Re: Preventing type extensions

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

Florian Weimer wrote on comp.lang.ada:
>>> Iterators are examples of pool-specific allocators in C++.
>
>> How so?  IIUC, iterators are not even a language construct, only an
>> idiom in the standard library.
>
> C++ is so flexible that you can express the concept of a pointer in a
> library.  In the same way, you could argue that C++ does not support
> numerics because it hasn't got a power operator. 8-)

Ah, I think I understand what you mean now: any type that has a
dereference operator (operator*() or  operator->(), IIRC) is a pointer
type, right? Sorry, I disagree with that. Pointer types are definite
and elementary. An arbitrary C++ type with operator* is not always
definite or elementary. This becomes important in the context of
safety-critical software; the fact that pointer types are elementary
guarantees that no dynamic dispatching occurs when you call an
operator of the type; that you can store them in registers, convert
them to and from hardware addresses and apply representation clauses
to them without worrying where to store the tag. The fact that they
are definite means you can store them efficiently in containers, with
shallow copy semantics.

Ah but perhaps C++ lacks indefinite types, too?

--
Ludovic Brenta.