Go: comparing anonymous structs

Question

I don't understand how go compares anonymous structures. I am trying to understand this piece of code:

package main

import (
    "fmt"
)

type foo struct {
    bar string
}

func main() {
    var x struct {
        bar string
    }
    var y foo
    fmt.Println(x == y) // this prints true
    equals(x, y) // this prints false

}


func equals(a, b interface{}) {
    fmt.Println(a == b)
}

Why does x == y yields true? They have a different type, so I would expect that they cannot be compared.

And, as they are equal, why casting them to interface{} makes them unequal?

Answer 1

Why does x == y yields true?

From the Go language specification:

Struct values are comparable if all their fields are comparable. Two struct values are equal if their corresponding non-blank fields are equal.

The zero-value for a string is "", so x.bar and y.bar are equal, and therefore x and y are equal.

---

why casting them to interface{} makes them unequal?

Again, from the same page in the language specification:

Interface values are comparable. Two interface values are equal if they have identical dynamic types and equal dynamic values or if both have value nil.