Why does calling methods on a protobuf Message throw a pure virtual method called error?

Question

I'm trying to use the Google Protobuf library and I want to store a bunch of different message types together in a container and get their names as I pull them out of the container. I think I can use the interface type google::protobuf::Message to do this. Here is what I have so far.

#include <iostream>
#include "addressbook.pb.h"

using namespace std;

int main(void) {
  vector<shared_ptr<google::protobuf::Message>> vec;
  
  {
    tutorial::AddressBook address_book;
    vec.push_back(shared_ptr<google::protobuf::Message>(&address_book));
  }

  cout << "Typename is " << vec.back()->GetTypeName() << endl;
  return 0;
}

Calling GetTypeName throws the following error:

pure virtual method called
terminate called without an active exception
Aborted (core dumped)

Note this is me playing around with the tutorial found here: https://developers.google.com/protocol-buffers/docs/cpptutorial

You can not instantiate a `protobuf::Message` object, because it is an abstract interface class with no implementation. You have to derive your own implementation adhering tot hat interface. — Devolus, Jan 05 '22 at 16:51
I'm not instantiating an `Message` object though. I have a pointer to it. — TenderShortGoldenRetriever, Jan 05 '22 at 16:55
Apart from anything else you pass ownership of the locally scoped object `address_book` to a shared pointer and then access that object *after* it has gone out of scope. You have undefined behaviour. — G.M., Jan 05 '22 at 16:59
`address_book` is on the stack it will be deleted when it goes out of scope, nothing can prevent that. Just create your book with `std::make_shared` — al3c, Jan 05 '22 at 17:02

score 0 · Accepted Answer · edited Jan 13 '22 at 21:50

0

address_book is on the stack it will be deleted when it goes out of scope, no smart pointer can prevent that.

Just create your book with std::make_shared, that will be on the heap and its lifetime will be managed from the std::shared_ptr.

{
    auto address_book = shared_ptr<google::protobuf::Message>(new tutorial::AddressBook);
    vec.push_back(address_book);
}

edited Jan 13 '22 at 21:50

TenderShortGoldenRetriever

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answered Jan 06 '22 at 09:40

al3c

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Why does calling methods on a protobuf Message throw a pure virtual method called error?

1 Answers1