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() } }
