Re: Help (gnat 3.04)

[nasser@apldbio.com (Nasser Abbasi)]

   From: cktan@kelly.teleport.com (Chong-Kwan Tan)

   Hi,

   I am trying to link all items in an array into a list and gnat 3.04 gives
   me this error:
       % gnatmake tt.adb
       gcc -c tt.adb
       tt.adb:21:37: object has deeper accessibility level than access type
       gnatmake: "tt.adb" compilation error

   Could someone tell me why?

   Thanks,
   -tan

   ====================================================================
	1  procedure TT is
	2
	3     type Item_Type;
	4     type Item_Ptr_Type is access all Item_Type;
	5     type Item_Type is
	6        record
	7           Next           : Item_Ptr_Type;
	8        end record;
	9     type Item_Table_Type is
       10        array (Positive range <>) of aliased Item_Type;
       11
       12     type Pool_Type is
       13        record
       14           Item    : Item_Table_Type(1..10);
       15        end record;
       16
       17     procedure Init
       18       (Pool : in out Pool_Type) is
       19     begin
       20        for I in 2 .. Pool.Item'Last loop
       21           Pool.Item(I-1).Next := Pool.Item(I)'Access;
       22        end loop;        
       23        Pool.Item(Pool.Item'Last).Next := null;
       24     end Init;
       25
       26     P : Pool_Type;
       27  begin
       28     Init(P);
       29  end TT;
   -- 

You are passing P as In OUT, this seems to be the cause of the problem.
the only think I could think of is that at line 21, the right hand
side is an access to the local copy of pool.item(I) that is passed as "IN",
so this copy of the P object sits on the call frame of procedure Init, i.e.
it is a local copy of P that Init is processing, and so, the program is 
trying to take the address of a local object and assign it to a object 
whose dynamic life is longer (the right hand side), since pool is also
passed in as  OUT parameter, which refers to object outside the procedure 
Init, and so you get the error. 

i.e. Inside Init, when doing pool.item(I)'access, this is getting the address
of the IN copy of P.  So, I guess this means one can not pass in an
object as IN OUT if one intends to gets its address ? it is the only
thinh that would explain to me as it seems on the surface the access type
life time rules are obeyed...

Any way, to Init pool , just  call Init with pool as OUT only and that 
will work, since this way the adress that is being taken belong to an
object that lives as long as what is being assigned to, which is itself,
so Ada is happy, end every one is happy, and life is good.

  procedure T is

     type Item_Type;
     type Item_Ptr_Type is access all Item_Type;
     type Item_Type is
        record
           Next           : Item_Ptr_Type;
        end record;
     type Item_Table_Type is
        array (Positive range <>) of aliased Item_Type;

     type Pool_Type is
        record
           Item    : Item_Table_Type(1..10);
        end record;

     P : Pool_Type;

     Procedure Init_ver2 (Pool: out Pool_Type) is
     begin
        for I in 2 .. P.Item'Last loop
           P.Item(I-1).Next := 
                          P.Item(I)'Access;
        end loop;     
        P.Item(P.Item'Last).Next := null;
     end Init_ver2;


  begin
     Init_Ver2(P);
  end T;


-- 
Nasser Abbasi. C/C++/Ada Solaris. GeneAssist - A client/server 
application for Nucleic acid and protein sequence search and analysis.
PE-Applied BioSystem division. email:  nasser@apldbio.com   
MSEE, MSCS, MSCE, FM (and Karpov is my chess hero! ..).