Polymorphism

The idea of this section is to explain how inheritance may be implemented in order to enable polymorphism among messages. There are two methods, depending on the proto version that we are using:

• Proto 2 uses Extensions
• Proto 3 deprecates Extensions in favor of Any statement

Extensions are a range of tags reserved for future extended messages so you can use them for different purposes:

syntax = "proto2";

message Film {

    extensions 50 to 99;

}

message TerrorFilm {

    extend Film {

        required TerrorFilm film = 50;

    }

    bool has_zombies = 1;

}

message ScienceFictionFilm {

    extend Film {

        required ScienceFictionFilm film = 51;

    }

    bool has_jedis = 1;

}

In terms of binary serialization the final message will look the same as a non extended message, but in terms of code generated for accessing those fields, it’s a different story. Here is an example for setting the extended field film in a Java implementation.

ScienceFictionFilm...