Hi Liam,
Currently there's no way to do this.
But I think it would lead to a different kind of type unsafety anyway: If
you accept a T::Builder, the implication is that it doesn't need to be the
root builder of the message. If someone passed an inner object, or an
orphan, what should happen?
What I'd suggest instead is that you encapsulate MessageBuilder into your
own API, such that people can only construct a MessageBuilder of the right
type. Two ways you might do that:
1) You could define a type `FooMessage` which privately contains a
MallocMessageBuilder and which has a getRoot() method that only returns the
appropriate builder type. Then have your project require a `FooMessage`
instead of a `MesasgeBuilder`. If you have a lot of root types, you could
of course make this a template.
template <typename T>
class TypedMessage {
public:
typename T::Builder getRoot() { return inner.getRoot<T>(); }
private:
capnp::MallocMessageBuilder inner;
};
2) If your framework controls the scope in which the message is supposed to
be constructed (like, your framework calls "up" to the application to
request a message), then it could allocate the MessageBuilder internally
and pass the T::Builder up to be filled in, so the app never sees the
MessageBuilder. This is what Cap'n Proto RPC does, for example. This has
the neat advantage that your framework can play with message allocation
strategies. E.g. Cap'n Proto RPC aims to be able to allocate messages
directly inside a shared memory segment, if using a shared memory
transport, without the application knowing that this is the case.
-Kenton
On Wed, Nov 22, 2017 at 1:10 PM, Liam Staskawicz <[email protected]> wrote:
> Hello,
>
> Part of my current project provides a mechanism to transmit capnp messages
> and I'd like to accept a T::Builder as input, rather than a MessageBuilder
> if possible, to help avoid messages of the incorrect type being sent on a
> channel.
>
> Is it possible to access the MessageBuilder that a T::Builder is
> associated with through the T::Builder? Is there a better way to approach
> this issue?
>
> Thanks!
> Liam
>
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