golang-解引用后从struct获取接口实现实例

Question

Create a struct that implements an interface. Encapsulate it into another struct. Fetch the struct / dereference it.

1. Fetching gets pointer to the implementation of the interface.
2. Dereferencing the pointer results in another point to the interface.

Why wouldn't dereferencing the pointer to the implementation gives the implementation instance?

package main

import (
    "fmt"
    "net/http"

    "golang.org/x/net/http2"
)

func main() {
    transport := &http2.Transport{}
    client := &http.Client{Transport: transport}

    tmp1 := client.Transport
    tmp2 := &client.Transport
    tmp3 := &tmp1

    fmt.Printf("%T
%T
%T
", tmp1, tmp2, tmp3)

    //dialTLS := tmp1.DialTLS
}

This code outputs...

*http2.Transport
*http.RoundTripper
*http.RoundTripper

As well, trying to access a property of the tmp1 (uncommenting dialTLS := tmp1.DialTLS) results in compile error...

tmp1.DialTLS undefined (type http.RoundTripper has no field or method DialTLS)

...even though fmt.Printf("%+v", tmp1) outputs...

&{DialTLS:<nil> TLSClientConfig:0xc8203ec8c0 ConnPool:<nil> DisableCompression:false MaxHeaderListSize:0 t1:<nil> connPoolOnce:{m:{state:0 sema:0} done:0} connPoolOrDef:<nil>}

What I am attempting to do is access DialTLS in the Transport instance.

Answer 1

You have a type mismatch here. If you look at the documentation, you'll see that the field Transport of the Client struct returns an interface RoundTripper:

type Client struct {
    // Transport specifies the mechanism by which individual
    // HTTP requests are made.
    // If nil, DefaultTransport is used.
    Transport RoundTripper

    // ...
}

So, the type of tmp1 is http.RoundTripper, though the underlying type is *http2.Transport which implements the RoundTripper interface.

As for tmp2 and tmp3, they are seen as pointers to a RoundTripper and not as **http2.Transport.

In order to retrieve the DialTLS field, you have to use type assertions to convert a RoundTripper back into a Transport:

dialTLS := client.Transport.(*http2.Transport).DialTLS