Re: metatable - tostring,tonumber

[Tom Wrensch <twrensch@...>]

Here's a correction to my own mail:

I forgot that only tables and userdata can have metatables, so the code I
gave fails for numbers, strings, and such. To fix it the type of the
object must be checked using the type() function. So tostring might be:

    _tostring = tostring
    function tostring(obj)
        local tsf = _tostring
        if type(mt)=="table" or type(mt)=="userdata" then
            local mt = metatable(obj)
            if mt~=nil then
                tsf = mt.tostring or tsf
            end
        end
        return tsf(obj)
    end


Perhaps it would make sense to modify the metatable function so it returns
nil for objects that are not userdatas or tables?

  - Tom

On Tue, 19 Mar 2002, Tom Wrensch wrote:

> On Tue, 19 Mar 2002, nikdo79 wrote:
> 
> > Hi there,
> > 
> > there's no entry in the Metatable for tostring/tonumber?
> > I can't make a Object-Table to say it's class when being tostring'ed?
> > so that e.g.:
> 
> You can add this fairly easily: (untested code)
> 
>     _tostring = tostring
>     function tostring(obj)
>         local tsf = _tostring
>         local mt = metatable(obj)
>         if mt~=nil then
>             tsf = mt.tostring or tsf
>         end
>         return tsf(obj)
>     end
> 
> The version for tonumber is similar: (again, untested)
> 
>     _tonumber = tonumber
>     function tonumber(obj)
>         local tnf = _tonumber
>         local mt = metatable(obj)
>         if mt ~= nil then
>             tnf = mt.tonumber or tnf
>         end
>         return tnf(obj)
>     end
>         
> Of course you can also implement these in 'C' if you need to.
> 
>   - Tom
> 
> > myobject=Car.New{TopSpeed=160,Brand='Opel',Type='Corsa'}
> > 
> > write(tostring(myobject))
> > 
> > would result in:
> > Car Object: Opel Corsa, 160 km/h max
> > 
> > That's right, isn't it?
> > 
> > Dom
> > 
> > 
> 
>