2. The general problem that I was trying to illustrate is that shared ownership requires either reference counting or avoiding it through copying if you don't have automatic memory management.
Go is GCed language. Show me the code that will do what you claim (map[var1] = var2 and use var1(type string) and var2(type string) in later code) in Go.
let addr s = 2 * (Obj.magic s) + 1
let main () =
let table = Hashtbl.create 0 in
let key = "foo" and value = "bar" in
Hashtbl.add table key value;
Hashtbl.iter (fun key' value' ->
Printf.printf "%b %b %x %x %x %x\n"
(key == key') (value == value')
(addr key) (addr key') (addr value) (addr value')
) table
let () = main ()
true true 10cbd2490 10cbd2490 10cbd2378 10cbd2378
Note that == is reference equality in OCaml.
I'm honestly baffled why you think that there is a need to copy the variables or why you can't use them later or whatever you believe there.
> I'm honestly baffled why you think that there is a need to copy the variables or why you can't use them later or whatever you believe there.
What you wrote is SPECIAL case in SOME of the GC'd languages that have immutable strings. If you have language with mutable strings then this will not work. I know it will not work in Go also and they have immutable strings (so even not all gc'd languages with immutable strings optimize this) that's why I've asked for Go code in Go thread.
I don't really use Go and made a general comment about GC that was not limited to Go in a subthread about fairly general observations about GC, because somebody made a general statement about GC that was not limited to Go.
The language is not relevant for the observation I made. Nor does it really matter if we're storing strings or other heap-allocated objects. It's a question of lifetime management.
> What you wrote is SPECIAL case in SOME of the GC'd languages that have immutable strings.
OCaml strings are mutable, actually. But you seem to be confusing lifetime issues with aliasing issues, anyway.
The underlying problem is that without copying C++ would not know when to free the strings. It would be whenever they went out of scope in the calling function or when they were deleted from the map, whichever is later; the alternative is reference counting (also expensive, and not used by std::string). A GCed language can avoid that because the garbage collector will only free them when they are no longer reachable from either (or any other location).
https://news.ycombinator.com/item?id=14116435
You replied "No" to that what I wrote about Go which is not true. I never even once wrote about lifetimes. Don't you see it? Look closely again what I wrote in my comments. I think you are confused about what I was disagreeing because you didn't read carefully what I wrote. I agree with what you wrote about lifetimes 100% and I was before that discussion started because I write C++ and those things are basic knowledge. But it wasn't about lifetimes from the beginning...
> The language is not relevant for the observation I made. Nor does it really matter if we're storing strings or other heap-allocated objects.
No, because what you wrote is only true depending on the language and depending on where those strings are stored and in which language, because in some cases they will be copied so what you wrote at the beginning would not be entirely true. And I disagreed with that only, I didn't mention lifetimes even once.
If instead of what you wrote you would write (big letters to show diff):
- in C++, m[a] = b, IN SPECIAL CASE where m is a map, and a and b are strings AND A AND B OUTLIVE MAP ASSIGNMENT requires that both a and b are being copied UNLESS YOU USE SHARED OWNERSHIP. This is not necessary in SOME OF GCed languages in SPECIAL CASES.
Then I would have no reason to disagree with you in the first place.
Then you wasted a lot of time for both of us by getting sidetracked by a detail that I hadn't even mentioned in my original comment. This is about the issues with having one object being referenced from multiple locations. I gave a concrete example to illustrate this issue, and you spent an entire subthread arguing the specifics of the example without seeming to understand the issue it was meant to illustrate. This is not specific to maps or strings. It's a general issue of manual memory management whenever you're dealing with shared ownership of the same object.
> You replied "No" to that what I wrote about Go which is not true.
I think you are reading to much into an expression of disagreement. You kept fixating on the specifics of go, I was trying to get back to the semantics of GCed languages vs. manual memory management. That's what my "no" was about.
Edit: I also did a quick test for Go, just to end that part of the argument, too. Contrary to your statement, Go doesn't seem to copy strings when adding them to maps, either. While there's no way to take the address of a string literal in Go, you can add lots of instances of a very large string and check the memory usage (compared to an empty string). It turns out to be nearly the same.
> I think you are reading to much into an expression of disagreement. You kept fixating on the specifics of go, I was trying to get back to the semantics of GCed languages vs. manual memory management. That's what my "no" was about.
There was only one sentence in that comment to which you could write No, next time if you want to get back to lifetimes in discussion with someone, write "I want to discuss lifetimes" instead of writing "No" to something that someone wrote, you see a difference? Words matter.
> I also did a quick test for Go, just to end that part of the argument, too. Contrary to your statement, Go doesn't seem to copy strings when adding them to maps, either. While there's no way to take the address of a string literal in Go, you can add lots of instances of a very large string and check the memory usage (compared to an empty string). It turns out to be nearly the same.
No, you are wrong. Look at runtime hashmap implementation and generated assembly (go tool objdump binary_name > asm.S) for function that do what I was writing in my comments. You will see that there is a copy.
Because you were derailing the discussion and I was trying to get it back on track. My whole original comment was about lifetimes and GC vs. manual memory management. You were getting sidetracked by details that weren't relevant for that point, which was specifically about GCed languages vs. manual memory management in general and didn't even mention Go [1].
> No, you are wrong. Look at runtime hashmap implementation and generated assembly (go tool objdump binary_name > asm.S) for function that do what I was writing in my comments. You will see that there is a copy.
If this were true, the following program would take >65 GB of memory. In reality, it requires some 110 MB.
package main
import ("strings"; "fmt")
func main() {
m := make(map[int]string)
value := strings.Repeat(".", 65536)
for i := 0; i < 1000000; i++ {
m[i] = value
}
fmt.Println(len(m))
}