Wireshark · Wireshark-dev: Re: [Wireshark-dev] Changes to internal Lua classes implementation (Pinfo, TvbRange, etc.)

[Peter Wu <peter@xxxxxxxxxxxxx>]

On Mon, Oct 03, 2016 at 10:39:08AM +0100, Graham Bloice wrote:
> On 3 October 2016 at 00:51, Peter Wu <peter@xxxxxxxxxxxxx> wrote:
> 
> > Hi,
> >
> > To fix bug 12968 (__gc being called for tables, resulting in lua_error
> > while exiting), I modified the way how classes are registered in
> > https://code.wireshark.org/review/18026
> >
> > In these changes I also reduced the use of macros, following Joãos
> > concerns. Macros like WSLUA_META should most likely become typedefs.
> > Below I will give a motivation for the changes (feedback welcome!).
> >
> >
> > Consider class Address with function Address.ip and metamethods like
> > address.__tostring. Previously you could have these strange invocations:
> >
> >     -- note: "Address" is the global class, "address" is an instance.
> >     tostring(Address)   -- invokes metamethod __tostring (error!)
> >     address.ip("foo")   -- invokes "static" class function
> >     tostring(FileHandler)   -- error
> >     filehandler.new(...)    -- huh?
> >
> > In the proposed change, there will be different method tables and
> > metatables for the class ("Address") and its instances ("address").
> > Further modification could disable the above badness, allowing just:
> >
> >     tostring(address)
> >     Address.ip("foo")
> >     tostring(filehandler)
> >     FileHandler.new(...)
> >
> > This might break some dissectors that use non-documented invocations,
> > but enables use of separate __call functions for example:
> >
> >     field = Field("x")  -- __call in metatable of class Field
> >     fields = field()    -- __call in metatable of instance of Field
> >
> > Also changed is that attributes are no longer visible on the class, only
> > its instances:
> >
> >     filehandler.read_open = x   -- ok, sets callback attribute
> >     -- previously failed in the setter callback out because "self" is
> >     -- not a FileHandler instance. Now it will already fail while
> >     -- looking up the attribute (in the __newindex metamethod).
> >     FileHandler.read_open = x
> >
> >
> >
> > Distinct metatables are good, but what about different method tables?
> > The proposal will also disable invocations like Tvb.len(tvb) assuming
> > that nobody wants to do it (tvb:len() is saner).
> >
> > Any objections with removing __setters/__getters/__methods?
> > I doubt that dissectors use this. Users can read the WSDG and study the
> > source code to discover available functions.
> >
> >
> > Future work, in order (assuming the current proposal):
> >  - grep for WSLUA_REGISTER_ATTRIBUTES, change their ClassName_register
> >    functions to use the new wslua_class definition mechanism, including
> >    setting the "attrs" member.
> >  - Remove wslua_reg_attributes and macros.
> >  - grep for WSLUA_REGISTER_META and WSLUA_REGISTER_CLASS to change the
> >    remaining ClassName_register files.
> >
> > If you feel that the Lua core code needs some love, help is appreciated
> > in these tasks :-)
> > --
> > Kind regards,
> > Peter Wu
> >
> 
> As a non-lua programmer most of that goes straight over my head, but I'm
> intrigued\concerned about methods that are class based (i.e. similar to C++
> class static methods ??) and instance based that differ only by the case of
> the initial method letter.
> 
> Is this normal for Lua so won't be a surprise for users, or is it likely to
> surprise\confuse users and cause lots of issues and support queries?
>
> -- 
> Graham Bloice

So far all methods have lowercase letters which seems to be a fine
convention. If by "case" you refer to the "field" and "Field" examples
above, there is a class "Field" that results in an instance "field" on
invocation.

In C++ you can invoke the static and non-static (instance) methods on an
instance, but instance methods cannot be invoked on the class itself:

    C c;
    c.static_fn();  // note: in the Lua proposal this is also forbidden
    c.normal_fn();
    C::static_fn();
    C::normal_fn(); // error: cannot call member function without object

The Lua proposal seems to match Ruby which also distinguishes class
methods from normal methods on instances. Unfortunately I was not able
to find examples of Lua classes where they class methods in addition to
instance methods. What would be intuitive to you?
-- 
Kind regards,
Peter Wu
https://lekensteyn.nl