Re: calling an ada procedure from C++

["Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net>]

"Ludovic Brenta" <ludovic.brenta@insalien.org> wrote in message
news:m38ymezgo4.fsf@insalien.org...
>
> I think some compilers will also return small structures in processor
> registers.  They will neither use the stack not any static memory, if
> they can help it.

Yes, come to think of it, I have seen that for small structures, and I
should have included that in my list of mechanisms.

> The basic point remains: how each compiler passes structures between
> efunctions is compiler-specific.  It is however possible, both in C
> and in Ada, to specify the exact bit pattern for a stucture, and thus
> to pass the structure back and forth.

Even that is not necessarily true for all C compilers! The standard allows a
great deal of lattitude in the placement of bit field. According to the 89
standard (I'm not sure about C99), if you define

    struct Jello
    {
    unsigned Flavor : 5
    unsigned Servings : 3
    };

the compiler is free to implement this by placing Flavor in the high order
bits or in the low order bits. This creates a portability problem that has
resulted in the underuse of bit fields.

> I think the most portable way
> would be to pass pointers to the structures (as earlier suggested) but
> if the OT uses GNAT (where the compiler handles both C and Ada), there
> may be more efficient ways using representation clauses and Pragma
> Convention.
>
> BTW, C++ is a different beast altogether.  I would avoid it if
> possible, especially if the C++ struct is really a class with methods
> and parents or descendants.  Better stick with a strictly C struct.

The ACT people have provided support for interfacing to C++. In general, the
ACT and GNU people are to be congraduated for tackling the issue of
multi-language OO programming.

> My 2 cents.
>
> -- 
> Ludovic Brenta.