Inheritance

Like all object-oriented programming languages, Lua++ supports full class inheritance. To inherit all the heritable properties of a class in Lua++, one would separate the class name from the base class using a colon in this way: tag : base-class, and for multiple class inheritance, one would separate each base class with a comma: tag: base-1, base-2, ... as shown in the following example:

class Person {
    firstName: string,
    lastName:  string,

    constructor(firstName: string, lastName: string)
        self.firstName = firstName
        self.lastName = lastName
    end,

    function getName(): string
        return self.firstName .. " " .. self.lastName
    end,

    function getDescription(): string
        return "This is " .. self.getName() .. "."
    end
}

Here we define a base class Person, with the firstName and lastName properties. This class has two public methods, getName(), and getDescription(). As mentioned earlier, to inherit a class you use the : operator. For example, the following Employee class inherits properties and methods from the Person class:

class Employee : Person { 
    
}

Since the Person class has a constructor that initializes the firstName and lastName properties, you need to initialize these properties in the constructor of the Employee class by calling its parent class’ constructor. This can be done by using the base() keyword.

class Employee : Person {
    job: string,    
 
    constructor(firstName: string, lastName: string, 
                job: string)
        self.job = job
        
        -- Call the constructor of the person class
        base(firstName, lastName)
    end
}

The following creates an instance of the Employee class which inherits all the methods and properties of the Employee class:

local employee: Employee = Employee("Max", "Prihodko", "Programmer")

Lua++ also allows you to override methods inherited by a base class. This can be done by using the : symbol after the name definition of the class.

class Employee : Person {
    job: string,    
 
    constructor(firstName: string, lastName: string,
                job: string)
        self.job = job
        
        -- Call the constructor of the person class
        base(firstName, lastName)
    end,

    function describe(): string
        return base.describe() .. " I am a " .. 
                                    self.job .. "."
    end
}

local employee: Employee = Employee("Max", "Prihodko",
                                    "Programmer")

print(employee.describe())

The output of this fragment of code will be:

This is Max Prihodko. I am a Programmer.