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From: mg@harp.camb.inmet.com (Mitch Gart)
Subject: parameters in bindings
Date: 1996/03/22
Message-ID: <DooE1w.MCH.0.-s@inmet.camb.inmet.com>#1/1
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sender: news@inmet.camb.inmet.com (USENET news)
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organization: Intermetrics, Inc.
newsgroups: comp.lang.ada
Date: 1996-03-22T00:00:00+00:00
List-Id: <comp.lang.ada>

Suppose we have a C function

    int f (int * param1, int ** param2);

(For the non-C crowd this is a function named f which takes 2 
parameters and returns an integer.  The first parameter is a 
pointer to an integer and the second is a pointer to a pointer 
to an integer.)

How do we best interface to this in Ada 95?

Method 1 uses "in out" parameters and would be the best Ada
interface (ignoring the question about "in out" versus "out"):

    type int_access is access all int;		-- int defined somewhere

    function f (param1: in out int;
		param2: in out int_access) return int;

This is unfortunately illegal, out parameters are not allowed for 
functions.  There have been long threads discussing this restriction
and it sounds like it will not be changed.

Method 2 uses anonymous access types.  I have used this method all
over the place in a number of bindings such as X11Ada and Win32Ada.

    type int_access is access all int;		-- int defined somewhere

    function f (param1: access int;
		param2: access int_access) return int;

I have recently learned this is a fundamental error because an Ada 95 
rule makes passing null for param1 or param2 illegal (see LRM 4.2(7)
and 4.6(49)).  Many of the C functions in the bindings allow null
to be passed, with a meaning like  "if the parameter is non-null assign
a result to the place where it points".

Method 3 uses all named access types:

    type int_access is access all int;		-- int defined somewhere
    type int_access_access is access all int_access;

    function f (param1: int_access;
		param2: int_access_access) return int;

This works but to me it seems ugly because extra access types are 
introduced which are otherwise unnecessary.

Because of the illegal-null problem in method 2 I guess I'm going to
make a wholesale change to the bindings and use method 3.  Before I
make that change does anybody have any better ideas?

- Mitch Gart
- mg@inmet.com