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- Subject: Re: Re: mettables and the io library
- From: Luiz Henrique de Figueiredo <lhf@...>
- Date: Tue, 19 Aug 2003 07:33:11 -0300
>I would dearly love a slow, painstakingly detailed explanation of
>the metable implementation.
File handles are represented as userdata. To be able to use the syntax
f:read() on for file handles, we must set an __index metamethod for file
handles. We do this by adding an "__index" field to the metatable of file
handles. The value of this field is the table itself because that's where we
store the read functions and its friends. (Note that he value of the "__index"
field can be a function or a table; if it is a table the fields are resolved
in the given table.) We might have stored read and friends in another table,
but metatables are ordinary tables so we use it.
The fields "_input" and "_output" are used to store the current values of
file handles returned by io.input() and io.output().
A similar, but simpler, technique is used in my bindings for C libraries that
receive handles as their first argument. For instance, you can write both
md5.digest(d) or d:digest(), where d is an md5 context, because the metatable
for md5 contexts has an "__index" field that points to the md5 table. So,
d:digest(), which is sugar for d.digest(d), gets translated to md5.digest(d).