Re: object --> metatable binding

[spir <denis.spir@...>]

Richard Hundt <richardhundt@gmail.com> wrote:

> spir wrote:
> > Hello,
> > 
> > How is the binding of a metatable to an object recorded? Is there an external table of (object:metatable) pairs?
> > Additional question: why isn't it set on the object itself (actually, it may be, but I couldn't access it)?
> 
> Well, metatables live up to their name in that they are meta-level 
> entities, so isolation from the members of the table itself is 
> warranted, but a metatable is actually set directly on the object (where 
> "object" here is userdata or table). See lobject.h in the Lua sources, 
> for example:
> 
> typedef struct Table {
>    CommonHeader;
>    lu_byte flags;  /* 1<<p means tagmethod(p) is not present */
>    lu_byte lsizenode;  /* log2 of size of `node' array */
>    struct Table *metatable;a /* <== HERE */
>    TValue *array;  /* array part */
>    Node *node;
>    Node *lastfree;  /* any free position is before this position */
>    GCObject *gclist;
>    int sizearray;  /* size of `array' array */
> } Table;

Thank you, very informative.

> Other types: string, number, function, cfunction and lightuserdata share 
> metatables which are saved in the global_State struct...
> 
> > 
> > Also is there another idiom for:
> >    setmetatable(t1, getmetatable(t0))
> 
> ... so you can't really get around calling getmetatable and setmetatable 
> functions (or Lua API equivalents), because the metatable lookup and 
> setting is type dependent.
> 
> However, I seldom need to copy a metatable from one table to another, I 
> prefer to keep them in variables.

Yes, that makes sense. Guess I'll do the same from now on (esp. for "type" cloning).

> Hope that helped.

Very much. Things are clear, now :-)

> Cheers,
> Richard Hundt
> 


Denis
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la vita e estrany

http://spir.wikidot.com/