Type assertions

A type assertion provides access to an interface value's underlying concrete value. 

  1. t := i.(T)

This statement asserts that the interface value i holds the concrete type T and assigns the underlying T value to the variable t.

If i does not hold a T, the statement will trigger a panic.

To test whether an interface value holds a specific type, a type assertion can return two values: the underlying value and a boolean value that reports whether the assertion succeeded. 

  1. t, ok := i.(T)

If i holds a T, then t will be the underlying value and ok will be true.

If not, ok will be false and t will be the zero value of type T, and no panic occurs.

Note the similarity between this syntax and that of reading from a map.

type-assertions.go

  1. package main
  3. import "fmt"
  5. func main() {
  6.     var i interface{} = "hello"
  8.     s := i.(string)
  9.     fmt.Println(s)
  11.     s, ok := i.(string)
  12.     fmt.Println(s, ok)
  14.     f, ok := i.(float64)
  15.     fmt.Println(f, ok)
  17.     f = i.(float64) // panic
  18.     fmt.Println(f)
  19. }