Re: "Must instantiate controlled types at library level." Why?

["Peter C. Chapin" <pchapin@sover.net>]

Martin Krischik <krischik@users.sourceforge.net> wrote in
news:2780491.NPbR8AFya6@linux1.krischik.com: 

> Well, here you have a good example of the difference of Ada and C++. Ada
> has a complete ban on the construct while C++ goes out and about to ban
> only the tricky part.

It's interesting to reflect on these different approaches. C++ can get 
away with imposing such restrictions on nested functions because nested 
functions are rarely used in C++. In Ada similar restrictions on nested 
procedures would be entirely unacceptable. Consequently Ada imposes 
restrictions in a different place in order to avoid the same fundamental 
problem.

> Just in case you did not know: 
> 
> P is tagged ....
> P_Class is access P;
> C is new P ....
> 
> A_C : C := ...
> A_P : P'Class := P'Class (A_C); 
> A_P_Ptr : P_CLass := new P'CLass'(A_C);
> 
> A_P and A_P_Ptr will contain a copy of C.

Interesting. I don't completely follow the code above because I haven't 
gotten to this material yet in my study. In the statement

A_P : P'Class := P'Class(A_C);

Is A_C "sliced" into a P (to use a bit of C++ terminology)? I'm not 
certain what declaring A_P as type P'Class is doing for me.

> PS: I hope you don't mind my other post to much. You do dig deeper and
> you do know the ISO standard so my rant about the average C++ programmer
> does not apply to you.

No offense taken. Certainly C++ is a rather twisted language, especially 
syntactically. I'm enjoying Ada because it seems much cleaner while still 
being very powerful and expressive.

Peter