They are well-known by their fully-qualified name, e.g.
"google.protobuf.DoubleValue". And you can get the fully-qualified names
via C++ reflection.
So you're marshaling logic woudl need to special-case the relevant message
names and subsequently can make assumptions about their structure (since
they are "well known"). As far as "auto-unboxing", keep in mind that one
significant use case for these well-known types is that they can be nil,
whereas there is obviously no analog for the unboxed primitive type (zero
is the closest, but then you cannot distinguish between whether the
original was nil or explicit zero).
*Josh Humphries*
jh...@bluegosling.com
On Fri, Jul 27, 2018 at 2:08 PM Chris Buffett
wrote:
> I'm wondering if protobuf supports a way to determine if a type is a
> well-known type via reflection in C++? I'm working on a marshalling layer
> for a custom encoding format and I need to unbox well-known types into
> their primitive type (e.g., Double to double). I've so far been unable to
> find any information on if it's possible to definitively tell if a type is
> well known or not, other than .
>
> For example, if I have the following protobuf,
>
> message A
> {
> message B
> {
> double val = 1;
> google.protobuf.Doublevalue prev_val = 2;
> }
>
> repeated B values;
> }
>
> I need to convert into something with the following format
> class B
> {
> double val;
> double prev_val;
> }
>
> class A
> {
> list values;
> }
>
> The issue I see is that based on the protobuf format, the output would be
> ambiguous. I could also generate the following because I'm unable to tell
> whether I should unbox the type or generate a wrapping class.
>
> class B
> {
> double val;
> class C
> {
> double prev_val;
> }
> }
>
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