Slice of pointers, when passed to object, gets pointers with other addresses

Question

I'm trying to pass a slice of pointers to structs that implement the interface LogicAdapter. Here's my code:

main.go:

var adapters[]LogicAdapter
    adapter1 := &ExampleAdapter{}
    fmt.Printf("Addr: %p\n", adapter1)
    adapters = append(adapters, adapter1)
    bot := ChatterBot{"Charlie", MultiLogicAdapter{adapters}}
    bot.getResponse("test", 0)

multiadapterlogic.go:

type MultiLogicAdapter struct {
    adapters []LogicAdapter
}

func (logic *MultiLogicAdapter) process(text string, session int) string {
    //response := logic.adapters[0].process(text, session)
    response := ""
    for _, adapter := range logic.adapters {
        fmt.Printf("Addr: %p\n", &adapter)
    }
    _ = response
    return ""
}

The output of main is:

Addr: 0x5178f0

Addr: 0xc42000a340

I didn't include LogicAdapter because I don't think it's necessary. I don't like to fill this with much code, but here's ChatterBot if it makes things easier to understand (but keep in mind that bot.getResponse calls process, that's it)

chatterbot.go

type ChatterBot struct {
    Name string
    MultiLogicAdapter
}

func (bot *ChatterBot) getResponse(text string, session int) string {
    response := bot.process(text, session)
    _ = response
    return ""
}

First of all, I had the idea of storing pointers to LogicAdapters, in this way

var adapters[]*LogicAdapter

but everytime I tried to insert the adapter1 pointer there, I got:

*LogicAdapter is pointer to interface, not interface

so I discovered this and learned this:

When you have a struct implementing an interface, a pointer to that struct implements automatically that interface too. That's why you never have *SomeInterface in the prototype of functions, as this wouldn't add anything to SomeInterface, and you don't need such a type in variable declaration (see this related question).

So I decided to leave the adapters[] declaration as you see in the code. The problem is that besides adapters storing a pointer to adapter1, when it gets printed in MultiLogicAdapter, it's another address. I know that I'm passing adapters to MultiLogicAdapter as a copy, but the copy would have the same references to adapter1. So what's happening?

Why I'm iterating through pointers? Because of this: https://www.goinggo.net/2013/09/iterating-over-slices-in-go.html If I don't, I'll be creating lots of unecessary copies.

fmt.Printf("Addr: %p\n", adapter1) in first case and fmt.Printf("Addr: %p\n", &adapter) in second. &adapter is pointer to pointer — Uvelichitel, Feb 05 '17 at 10:53
To add to @Uvelichitel's comment: The first printf is printing the address of an adapter, the second is printing the address of a local variable containing the address of an adapter. — Charlie Tumahai, Feb 05 '17 at 17:21
@CeriseLimón so when we iterate over something, we're iterating over pointers? — , Feb 05 '17 at 18:06
Range iterates over values from the slice. In the for loop, your code prints the address of one of these values. You should simply print the value. `fmt.Printf("Addr: %p\n", adapter)` will give the result you are expecting. — Charlie Tumahai, Feb 05 '17 at 19:17

Josef Grahn · Accepted Answer · 2017-02-05T19:34:07.530

In var adapters []LogicAdapter, you have an array of interfaces. The interface is itself essentially an object carrying a pointer to the object you assigned to it (with some extra information to track its type etc.). So when you append an *ExampleAdapter to it, you will create an interface instance holding a pointer to an ExampleAdapter.

Later, when you do for _, adapter := range logic.adapters {...}, the variable adapter declared in the loop will be a copy of the interface "object" in the slice. So when you take a pointer to it with &adapter you get a pointer to the interface, not to the ExampleAdapter the interface contains a pointer to.

For a more detailed exposition of the internals of interfaces, see for example https://research.swtch.com/interfaces.

Slice of pointers, when passed to object, gets pointers with other addresses

1 Answers1