返回具有数据成员的多态类型

Question

I'm trying to write a function getTargetServer() to return a polymorphic type that has both a data member URL and a method Close(). This would be a generalization of the *Server returned from httptest.NewServer() but I want to alternatively be able to return a custom type for which Close() is a NOP.

type externalTestServer struct {
    URL string
}

func (externalTestServer) Close() {}

func getTargetServer() *externalTestServer {
    if urlbase, ok := optionals["urlbase"].(string); ok {
        return &externalTestServer{URL: urlbase}
    } else {
        testServer := httptest.NewServer(newMyServerHandler())
        // return testServer // ### Error ###
        return &externalTestServer{URL: testServer.URL}
    }
}

func Test_health_check(t *testing.T) {
    testServer := getTargetServer()
    defer testServer.Close()
    response, err := http.Get(testServer.URL + "/health")
    assert.NilError(t, err)
    assert.Assert(t, cmp.Equal(response.StatusCode, http.StatusOK))
}

This works like a charm except that Close() is always a NOP. When I uncomment the indicated ### Error ### line in order to return a closable *Server, I get the following error message:

cannot use testServer (type *httptest.Server) as type *externalTestServer in return argument

I understand the error, but haven't discovered a solution that lets me return a polymorphic type that generalizes *Server

---

Note: "A Tour of Go" defines an interface type as follows:

An interface type is defined as a set of method signatures.

Therefore, returning a simple interface will not allow for directly-accessible data members.

Answer 1

You could create a struct that has a field which is a string URL and a field that is a Close func. The Close func can be implemented by either externalTestServer or httptest.Server:

type server struct {
    URL   string
    Close func()
}

    if urlbase, ok := optionals["urlbase"].(string); ok {
        extServer := &externalTestServer{URL: urlbase}
        return &server{
            URL:   urlbase,
            Close: extServer.Close,
        }
    }
    testServer := httptest.NewServer(newMyServerHandler())
    return &server{
        URL:   testServer.URL,
        Close: testServer.Close,
    }