Re: [PATCH 15/16] gdb: make cmd_list_element var an optional union

[Lancelot SIX <lsix@lancelotsix.com>]

On Thu, Jul 29, 2021 at 11:47:10AM -0600, Tom Tromey wrote:
> >>>>> "Lancelot" == Lancelot SIX <lsix@lancelotsix.com> writes:
> 
> Lancelot> Would you prefer to have something like var.get<var_string> () return a
> Lancelot> string happily if var is lets say a var_filename but fail if it is a
> Lancelot> var_boolean?
> 
> Yeah.
> 
> Lancelot> I guess it is possible, but I did intentionally went for a very explicit
> Lancelot> approach.  The idea is to have the user of the class state that he is
> Lancelot> aware he will access a data of a given type that can be interpreted in
> Lancelot> different ways.  This kind of makes more sense when dealing with the
> Lancelot> various var_*integer* types where the same value can have different
> Lancelot> meaning depending on the effective var_type.
> 
> I don't understand how to reconcile this with the idea that one could
> pass in a bunch of var_ parameters and get some result back.
> Like does get<var_integer, var_zinteger> work?

Yes, given that both var_integer and var_zinteger use a int*, this would
work. However, something like get<var_uinteger, var_zuienteger> will
fail to compile because var_uinteger uses unsigned int* while
var_zuinteger uses a int*.

> 
> I guess the argument would be that the author of that line is supposed
> to understand what's happening and that the meaning of the value might
> "vary"?

It is the original idea.  Multiple "var_types" can share the same
storage type while still being different logical types (a given value is
to be interpreted differently based on the actual type).  The original
idea was to use it in a 'get<var_sometype>' way but this was conflicting
with how this is used.  I followed that path to see where it lead.

> For me that seems to add a bunch of extra template code but
> without any real gain in safety.

I’ll revisit all of this in a V2 later.  I was worried about the amount
of template boilerplate this adds compared to the value it brings, I
should have worried a bit more!  If I strip the extra template noise,
this can come down to some get_string/set_string/get_int/set_int and so
on.  The 'get<var_?>' can be a light abstraction on top of that, if ever
needed (I am not sure it is).

Thanks for the comments, much appreciated.

Best,
Lancelot.

> 
> Tom