Re: Access to tagged type parameters

[Jonathan DeSena <jonathan.desena@jhuapl.edu>]

Ted Dennison wrote:

> In article <pybf7.17453$ZM2.1548231@newsread2.prod.itd.earthlink.net>,
> David Brown says...
>>
>>Jonathan DeSena <jonathan.desena@jhuapl.edu> wrote:
>>
>>> "All formal parameters belonging to tagged
>>> types are implicitly declared to be aliased. This allows a subprogram to
>>> create access values pointing to its tagged formal parameters using the
>>> 'Access attribute ..." (section 12.6.2, page 579 in my version)
>>> 
>>> non-local pointer cannot point to local object
>>
>>>      B_Access: A_Access_Type;
>>
>>The pointer access type A_Access_Type has a scope outside of the scope of
>>the object you getting the access value of.
> 
> Actually, it doesn't though. The actual pointed-to object is B, which is
> declared at the same scope as A_Access_Type.
> 
> It looks like the compiler is considering C (the formal parameter) to be
> the pointed to object instead of B (the actual parameter). I guess it can
> do that, but then I don't much see the point of the rule specifying that
> tagged types are always passed by reference. By this rule a tagged
> by-value pointer would be just as safe (and useful). I guess it still has
> use for 'Unchecked_Access...
> 
> FWIW, GreenHills tags the same line with the following error:
> LRM:3.10.2(28), The prefix to 'ACCESS shall not be statically deeper than
> that of the expected type, Continuing
> 
So, does that mean the original quote is incorrect, or just that some 
compilers do not impliment it correctly? 

>>If you use 'Unchecked_Access,
>>you will get the access you want, however, the it is up to you to make
>>sure it doesn't go outside of scope.
> 
> Another possibility is to use the "access" parameter mode instead of "in
> out". In that case, the accessability rules seem to behave as I would
> expect.
Unfortunately, the reason I want to do this is to be able to add the passed 
in tagged type, say type A_Type, to a routine which will add it's access 
value to an array of access values, say  A_Access_Type.  Thus I WANT to be 
able to do the equivalent of the assignment B_Access := B'Access. If B is 
type access to A_Type'Class, I cannot do B_Access:=B, or something. On the 
otherhand, if I use the type A_Access_Type as the paramater instead, the 
assignment B_Access:=B works (since B & B_Access are now the same type), 
but the routine is not inhereted to derived types of A_Type. This means 
that it is very difficult to use tagged types AND access types together and 
still take advantage of inheritance. (Does that paragraph make any sense, 
or do I need to clarify? It now seems unwieldy to me. Oh well) 

I suppose 'Unchecked_Access might do the trick, but I don't know enough to 
know where the pitfalls of using it are. Also, this seems overly 
complicated just because of using a tagged type.

I tried to remove as many access types as possible to simplify matters, but 
because I am using a recursive type, they are required (won't compile 
without them).  Because of Ada's strong typing (a good thing, I agree), 
once I use access types, they seem to proliferate throughout the package.

Thanks again,
jtd