06-underlying-value.md

Underlying value

Last time on Go interface values:

• Go interfaces are really two uintptr values that address the underlying type and value stored in the interface
• The address of the type points to an internal Go type where an equivalent to a reflect.Kind value is stored
• Type information is global and shared by all values stored in interfaces
• The address of the value may sometimes be shared even across distinct types such as int32 and int64?

Okay, now that we are all caught up, let's take a quick look at the example from the previous page (Golang playground):

package main

func main() {
	println(interface{}(int32(3)))
	println(interface{}(int32(5)))
	println(interface{}(int64(3)))
	println(interface{}(int64(5)))
}

We learned the value addresses for int32(3) and int64(3) are the same as well as int32(5) and int64(5):

• 0x476598: value address for int32(3) and int64(3)
• 0x4765a0: value address for int32(5) and int64(5)

The reason for this is because we stored constant values in interfaces, and Go's compiler is smart enough to reference the same value multiple times when it is small enough to fit into the width of a larger type (as both 3 and 5 fit into both an int32 and int64). If we were to refactor things a bit so 3 and 5 are no longer constant (Golang playground):

package main

func main() {
	a, b := int32(3), int32(5)
	x, y := int64(3), int64(5)
	println(interface{}(a))
	println(interface{}(b))
	println(interface{}(x))
	println(interface{}(y))
}

We see all the value addresses are distinct from one another:

(0x459d80,0xc00003475c)
(0x459d80,0xc000034758)
(0x459dc0,0xc000034760)
(0x459dc0,0xc000034768)

There's one more thing. Consider the following example (Golang playground):

package main

func main() {
	a, b := byte(3), byte(5)
	x, y := uint8(3), uint8(5)
	println(interface{}(a))
	println(interface{}(b))
	println(interface{}(x))
	println(interface{}(y))
}

What do you think will happen? Join me on the next page and we will find out together!

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Next: Special values