golang多种类型的开关 [英] golang multiple case in type switch
问题描述
当我运行代码片段时,它引发一个错误:
$ b
a.test undefined(type interface {}方法)
看起来类型切换不起作用。
<$ p
$ b $
$ b func(this * A)test(){
fmt.Println(this)
}
type B struct {
$ b $ func main(){
var foo interface {}
foo = A {}
switch a:= foo 。(type){
case B,A:
a.test()
}
}
如果将其更改为
切换a:= foo。(type) {
案例A:
a.test()
}
这是由规格(强调我的):
$ b blockquote>
TypeSwitchGuard可能包含一个简短的变量声明。当使用该表单时,该变量将在每个子句中的隐式块的开始处声明。 在具有一种类型的案例列表中,该变量具有该类型;否则,该变量具有TypeSwitchGuard中表达式的类型
。因此,事实上,类型开关确实生效,但变量 a
保持类型界面{}
。
解决这个问题的一种方法是断言 foo
的方法 test()
,它看起来像这样:
< pre $
$ b $ struct $ {
a int
$ b $ func(this * A)test(){
fmt.Println(this)
}
type B struct {
A
}
类型测试器接口{
test()
}
func main(){
var foo interface {}
foo =& B {}
如果a,ok:= foo。(tester); OK {
fmt.Println(foo has test()method)
.test()
}
}
when I run the code snippet bellow, it raise a error
a.test undefined (type interface {} is interface with no methods)
It seem the type switch does not take effect.
package main
import (
"fmt"
)
type A struct {
a int
}
func(this *A) test(){
fmt.Println(this)
}
type B struct {
A
}
func main() {
var foo interface{}
foo = A{}
switch a := foo.(type){
case B, A:
a.test()
}
}
If I change it to
switch a := foo.(type){
case A:
a.test()
}
it's ok now.
This is normal behaviour that is defined by the spec (emphasis mine):
The TypeSwitchGuard may include a short variable declaration. When that form is used, the variable is declared at the beginning of the implicit block in each clause. In clauses with a case listing exactly one type, the variable has that type; otherwise, the variable has the type of the expression in the TypeSwitchGuard.
So, in fact, the type switch does take effect, but the variable a
keeps the type interface{}
.
One way you could get around this is to assert that foo
has the method test()
, which would look something like this:
package main
import (
"fmt"
)
type A struct {
a int
}
func (this *A) test() {
fmt.Println(this)
}
type B struct {
A
}
type tester interface {
test()
}
func main() {
var foo interface{}
foo = &B{}
if a, ok := foo.(tester); ok {
fmt.Println("foo has test() method")
a.test()
}
}
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