Use conditional casting with symtab_node

[Lawrence Crowl]

On 9/19/12, Gabriel Dos Reis <gdr@integrable-solutions.net> wrote:
> On Wed, Sep 19, 2012 at 1:39 PM, Lawrence Crowl <crowl@googlers.com> wrote:
>> On 9/19/12, Gabriel Dos Reis <gdr@integrable-solutions.net> wrote:
>>> On Sep 19, 2012 Richard Guenther <richard.guenther@gmail.com> wrote:
>>> > Indeed.  Btw, can we not provide a specialization for
>>> > dynamic_cast <>?  This ->try_... looks awkward to me compared
>>> > to the more familiar
>>> >
>>> > vnode = dynamic_cast <varpool_node> (node)
>>> >
>>> > but yeah - dynamic_cast is not a template ... (but maybe there
>>> > is some standard library piece that mimics it?).
>>>
>>> No, it is a language primitive.
>>>
>>> but we can define out own operation with similar syntax that allows
>>> for specialization, whose generic implementation uses dynamic_cast.
>>>
>>>   template<typename T, typename U>
>>>   T* is(U* u) {
>>>       return dynamic_cast<T*>(u);
>>>   }
>>
>> At this point, dynamic_cast is not available because we do not
>> yet have polymorphic types.  There has been some resistance to
>> that notion.
>
> Hmm, when did we rule that out?

We have not ruled it out, but folks are, rightly, concerned about any
size increase in critical data structures.  We are also currently
lacking a gengtype that will handle inheritance.  So, for now at
least, we need a scheme that will work across both inheritance and
our current tag/union approach.

> We currently implement dynamic_cast using the poor man's simulation
> based on tree_code checking.  We can just as well give such
> simulation the is<> notation.
>
>> Absent dynamic cast, we need to specialize for various type
>> combinations.  Function template specialization would be handy,
>> but C++ does not directly support that.  We could work around
>> that.
>
> We can always use the standard workaround: call a static member
> function of a class template that can be specialized at will.
>
>> However, in the end, the fact that try_whatever is a member
>> function means that we can use a notation that depends on context
>> and so can be shorter.  That is, we can write 'function' instead of
>> 'cgraph_node *'.

-- 
Lawrence Crowl