Class Inheritance

Inheriting classes within the config is a concept that can easily go wrong. The purpose of this document is to clarify the cause of some common errors and contains instructions on how to resolve these mistakes.

Terms

• A config is a config.cpp or config.bin file that is loaded during the game start. It is does not refer to a Model Config.
• The master config is the in-game one and only config built from all other configs. This includes both the base game configs as well as addon configs. It can be viewed through the Splendid Config Viewer.
• bin or config.bin is the father of all other configs. It forms the first state of the master config.bin and consists of the base game configs.
• child configs are added on top of the config.bin. The order in which they get added is defined by the requiredAddons array within the CfgPatches (see addon loading order).

Basic config concepts

The config is a hierarchical structure based on classes that provides almost all information necessary for the game. Objects, their behaviour, user interface elements and even which functions to run on game start are defined through the config.

Classes can contain either child classes (see below) or contain properties. Properties work similar to script variables, they have a name and a value. However unlike script variables properties can only have either number or string values, or (one- or multidimensional) arrays of number or string values.

Parent and child classes

Due to the hierarchical nature of the config, classes can have parent, child and sibling classes:

class Isaac {
    class Esau {
        gender = "male";
        firstborn = 1;
        ...
    };
    class Jacob {
        gender = "male";
        firstborn = 0;
        ...
    };
};

In this example, the parent class of Esau is Isaac, Esau and Jacob are child classes of Isaac, and Jacob is a sibling class of Esau.

Inherition

A class can inherit properties from sibling classes by defining it as a base class. This means that all properties of that base class will also exist within your class and, unless you overwrite them, will have the same values as in the base class. This is extremly useful for quickly writing sibling classes that share properties, or to build a class on top of another class that has the same properties with only minor value changes.

Taking the example from above we can either create a common ChildMaleBase base class:

class Isaac {
    class ChildMaleBase {
        gender = "male";
    };
    class Esau: ChildMaleBase {
        firstborn = 1;
        ...
    };
    class Jacob: ChildMaleBase {
        firstborn = 0;
        ...
    };
};

Or we can use Esau as a base class for Jacob and overwrite the firstborn property:

class Isaac {
    class Esau {
        gender = "male";
        firstborn = 1;
        ...
    };
    class Jacob: Esau {
        firstborn = 0;
        ...
    };
};

Both will have the same result in the master config, but note that in the latter example Jacob is dependant on Esau which is logically not correct - one can exist without the other. Especially when having more than one sibling class, you should consider creating a base class for good practice.

External base classes

Especially when writing the config for a new vehicle or changing the config of an existing vehicle, you will sooner or later come on contact with external base classes. External base classes are simply base classes that you did not define within your own config but were defined either by the base game or another addon.

It's a simple concept to understand but not as easy to implement correctly.

When using an external base class the first thing you have to do is declaring it. This tells the game engine that there is a sibling class within the master config that your class is somehow based on. It is only important to declare the base class of your class, there is no need to declare the base class of the base class and so on:

class externalBaseClass;                   //declare external base class
class myClass: externalBaseClass {         //define that you inherit from it
    ...                                    //start using it
};

Now what if you want to access a child class of the base class of your own class? At this point, you are accessing the contents of the base class, and thus you need to declare the base class of the base class as well:

class externalBaseClass;
class externalCoolClass: externalBaseClass {
    class externalChildClass;
};
class myClass: externalCoolClass {
    class externalChildClass: externalChildClass {
        ...
    };
    ...
};

This might seem complicated at first but you only have to follow these two rules:

• If your (child) class is based on an external class, declare the external class first
• If you open the brackets of an external class, declare the base class of that class as well

Addon loading order

When you are using external base classes it is extremly important to define what addons your addon depends on, or in other words, which external base classes need to be already loaded when your config gets loaded. Not defining the load order will guarantee you to end up in a world of pain and horror.

Defining which addons are required by your config is easily done through the CfgPatches:

class CfgPatches {
    class MyAddon {
        ...
        requiredAddons[] = {"ExternalAddonA", "ExternalAddonB", ...};
        ...
    };
};

This ensures that whatever changes you made or whatever classes based on some other classes you defined are applied on top of the external addons child config. Without this there is no guarantee that your config gets applied correctly.