Re: Multidimensional Arrays

[Lorenzo Donati <lorenzodonatibz@...>]

On 22/07/2011 19.11, Steve Litt wrote:
[...]
> If I understand the situation correctly, Lua has exactly two types for
> complex data:
>
> 1) Tables
> 2) Metatables
>
> All other complex data is built from those two and doesn't exist in
> the language.

Nope. The only "structured" data type are tables. Metatables are not 
another type. Metatables are plain Lua tables. They are attached to 
other types in order to modify the behaviour of some operations on those 
types.

Every table can have its own metatable (which, as I said, is a plain 
table itself). Example:

meta = {} -- plain table
t = {}
-- the following sets the table "meta" as the metatable for table "t"
setmetatable( t, meta )

The previous example is fairly silly, because an empty metatable doesn't 
do much.

A more interesting case is this:

meta = {}
meta.__call = function( self, ... )
 -- the first arg (self) gets the
 -- table which was "called" (see below)
 print( ... )
end
t = {}
setmetatable( t, meta )

In this case the metatable has a so-called metamethod "__call" which 
renders the table on which the metatable is attached "callable". When 
"called" the table t dispatches its arguments to the metamethod 
"__call", so that the call:

t( "hello", "there", 3.14 )

is equivalent to the call:

meta.__call( t, "hello", "there", 3.14 )

and thus prints:

hello	there	3.14


Adding appropriate metamethods to "meta" above you can customize many 
other aspects of the behaviour of "t", but "meta" remains still a plain 
table.

Note: only tables and userdata can have individual metatables, all other 
types have a single metatable per type (not per-object).
Anyway, to set a metatable for types other than tables, you must use 
debug.setmetatable or resort to C API.


[...]

> SteveT

-- Lorenzo