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Re: pointer confusion
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From:
ToddAndMargo via perl6-users
Date:
December 6, 2022 23:40
Subject:
Re: pointer confusion
Message ID:
82497e41-6888-158c-e5ba-b40fba52c32b@zoho.com
On 12/5/22 16:35, Ralph Mellor wrote:
> On Mon, Dec 5, 2022 at 10:20 PM ToddAndMargo via perl6-users
> <perl6-users@perl.org> wrote:
>>
>>> use NativeCall;
>>> my Pointer $foo .= new: 42;
>>> say $foo; # NativeCall::Types::Pointer<0x2a>
>>> print $foo; # NativeCall::Types::Pointer<5895604297984>
>
> ----
>
> `say` concatenates the `.gist`s of each of its arguments. In this case
> `say` only has one argument, `$foo`, which contains a `Pointer`.
>
> The `.gist` of a `Pointer` is a concatenation of:
>
> * The full type name (`NativeCall::Types::Pointer`)
>
> and
>
> * `<...>` enclosing the pointer's value (the address it represents)
> in HEXADECIMAL literal form.
>
> ----
>
> `print` concatenates the `.Str`s of each of its arguments. In this case
> `print` only has one argument, again `$foo` which contains a `Pointer`.
>
> The `.Str` of a `Pointer` is a concatenation of:
>
> * The full type name (`NativeCall::Types::Pointer`) as with `say`.
>
> and:
>
> * `<...>` enclosing some OTHER number (NOT the pointer's value,
> ie not the address the `Pointer` represents) in DECIMAL literal form.
>
> I think this OTHER number is a number associated with the `ObjAt` of
> the `Pointer`.
>
> `ObjAt` is a high level Raku Object ID that's unique to the associated object.
>
> See https://docs.raku.org/type/ObjAt
>
> The key thing is that the integer number included in the `.Str` of a `Pointer`
> is NOT the address that that `Pointer` points to. It's something completely
> unrelated. Unless you know you need to pay attention to its `ObjAt` you
> are best advised to ignore it.
>
> ----
>
> To return to the outputs of the `say`/`.gist` and `print`/`put`/.Str` forms
> and add the `.raku` at the start:
>
> ```
> ...Pointer.new(42)
> ...Pointer<0x2a>
> ...Pointer<5895604297984>
> ```
> You thought the three numbers should be the same integer.
>
> That's part of the problem. It's a surmise that leads to surprise.
>
> ----
>
> Your original exploration was in the REPL.
>
> By default this REPL line and display uses `.gist`/`say`
> ```
> [3] > my Pointer $Ptr2Ptr = NativeCall::Types::Pointer[BYTES].new($x);
> NativeCall::Types::Pointer<0xfe45ddcc>
> ```
> (You can change from the default but let's move on.)
>
> If a REPL line explicitly writes to STDOUT, then the REPL uses
> that instead. So this line does NOT use `.gist`/`say`:
>
> ```
> [4] > print "x = <" ~ $x ~ "> <0x" ~ $x.base(16) ~ "> Ptr2Ptr <" ~
> $Ptr2Ptr ~ ">\n";
> x = <4265991628> <0xFE45DDCC> Ptr2Ptr
> <NativeCall::Types::Pointer<4852306736864>>
> ```
> You wrote `print`, and, as explained at the start, if you do that, the
> REPL accepts its output and displays that as is, no `.gist` involved.
>
> In the meantime, the `$Ptr2Ptr` is concatenated with `...<" ` and `">..."
> either side of it via two `~`s. The `~` op calls `.Str` on its operands.
>
> As explained above the `.Str` of a `Pointer` is not its `.gist` and instead
> returns a string that includes an `ObjAt` integer inside the `<...>` and
> that integer is not the address the `Pointer` points to.
>
> ----
>
> So, factors that are confusing.
>
> Please sleep on what you've learned, and perhaps rethink it through
> the next day before deciding whether you now find it makes sense or
> if it remains confusing. Then I'll follow up if you still find it confusing.
>
> Hth!
>
> --
> raiph
Yes it does. Thank you!
Oh, I figured out the "*ppXXXXX" double pointer.
It is "Point[Pointer]", for when one pointer is
not enough!
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