Re: Creating properties in Lua-only classes

Now, once more on the subject of creating/modifying properties in Lua:
overriding functions will work for modifying *existing* properties, but it
won't allow you to create *new *ones.  After some more digging and finding
mysterious references in the changelog to the "Lua property() function", I
eventually found some undocumented functionality; here is an example of
creating a brand new property in a Lua-only class:


class 'Window'

function Window:getAspectRatio()
    return self.width / self.height
end

function Window:setAspectRatio(aspect)
    self.height = self.width / aspect
end

function Window:__init(width, height)
    self.width = width
    self.height = height
    -- first parameter is getter, second parameter is (optional) setter
    self.aspectRatio = property( Window.getAspectRatio, Window.setAspectRatio )
end

-- normal property usage
win = Window(640, 480)
print( win.aspectRatio ) -- prints 1.3333333333333
win.aspectRatio = 1.25
print( win.height ) -- prints 512


Note that getters seem to be restricted to a single return value.



On Mon, Feb 21, 2011 at 3:59 PM, Kitten, Nicholas
<[email protected]>wrote:

> Wow, I just realized I was overthinking this way too much.  If I have the
> following forwarding class using wrap_base:
>
> struct WindowWrapper : Window, luabind::wrap_base
> {
>     WindowWrapper()
>         : base()
>     {}
>
>     virtual void SetHeight(int h)
>     {
>         call<void>("SetHeight", h);
>     }
>
>     static void BaseSetHeight(Window* ptr, int h)
>     {
>         return ptr->Window::SetHeight(h);
>     }
> };
>
>
> Then simply overriding SetHeight( ) in Lua will automatically work, because
>
> win.height = 3
> -- calls Window::SetHeight( )
> -- which calls Lua's version of SetHeight( ), in this case
> SubWindow:SetHeight( )
>
>
> So as long as I maintain a consistent naming convention (property "height"
> corresponds to "GetHeight" and "SetHeight"), then clients are free to
> override the getters and setters in Lua.
>
>
>
> On Mon, Feb 21, 2011 at 3:33 PM, Kitten, Nicholas <[email protected]
> > wrote:
>
>> I'll rephrase my question, with code this time:
>>
>> I have a C++ class, bound to Lua along these lines:
>>
>> luabind::class_<Window>( "Window" )
>>
>> .def( luabind::constructor<>( ) )
>>
>> .property( "height", &Window::GetHeight, &Window::SetHeight )
>>
>> .property( "width", &Window::GetWidth, &Window::SetWidth );
>>
>> /** wrap_base forwarding class omitted **/
>>
>>
>> Of course, in Lua this means I can do the following:
>>
>> win = Window()
>> win.width = 320        -- calls setter
>> print( win.width )       -- calls getter
>>
>> Now, I would like to allow a client to subclass Window from within Lua,
>> overriding the setters and/or getters with Lua functions, something like
>> this:
>>
>> class 'SubWindow' (Window)
>>
>> function SubWindow:SetHeight( newHeight )
>>
>> Window.SetHeight( self, newHeight )
>> --  other stuff
>>
>> end
>>
>> function SubWindow:SetWidth( newWidth )
>>
>>  Window.SetWidth( self, newWidth )
>> --  other stuff
>>
>> end
>>
>>
>> function SubWindow:__init( )
>>
>> Window.__init(self)
>> --[[
>>     magically bind derived SetHeight( ) and SetWidth( ) to __newindex
>> metamethod
>> --]]
>>
>> end
>>
>> win = SubWindow()
>> win.width = 320        -- calls SubWindow's setter
>> print( win.width )       -- calls Window's getter
>>
>>
>> As you can see, the only way I can think to do this would be by overriding
>> the __index and __newindex metamethods on the derived class, but because
>> Luabind classes are userdata, this seems impossible at the moment.  I know
>> the last comment I read from this list on the subject said it might possibly
>> be implemented with version .9 (even though firm plans were only for 1.0) -
>> has that happened?
>>
>> If not, is there a distinct mechanism within Luabind to override
>> properties in derived classes?  I remember seeing a comment in the source
>> saying that properties should "always be overridden, not overloaded", but I
>> couldn't discern how the overriding would happen in the first place.
>>
>> Thanks,
>> -Nick
>>
>>
>>
>> On Tue, Feb 8, 2011 at 12:06 PM, Kitten, Nicholas <
>> [email protected]> wrote:
>>
>>> Does anyone know how I would go about creating a property within a
>>> Lua-created class (using Lua functions for getters and setters)?  I'm
>>> guessing there's support in the metatables for doing it, but I've never seen
>>> an implementation.
>>>
>>> Thanks,
>>> -Nick
>>>
>>
>>
>

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