Golang's invariance

func-inv.go

/* "Invariance" in golang
 * http://play.golang.org/p/ww9UEEKI97
 *
 * Output:
 * > B::foo
 * > C::foo
 * > B::foo
 * > B::bar
 * > B::bar
 */

package main

import "fmt"

type B struct{}

func (b *B) bar() {
	fmt.Println("B::bar")
}

func (b *B) foo() {
	fmt.Println("B::foo")
}

type C struct {
	*B
}

func (c *C) foo() {
	fmt.Println("C::foo")
}

func callBMethod(h func(*B)) {
	b := &B{}
	h(b)
}

func callCMethod(h func(*C)) {
	c := &C{}
	h(c)
}

func main() {
	callBMethod((*B).foo)
	callCMethod((*C).foo)

	bh := (*B).foo
	cbh := func(c *C) { bh(c.B) }
	callCMethod(cbh)

	/* The following statement causes an error:
	 * ./test.go:37: cannot use (*B).foo (type func(*B)) as type func(*C) in
	 * argument to callCMethod
	 */
	//callCMethod((*B).foo)

	callBMethod((*B).bar)
	callCMethod((*C).bar)
}

inheritance.go

/* "Inheritance" in embedded structs
 * http://play.golang.org/p/oF3puVgCZk
 *
 * Output:
 * > B::foo
 * > C::foo	B::foo
 * > D::foo	C::foo	B::foo
 */

package main

import "fmt"

type I interface{
  foo()
}

type B struct{}

func (b B) foo() {
	fmt.Println("B::foo")
}

type C struct {
	B
}

func (c C) foo() {
	fmt.Print("C::foo\t")
	c.B.foo()
}

type D struct {
	C
}

func (d D) foo() {
	fmt.Print("D::foo\t")
	d.C.foo()
}

func callFooMethod(o I) {
	o.foo()
}

func main() {
	callFooMethod(&B{})
	callFooMethod(&C{})
	callFooMethod(&D{})
}