Oneof

Oneof becomes very handy in terms of space optimization when you have many optional fields in a message and know that at most only one will be set. The implementation of Oneof shares the same memory space for all the fields, so it doesn’t need to reserve space for the other tags having empty fields. Of course with the language implementation, you should have a WhichOneOf method in order to determine what was the set value.

syntax = "proto2";

message Film {
    message Character {
        oneof has {
            string laser_gun_model = 1;
            int32 laser_sword_power = 2;
        }
    }
}

In this example setting laser_gun_model or laser_sword_power allows you to know which type of weapon the Character has. Looking into the value in the example provides you with additional information (gun model as a string, or laser power as an integer).