Type Assertions in Go

When working with interfaces in Go, every once-in-awhile you'll need access to the underlying type of an interface value. You can cast an interface to its underlying type using a type assertion.

The example below shows how to safely access the radius field of s when it is not known that s is a circle:

type shape interface {
	area() float64
}

type circle struct {
	radius float64
}

func (c circle) area() float64 {
	// ...
}

func printShapeInfo(s shape) {
	c, ok := s.(circle)
	if ok {
		radius := c.radius
		fmt.Printf("s is a circle, radius: %v\n", radius)
		return
	}
}

In printShapeInfo:

• We want to check if s is a circle in order to cast it into its underlying concrete type
• We know (from the function signature) that s is an instance of the shape interface, but we do not know if it's also a circle
• c is a new circle struct cast from s
• ok is true if s is indeed a circle, or false if s is NOT a circle