Type Assertions

Sometimes you have an interface value, but you need to access methods or fields that are only available on the underlying concrete type. You can do this using a type assertion.

Syntax

c, ok := s.(circle)

• c: The value cast to the concrete type (circle).
• ok: A boolean that is true if the assertion succeeded, or false if it failed.

Safe Assertion

Always use the “comma ok” idiom to avoid a panic if the interface doesn’t actually hold the type you’re asserting.

func printCircleRadius(s shape) {
    if c, ok := s.(circle); ok {
        fmt.Printf("Radius: %v\n", c.radius)
    } else {
        fmt.Println("Not a circle")
    }
}