Re: Using C++ in GCC is OK

[Basile Starynkevitch <basile at starynkevitch dot net>]

On Sun, 2010-05-30 at 17:26 -0700, Mark Mitchell wrote:
> I am pleased to report that the GCC Steering Committee and the FSF have
> approved the use of C++ in GCC itself. [...]
> 
> For example, I think it goes without question that at this point we are
> limiting ourselves to C++98 (plus "long long" so that we have a 64-bit
> integer type); C++0x features should not be used.  Using multiple
> inheritance, templates (other than when using the C++ standard library,
> e.g. std::list<X>), or exceptions also seems overly aggressive to me.
> We should use features that are relatively easy for C programmers to
> understand and relatively hard for new C++ programmers to misuse.  (For
> example, I think constructors and destructors are pretty easy and hard
> to misuse.)


Either I wrongly understand this last sentence, or I rather strongly
disagree with it, at least within our current practice inside GCC.

I would believe on the contrary that constructors & destructors can too
easily be misused, in particular for GGC garbage collected data that GCC
need to have. See also http://gcc.gnu.org/wiki/gengtype

In general, and independently of GGC, constructor should only
*initialize* the object, and should avoid doing any processing on it. So
ideally, a constructor should only initialize the own field of the class
(after having implicitly called the parent constructor), and not do much
else. I believe it can be tempting for newcomers to add a lot more
processing inside constructors, and this IMHO should be avoided.

Symetrically, destructors should clear objects; however, we should be
very careful about destructors of GTY-ed data.

At last, there is a very important issue when switching to C++. What is
our "ideal" class hierarchy? Do we aim at a large forest, or on the
contrary at a single tree of classes, so we have  a single root class,
providing common services (dump or debug printing, serialization, ...)
or not? What is the relation between our C++ classes & gengtype or GGC
(I am strongly convinced that even with C++ we need a garbage collection
to tame the complexity of GCC and of our circularities in data).

Actually, I would believe that many of our C++ class declarations should
be generated by a gengtype like tool, which would add support for
garbage collection & for some meta-data on the classes. Generating the
class declarations [perhaps even from an input which has a simpler
syntax -e.g. an S-expr syntax- than what gengtype parses today] is far
more easier than extending gengtype to parse all the class syntax
permitted in C++. That could be difficult.

There is also the case of RTTI & dynamic_cast? Do we allow them or not?

What about data structures with a last field which is an array (e.g. in
gimple.h the gimple_statement_with_ops).

Regards.

-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
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