在Lua中进行类型检查的OOP

Question

I'm trying to implement classes with inheritance in Lua. Additionally, when creating new objects, there needs to be some kind of "type-checking".

One particular requirement is to use a specific syntax for creating a class (example):

Class({Volkswagen, Vehicle, model=String, speed=Number, driving=Boolean})

Please note that the parameters are passed as a list with variable length. So I need to have a function that gets an arbitrary number of arguments:

function Class (arguments)
    name = arguments[1]
    superclass = arguments[2]
    ...
end

The first two arguments are the name of the class and the name of the class it is inherited from.

I'm sure the solution is simple, but I have absolutely no idea how to handle the key-value pairs of the attributes, for example...

print(arguments.model)
print(type(arguments.model))

...are both nil. Any ideas how i can get the type of the attributes (String, Boolean, Number)?

Answer 1

This line:

{Volkswagen, Vehicle, model=String, speed=Number, driving=Boolean}

Does not mean what you think it means. It's the values.

When you do {Volkswagen} , what you're doing is telling Lua to grab the value currently stored in the global table under the string name "Volkswagen" . I'll assume that there's some value stored there.

Your problem is that {model=String} is putting the value currently stored in the global table under the string name "String" into the table. Odds are good that there is no global variable named "String" . Which means you put nil in the table.

Unless LuaJava defines them, the "attributes" (String, Boolean, Number) do not exist. They're just empty values. You can't pass an empty value. You can pass a string , but that's different.

You didn't explain very well what exactly this Class function of yours should be doing, so I can't give you a hint on how to do what it is you're trying to do better.

Answer 2

If you're just looking for a generic Lua class system, here's my implementation: https://gist.github.com/1722329

It doesn't support type checking, but that should be easily implemented with a "TypedClass" class to act as a base for all strictly-typed classes.

Answer 3

For handling the key value pairs you may need to transfer the referenced table to a local table using the built in table function:

function Class(params)
local arguments={}

function transfer(k, v)
  arguments[k]=v
end

--transfer referenced table to local table
table.foreach (params, transfer)

print(arguments[1])
print(arguments[2])
print(arguments.model)
print(arguments.speed)
print(arguments.driving)

end

Class({"Volkswagen","Vehicle", model="" , speed=0 , driving=false} )