From: David Kastrup <dak@gnu.org>
Date: Sun, 23 Sep 2007 19:11:30 GMT
Subject: Re: [OT] Re: C++ *for Git*
Message-ID: <857imhmc2l.fsf@lola.goethe.zz>
In-Reply-To: <e5bfff550709231143m7eb351bx4cf1c60d1247cc3d@mail.gmail.com>

"Marco Costalba" <mcostalba@gmail.com> writes:

> On 9/23/07, David Kastrup <dak@gnu.org> wrote:
>> "Marco Costalba" <mcostalba@gmail.com> writes:
>>
>> > And BTW
>> >
>> > template <typename T>
>> >
>> > is the thing in C++ that more remembers me of opaque pointers and
>> > their use in C, the difference is that the first is fully type
>> > checked.
>>
>> Not really.  The difference is that the first generates new (and
>> optimized) code for every type which is something you can only do
>> using macros in C.  Class programming is similar to opaque pointers
>> (in particular concerning the generated code) but templates are really
>> more like macros, as their instantiation generates specialized code,
>> not at all like the handling of opaque pointers.
>
> Probably if I had written like this was more clear:
>
> template <typename T>  int some_function(T* p);

Huh?  How is this supposed to support your point?  There is nothing
like an opaque pointer involved here.  The point of opaque pointers is
that they can stand for a variety of types, whereas each template
instantiation can only substitute a single type.

> And regarding 'new' code for each type I would like to remember that
> template instantations of different types can be removed by
> compiler/linker when the instantations are the same (i.e. produce
> the same binary instuctions), this could happen for function
> templates that handle pointers, as example.

Hardly.  The type constraints/virtual function tables of any called
function depending on T will be different.  And if indeed nothing
depends on T at all inside of the template, it is pointless not to
declare it as void *p in the first place: the type of *p will never be
used then.

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum