How to serialize an array of polymorphic objects to file using Protocol Buffers?

Question

Having an array std::vector<BaseClass*> what is the correct way of saving this array to a file using Google's Protocol Buffers library?

BaseClass is a base class of hierarchy of classes, it has several sub classes. Is Google's Protocol Buffers even suitable for this purpose or maybe another library would be preferred?

Answer 1

Protocol Buffers allow for repeated fields these act like std::vector in your code. As for polymorphic objects, you can use the extension framework. See here under the extension header.

Answer 2

You could create a list-message MyList which contains elements of type Class . And there you need a specific message for each subclass:

message MyList{
    repeated Class entry = 1;
}

message Class{
    required BaseProperties baseProperties = 1;

    oneof{
        SubClassOne sub_one_properties = 2;
        SubClassTwo sub_two_properties = 3;
        ...
    }
}

message BaseProperties{
    //contains common properties of BaseClass
}

message SubClassOne{
    //contains specific properties of one this SubClass
}

message SubClassTwo{
    //contains specific properties of one this SubClass
}

If you don't like the oneof keyword, or are using an older libprotobuf, you can also insert an enum with typeinformations and add corresponding optional messagefields:

message Class{
    enum ClassType{
        SUB_CLASS_ONE = 1;
        SUB_CLASS_TWO = 2;
    }

    required ClassType type = 1;
    required BaseProperties baseProperties = 2;

    optional SubClassOne sub_one_properties = 3;
    optional SubClassTwo sub_two_properties = 4;
    ...
}

Answer 3

As it said above, you have to use extension mechanism to implement polymorphism. This link you may find useful http://www.indelible.org/ink/protobuf-polymorphism/