Re: Initialization Params for Controlled Types

[comar@cs.nyu.edu (Cyrille Comar)]

ncohen@watson.ibm.com (Norman H. Cohen) writes:
: In article <1994Nov4.134412.10010@unix.brighton.ac.uk>,
: je@unix.brighton.ac.uk (John English) writes: 
: 
: |> There is one subtle danger with using procedures as "constructors".
: |> Imagine the following: 
: |>   type BankAccount is tagged private;
: |>   procedure Open (Account : in out BankAccount;
: |>                Name    : in String);
: |>
: |> Here Open opens a bank account in a particular name.  Now if you
: |> derive a new type CurrentAccount from BankAccount to produce a
: |> BankAccount that allows overdrafts, you might have this: 
: |>   type CurrentAccount is new BankAccount with private;
: |>   procedure Open (Account   : in out CurrentAccount;
: |>                Name      : in String;
: |>                Overdraft : in Money);
: |>
: |> Here's where the danger is: the first procedure (Open for BankAccount)
: |> is a primitive of BankAccount and will be inherited by CurrentAccount.
: |> It's therefore possible to initialise a CurrentAccount as if it were
: |> a BankAccount, which might leave the overdraft limit uninitialised: 
: |>   C : CurrentAccount;
: |>   ...
: |>   Open (C, "Fred Bloggs");  -- oops!
: |>
: |> Okay, you can provide a default overdraft limit of 0.00, so this is
: |> ...
: |> C++ sidesteps this problem by making constructors non-inheritable.
: |> The only way to accomplish this in 9X is to make sure that Open isn't
: |> a primitive (e.g. by shoving it in a child package) which is fairly
: |> painful.

you have an exact equivalent in Ada. If you don't want to make Open a primitive
operation of BankAccount define it as a classwide operation:

  procedure Open (Account : in out BankAccount'Class;
                  Name    : in String);

It will not be inherited! I do not say that it is the perfect solution for your
problem, my point is that it is exactly equivalent to defining Open as a
non-virtual member function in C++.

-- 
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Cyrille Comar,                                  E-mail: comar@cs.nyu.edu
Gnat Project                                    US phone: (212) 998-3489