Digg: If (thought experiment) Go had "const" for variables meaning "you can only set this once"...
var x const int = 32 ...and you tried to use unsafe to get the address of x and remove its const-ness and then assign through it the value 9999 to x, what do you think would and should happen? a. x=9999, which seems what you expected b. x=32, a silent failure (frightening to debug!) c. panic, the observed behavior Even though this is just a thought experiment, it is VERY CLOSE to the true situation for Go's immutable strings. https://golang.org/ref/spec#String_types "Strings are immutable: once created, it is impossible to change the contents of a string." What you are trying to do is to make this impossible change. Don't do that. The result, even if you could do it, would not be Go. On Mon, Feb 26, 2018 at 5:44 AM, Rob Pike <r...@golang.org> wrote: > No. As I said, there are no guarantees. As it says in > https://golang.org/doc/go1compat: > > > - Use of package unsafe. Packages that import unsafe > <https://golang.org/pkg/unsafe/> may depend on internal properties of > the Go implementation. We reserve the right to make changes to the > implementation that may break such programs. > > > > > On Mon, Feb 26, 2018 at 1:15 PM, <d...@veryhaha.com> wrote: > >> >> >> On Sunday, February 25, 2018 at 7:38:22 PM UTC-5, Rob 'Commander' Pike >> wrote: >>> >>> The main rule about unsafe is that your program might work or might not. >>> There are no guarantees either way. That's why it's called 'unsafe' and why >>> you shouldn't use it. Your program that 'works' today could break tomorrow. >>> >> >> ok, but, is there always an unsafe alternative/variant existing later? >> >> >>> >>> -rob >>> >>> >>> On Mon, Feb 26, 2018 at 4:43 AM, Marvin Renich <mr...@renich.org> wrote: >>> >>>> * di...@veryhaha.com <di...@veryhaha.com> [180225 11:37]: >>>> > I think I get it. >>>> > Because the above program tries to modify the constant (or program) >>>> zone, >>>> > which is not allowed. >>>> > The following program works: >>>> >>>> But, note that the language spec does not guarantee it to work. The >>>> compiler is free to recognize what that first line is doing and optimize >>>> the assignment into a string in a R/O memory segment. The optimization >>>> is legal because the compiler does not have to recognize the use of >>>> unsafe to determine the programmer's intent to subvert the type system. >>>> >>>> > package main >>>> > >>>> > import "fmt" >>>> > import "unsafe" >>>> > import "reflect" >>>> > >>>> > func main() { >>>> > s := string([]byte{'k', 'e', 'e', 'p'}) >>>> > hdr := (*reflect.StringHeader)(unsafe.Pointer(&s)) >>>> > byteSequence := (*byte)(unsafe.Pointer(hdr.Data)) >>>> > fmt.Println(string(*byteSequence)) // k >>>> > *byteSequence = 'j' // crash here >>>> > fmt.Println(s) // expect: jeep >>>> > } >>>> >>>> ...Marvin >>>> >>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "golang-nuts" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to golang-nuts...@googlegroups.com. >>>> For more options, visit https://groups.google.com/d/optout. >>>> >>> >>> -- >> You received this message because you are subscribed to the Google Groups >> "golang-nuts" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to golang-nuts+unsubscr...@googlegroups.com. >> For more options, visit https://groups.google.com/d/optout. >> > > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- Michael T. Jones michael.jo...@gmail.com -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
