Q: pointers to capnp::Builder

[Stefan Kaps]

Hi,

i would like to use pointers to builders in my exporter, so I can pass nullptr in case of "please simply do a check, but do not build capnp-data".  Is this possible at all?

If not, are there any suggestions you have for me how to provide such functionality?

Thanks a lot in advance,

Stefan

[Kenton Varda]

Hi Stefan,

A Builder already acts like a pointer: You can copy it by value, and the copy manipulates the same underlying message.

If you just want to express the type "capnp::Builder or null", then I suggest using `kj::Maybe<capnp::Builder>`, which serves exactly this purpose. A `kj::Maybe<T>` can be assigned to any `T` or to `nullptr`. You can unpack a Maybe with `KJ_IF_MAYBE()`. Check out the comments in `kj/commen.h` and copious examples throughout the KJ and Cap'n Proto source code to learn more.

-Kenton

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[Stefan Kaps]

Oh, very cool.  I had not yet stumbled upon it.  This is definitly better than my solution, as it most likely won't generate any code where it's not needed.

Just an hour ago I wrote my own little hacky thing: a simple asm function that returns its first parameter.

extern "C" {
    myT *refPtr( myT & );
};

As the asm simply does

    mov rax, rcx
    ret

Works fine as we know under the hood references are pointers.  I would hope the CPU itself would optimize around the call-ret, seeing only one uOp being produced before reaching the "ret".

Am Donnerstag, 5. September 2019 00:43:18 UTC+2 schrieb Kenton Varda:

Hi Stefan,

A Builder already acts like a pointer: You can copy it by value, and the copy manipulates the same underlying message.

If you just want to express the type "capnp::Builder or null", then I suggest using `kj::Maybe<capnp::Builder>`, which serves exactly this purpose. A `kj::Maybe<T>` can be assigned to any `T` or to `nullptr`. You can unpack a Maybe with `KJ_IF_MAYBE()`. Check out the comments in `kj/commen.h` and copious examples throughout the KJ and Cap'n Proto source code to learn more.

-Kenton

On Wed, Sep 4, 2019 at 11:10 AM Stefan Kaps <st0f...@gmail.com> wrote:

Hi,

i would like to use pointers to builders in my exporter, so I can pass nullptr in case of "please simply do a check, but do not build capnp-data".  Is this possible at all?

If not, are there any suggestions you have for me how to provide such functionality?

Thanks a lot in advance,

Stefan

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[Kenton Varda]

Hmm, can't that function be equivalently written as:

myT* refPtr(myT& ref) { return &ref; }

?

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[Stefan Kaps]

Yes, indeed.  I was troubling my mind of the lifetimes of those references.  And without thinking about it, I just went to work, hehe.  But indeed, as I do recursion in that case, the Builder's lifetime does never end before the called function returned.

Thus my asm is garbage and I don't even need

kj::Maybe()

...