Go Type Assertion

External

• https://go.dev/ref/spec#Type_assertions

Internal

• Go Language
• Go Interfaces

Overview

A type assertion is an expression that probes whether an interface is of a certain concrete type, and if it is, returns a variable of that type (see here for the code used in the example).

var i SomeInterface

v, ok := i.(*SomeImplementation)

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.

Actually, it worked with:

v, isTypeName := i.(*TypeName)

Why is that? Probably because the interface variable contained a pointer to the actual type.

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 Switch

A type switch is a new control structure introduced by Go.

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)
}