Three new books, Go Optimizations 101, Go Details & Tips 101 and Go Generics 101 are published now. It is most cost-effective to buy all of them through this book bundle in the Leanpub book store.

const 4

What is the behavior of the following program?

package main

import "unsafe"

const S = "go" // S[1]-S[0] == 8

func main() {
	var x *[8][8]byte
	println(unsafe.Sizeof((*x)[S[1]-S[0]][S[1]-S[0]]))
}

Choices:

It prints 1.
It panics at run time.
It fails to compile.

Answer: it princs 1.

Run it on Go play.

Key points:

For a constant string S, the expression S[i] is always treated as a non-constant (please read the stirngs in Go article or Go Details and Tips book). So the program compiles.
unsafe.Sizeof calls are evaluated at compile time. In the evaluations, only the type information of the arguments are used. The expressions *x and S[1]-S[0] etc are never evaluated at run time. So no "index out of range" panics will occur.

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Articles in this book:

slice: 1, 2, 3
map: 1
channel: 1
loop: 1, 2, 3, 4
switch: 1
defer: 1, 2
panic/recover: 1, 2
reflect: 1, 2
const: 1, 2, 3, 4
operator: 1, 2, 3
function call: 1
scope: 1, 2
nil: 1
embedding: 1

(more are coming ...)