A macro for safely accessing fields deep in protobufs

[mbautin]

It would be good to have a macro to replace something like

        replicate->has_write_request() &&
            replicate->write_request().has_write_batch() &&
            replicate->write_request().write_batch().has_transaction());

with

PB_HAS_NESTED_FIELD(replicate, write_request, write_batch, transaction)

and similarly PB_NESTED_FIELD and PB_MUTABLE_NESTED_FIELD to get the actual field.

We can even avoid repeated function calls (in the example above, to replicate->write_request()) in this macro's implementation.

[robertsami]

@mbautin might be missing something here, but wouldn't the following two always evaluate to the same value?

replicate->has_write_request() &&
            replicate->write_request().has_write_batch() &&
            replicate->write_request().write_batch().has_transaction();

vs.

replicate->write_request().write_batch().has_transaction();

since replicate->write_request() would return a default value if not present, which would lead to replicate->write_request().write_batch() returning default value as well, which would lead to replicate->write_request().write_batch().has_transaction() returning false