Re: Preventing type extensions

[Florian Weimer <fw@deneb.enyo.de>]

* Ludovic Brenta:

> Florian Weimer writes on comp.lang.ada:
>> * Ludovic Brenta:
>>
>>> Another area where Ada helps certification is where it distinguishes
>>> pool-specific from general access types.  C and C++ are both completely
>>> helpless in this respect.  Consider:
>>>
>>> type T_Access is access T'Class; -- no equivalent in C++
>>> type T_General_Access is access all T'Class; -- equivalent to T* in C++
>>
>> 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-)

>> It's also common to define your own pointer types in C++.
>
> Do you mean that
>
> typedef T* T_Pointer;
>
> is common?

Many C++ libraries have their own variants of the std::shared_ptr and
std::unique_ptr templates.

>> It's hard to beat the C++ type system in terms of expressiveness.
>
> I disagree; I just showed the opposite in my previous post.  C++ does
> not have specific pointer types, for example; only class-wide ones.

C++ does not need them because the language is so expressive that you
can *define* your own pointer types.  Their type constructor is
syntactically different from language-provided pointers.  But a
factorial function in Ada is syntactically different from built-in
operators, too.  Does that mean that Ada does not properly support the
factorial function?  Clearly not.