如果结构具有参数实现接口的功能，则结构不实现接口

Question

I have a package in which I have two interfaces

package main
type A interface {
    Close()
}
type B interface {
    Connect() (A, error)
}

I have also two structures which implements these interfaces

type C struct {
}

func (c *C) Close() {

}

type D struct {
}

func (d *D) Connect() (*C, error) {
    c := new(C)
    return c, nil
}

Next I have a function which as a parameter wants an object which implements interface B

func test(b B) {
}

Finally, at the main() function I create D structure object and want to call test() function

func main() {
    d := new(D)
    test(d)
}

If I try to build that package I have an error.

cannot use d (type *D) as type B in argument to test: *D does not implement B (wrong type for Connect method) have Connect() (*C, error) want Connect() (A, error)

It is simple example of my code where I use external package and want to mock structures for tests. Is it any solution to use interfaces instead of types?

Answer 1

The returned type for your Connect method should be A and not *C.

The way you defined the Connect method is that it should return an interface, not a specific type. You will still be able to return *C as it implements the A interface.

package main

type A interface {
    Close()
}

type B interface {
    Connect() (A, error)
}

type C struct {
}

func (c *C) Close() {
}

type D struct {
}

func (d *D) Connect() (A, error) {
    c := new(C)
    println("successfully created new C:", c)
    return c, nil
}

func test(b B) {
    b.Connect()
}

func main() {
    d := new(D)
    test(d)
}

Outputs

successfully created new C: 0xe28f0

Try it out yourself here