Why golang struct array cannot be assigned to an interface array

Question

I'm trying to achieve something as below.

package main

import (
    "fmt"
)

type MyStruct struct {
    Value int
}

func main() {
    x := []MyStruct{
        MyStruct{
            Value : 5,
        },
        MyStruct{
            Value : 6,
        },
    }
    var y []interface{}
    y = x // This throws a compile time error

    _,_ = x,y
}

This gives a compile time error:

sample.go:21: cannot use x (type []MyStruct) as type []interface {} in assignment

Why is this not possible?.If not is there any other way to hold generic object arrays in Golang?

Because array of a generic type is not equal to array of a struct. Just use var y interface{} instead of array. Type interface{} can be used to store any type of variable in Golang. — putu
– putu, Commented Jun 2, 2017 at 2:42
Possible duplicate of golang: slice of struct != slice of interface it implements? — Adrian
– Adrian, Commented Jun 2, 2017 at 13:24

Zak · Accepted Answer · 2017-06-02 08:20:01Z

39

interface{} is stored as a two word pair, one word describing the underlying type information and one word describing the data within that interface:

https://research.swtch.com/interfaces

Here we see the first word stores the type information and the second the data within b.

Struct types are stored differently, they do not have this pairing. Their fields of a struct are laid out next to one another in memory.

https://research.swtch.com/godata

You cannot convert one to the other because they do not have the same representation in memory.

It is necessary to copy the elements individually to the destination slice.

https://golang.org/doc/faq#convert_slice_of_interface

To answer your last question, you could have []interface which is a slice of interfaces, where each interface is represented as above, or just interface{} where the underlying type held in that interface is []MyStruct

var y interface{}
y = x

or

y := make([]interface{}, len(x))
for i, v := range x {
    y[i] = v
}

edited Jun 2, 2017 at 8:20

answered Jun 2, 2017 at 8:14

Zak

5,90825 silver badges36 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Adrian Moisa Over a year ago

Will this duplicate everything in RAM? I don't see pointers used so I guess everything is copied by value.

Zak Over a year ago

If you use the second option: "or just interface{} where the underlying type held in that interface is []MyStruct" then slices are already reference types, so there would be no copying of the underlying data. If you copy your data out of the []MySlice and into []interface{}, then yes, you will have a copy of the data.

Collectives™ on Stack Overflow

Why golang struct array cannot be assigned to an interface array

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related