Re: Object or table?

[Dirk Laurie <dpl@...>]

On Tue, Aug 23, 2011 at 03:24:34PM +0200, Stefan Reich wrote:
> My question arises when tables are sent as method arguments. There are
> two cases:
> 
> -It's a plain table. Then it should be copied to the receiving sandbox.
> -It's an object. Then it should be passed by reference to allow remote calling.
> 
> The question is: How do I distinguish a table from an "object"? There
> is no clear-cut difference in Lua as far as I understand.

The difference is this: 'table' is a Lua type. 'object' is not:
it is only a style of programming.  You have the privilege and the 
responsibility to define what you mean by 'object'.  

For instance, you might consider something to be an object if all 
functions that are intended to access it, sit inside its metatable.
No way of testing that, though.

> My current solution is to consider a table an object if it contains
> only functions. But maybe you just want to pass an array that happens
> to contain a function. (Function references are also possible.)
> 
> Maybe it would make sense to consider anything with a metatable (even
> if the metatable is empty) an object and anything without a metatable
> a table.
> 

If you take PiL §16.1, §16.2 as your model for object-oriented
programming, the following property is a possible definition:

    function isobject(a)
        local mt = getmetatable(a)
        return mt and mt.__index == mt
        end

Dirk