08-copy-on-store.md

Copy on store

The examples have gotten pretty complex, but that has been necessary in order to ask this question: What is being assigned to y in the following example?

var x int64
var y interface{}
x = 2048
y = x

Because of how interfaces "wrap" underlying types and values, we might be tempted to say The variable y is assigned x. Except that is not entirely accurate. Let's use a different example:

var x int64
var y int64
x = 2048
y = x

Same question. I think we would all answer The variable y is assigned the value of the variable x. That would be correct. And we know that modifying x later would not affect y (Golang playground):

package main

import "fmt"

func main() {
	var x int64
	var y int64
	x = 2048
	y = x
	fmt.Printf("x=%d, y=%d\n", x, y)

	x = 4096
	fmt.Printf("x=%d, y=%d\n", x, y)
}

The above example of course emits:

x=2048, y=2048
x=4096, y=2048

So why would think anything different should happen when y is an interface (Golang playground)?

var y interface{}

The program emits the same output as before:

x=2048, y=2048
x=4096, y=2048

This is because storing a value in an interface results in a copy of that value, just like when assigning a value to another variable of the same type. Even if the value is a pointer, you are creating a copy of that pointer.

This does raise the question though -- if an interface is just two addresses to the underlying type and value, where is the copied value of y? We know it's:

• not in the interface itself
• not in staticuint64s (see Special values) as the value is greater than 255

So where is it? It is in one of two locations, and we are about to take a look at the first one...

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