Re: gengtype needs for C++?

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 29 June 2010 00:19, Basile Starynkevitch wrote:
> On Mon, 2010-06-28 at 16:08 -0700, Ian Lance Taylor wrote:
>> Basile Starynkevitch <basile@starynkevitch.net> writes:
>>
>> > * I don't know exactly what should be wished with respect to templates.
>> > Tom Tromey (in CC) have a wonderful insight in
>> > http://gcc.gnu.org/ml/gcc/2010-06/msg00143.html but I probably did not
>> > understood all the details & the consequences. In particular I don't
>> > understand if the mark method in his example has to be supplied or
>> > generated.
>>
>> In Tom's interesting idea, we would write the mark function by hand for
>> each C++ type that we use GTY with.
>>
>> One way to make this approach work for plugins would be to write
>>
>> template<typename T>
>> void
>> mark(T& t)
>> {
>> ? t->mark();
>> }
>>
>> Then every plugin which invents new garbage collected types would have
>> to define a mark method for them.
>
> Isn't that precise statement a good use case for virtual methods?

No. Not at all.

> In other words, for that trick to work for plugins, don't we require the
> mark() method above to be virtual?

No.  The mark method just needs to do whatever is needed to mark the
type.  T& is not a reference to base. T doesn't need to derive from a
single base class. A virtual function is completely unnecessary.

The template function can be instantiated for any T, and will call a
function T::mark, it doesn't need to be virtual for that to work.

> I would be delighted with that, but I understood that many people don't
> want to have virtual methods.

There is no need to have virtual functions here.