Go : Check if type implements an interface

Sometimes there can be scenarios where it is needed to know if your type satisfies an interface or not. This can be easily achieved using a blank identifier.

package main

type shape interface {
    getNumSides() int
    getArea() int
}

type square struct {
    len int
}

func (s square) getNumSides() int {
    return 4
}

// func (s square) getArea() int {
//  return s.len * 2
// }

func main() {
    //Verify that *square implement shape
    var _ shape = square{}
}

In the above program, we have commented out the square’s getArea() function. On “go build” above program gives the error

main.go:25:6: cannot use square literal (type square) as type shape in assignment:
        square does not implement shape (missing getArea method)

On uncommenting the getArea() method the error goes away. To check if a pointer of the type implements the interface then it can be done as below

var _ shape = &square{}

OR

var _ shape = (*square)(nil)

First one will give the error

main.go:25:6: cannot use (*square)(nil) (type *square) as type shape in assignment:
        *square does not implement shape (missing getArea method)

while the latter one will give error

main.go:25:6: cannot use &square literal (type *square) as type shape in assignment:
        *square does not implement shape (missing getArea method)