Go Type Assertion
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External
Internal
Overview
An interface hides the differences between the types implementing it and emphasizes the commonality.
However, there are times you may want to know what exact concrete type exists behind the interface.
Type assertions can be used for type disambiguation:
var i SomeInterface
v, isTypeName := i.(TypeName)
Usage example:
type SomeInterface interface {
MethodA()
}
type TypeA struct {
s string
}
func (v *TypeA) MethodA() {
fmt.Println("TypeA.MethodA()")
}
type TypeB struct {
s string
}
func (v *TypeB) MethodA() {
fmt.Println("TypeB.MethodA()")
}
...
var i SomeInterface
i = &TypeA{"A"}
b := &TypeB{"B"}
a, isTypeA := i.(*TypeA)
fmt.Printf("%v, %t\n", a, isTypeA) // will print {A}, true
b, isTypeB := i.(*TypeB)
fmt.Printf("%v, %t\n", b, isTypeB) // will print <nil>, false
Type Assertion
Type Switch
Type assertion with switch
:
var i SomeInterface
i = TypeA{"A"}
switch v := i.(type) {
case TypeA:
fmt.Printf("TypeA: %v\n", v)
case TypeB:
fmt.Printf("TypeB: %v\n", v)
}