s", p.Name)
}
func main() {
var r Runner
r = Person{Name: "song_chh"}
var s Sayer
s = r
s.Say()
}
增加Sayer接口定义，包含Say()方法，在main函数中声明一个Sayer变量，并将Runner接口变量赋值给Sayer变量。因为Person实现了Say()方法，所以说Person既实现了是Runner接口，又实现了Sayer接口
执行命令
go tool compile -S interface.go
截取32行汇编代码
0x0062 00098 (interface.go:32) PCDATA $2, $3
0x0062 00098 (interface.go:32) LEAQ type."".Sayer(SB), DX
0x0069 00105 (interface.go:32) PCDATA $2, $2
0x0069 00105 (interface.go:32) MOVQ DX, (SP)
0x006d 00109 (interface.go:32) MOVQ AX, 8(SP)
0x0072 00114 (interface.go:32) PCDATA $2, $0
0x0072 00114 (interface.go:32) MOVQ CX, 16(SP)
0x0077 00119 (interface.go:32) CALL runtime.convI2I(SB)
0x007c 00124 (interface.go:32) MOVQ 24(SP), AX
0x0081 00129 (interface.go:32) PCDATA $2, $2
0x0081 00129 (interface.go:32) MOVQ 32(SP), CX
可以看到在执行期间，调用runtime.convI2I进行接口转换，接下来看下源代码
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 = (inter, tab._type, false)
r.data = i.data
return
}
函数参数inter表示接口的类型，由编译器生成，即type."".Sayer(SB)，i 是绑定实体的接口， r 是转换后新的接口，如果要转换的接口是同一类型，则直接把 i 的tab和data给新接口 r ，将 r 返回。如果要转换的接口不是同一类型，则通过getitab生成一个新的tab复制给r.tab，然后将 r 返回
那么具体来看一下getitab这个函数，还是先看源码
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
// 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.
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.
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()})
}
首先，用 t 保存全局itabTable的地址，使用t.find进行查找，这个是通用的查找
如果没有查找到，就会上锁，重新使用itabTable.find进行查找
再没有找到，就会根据具体类型typ和接口类型inter生成一个itab，并将这个新生成的itab添加到全局的itabTable中。如果具体类型并没有实现接口，根据canfail值返回nil或者painc
标签：类型转换,PCDATA,tab,19,interface,func,go From： https://www.cnblogs.com/iiiiiher/p/17080945.html