Re: [announcement] SYSAPI and SYSSVC for Windows

From: Robert A Duff (bobduff_at_shell01.TheWorld.com)
Date: 12/24/03

  • Next message: Stephen Leake: "Re: Progress with GNAT on bare MIPS" 
    Date: 23 Dec 2003 18:02:45 -0500

    "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

    > Ekkehard Morgenstern wrote:

    > > so I could've just used a tagged limited record?
    > >
    > > Like this:
    > >
    > > type T is tagged limited
    > > record
    > > A : My_Array_Type;
    > > end record;
    > >
    > > procedure F ( O : in out T ) is
    > > Ptr : My_Array_Cell_Ptr;
    > > begin
    > > Ptr := O.A(1)'Access;
    > > end;
    > >
    > > Right?
    >
    > No. It is unrelated. The thing you are getting access of has to be aliased.
    > So it is the array elements which has to be, for example:
    >
    > type T is limited private;
    > private
    > type Integer_Array is array (Integer range <>) of aliased Integer;
    > type T is limited record
    > A : Integer_Array (1..3);
    > end record;
    >
    > procedure F (O : in out T ) is
    > begin
    > ... O.A(1)'Access; -- This is OK, A(i) are aliased

    No, that's not quite good enough. The parameter O is considered to be
    nested within F, so you need 'Unchecked_Access instead of 'Access here.
    Whenever you use 'Unchecked_Access, you have to make sure you don't
    use dangling pointers -- so the & operator in C or C++ is more like
    'Unchecked_Access than 'Access in that regard.

    The point is: when you say 'Access, the compiler can prove that you
    don't have dangling pointers. Otherwise, you need 'Unchecked_Access
    (but then you better prove it yourself, or your program might do bad
    things).

    > end;
    >
    > If the array elements be tagged, then you would need not write "aliased" in
    > the array declaration.

    That's not quite right. Tagged *parameters* (like O in the above
    example) are automatically aliased. But other objects are aliased only
    if declared so (by the "aliased" keyword) or if allocated in the heap by
    "new" (whether tagged or not).

    IMHO, it was a mistake to make tagged parameters automatically aliased.
    We should, instead, have allowed the "aliased" keyword on parameters.

    Summary: To get an access value to an existing object, you must first
    make sure it's aliased (which means allocated on the heap, explicitly
    declared "aliased", or a tagged parameter). Then you must worry about
    accessibility level (which determines whether you should use 'Access or
    'Unchecked_Access).

    - Bob

  • Next message: Stephen Leake: "Re: Progress with GNAT on bare MIPS"

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