Re: Ada access vs C/C++ pointers and references

["Nick Roberts" <nick.roberts@acm.org>]

On 18 Aug 2004 15:27:47 -0700, Keith H Duggar <duggar@mit.edu> wrote:

> [re array indexing]
> As you can see, method two saves one instruction by storing
> and manipulating a memory address directly.

In a loop, there is well known optimisation known (for reasons which
mystify me) as 'strength reduction', which allows a loop such as:

    for (i=0;i<n;i++) { ... a[i] ... };

to be transformed into the equivalent of:

    for (p=&a[0];p<&a[n];p++) { ... *p ... };

before emitting machine code. So an Ada loop such as:

    for i in 1..N loop
       ... A(i) ...
    end loop;

can be expected to be compiled into machine code which is as
efficient as the code that could be produced by using direct address
manipulation.

> First, is it correct to say that Ada access types are more
> similar to C++ references (T&) than they are to C/C++
> pointers (T*) ?

No. Ada access types are very similar to C and C++ pointers, and are
typically implemented and used in (almost) exactly the same way.

The rough Ada equivalent of C++ references are 'aliased objects'.

> Second, does Ada provide any facilities for direct and raw
> memory access by memory address and pointer arithmetic?

Yes it does. There are standard packages System.Storage_Elements and
System.Address_To_Access_Conversions, which provide facilities for
converting between access types and addresses, and performing integer
arithmetic on addresses, as well as Interfaces.C and
Interfaces.C.Pointers which enable Ada programs to access C strings,
arrays, and pointers.

> Third, if Ada does not provide arithmetic pointers do you
> know if this has played any part in acceptance of Ada for
> systems programming or efficiency critical applications?

I don't know (sorry).

> Fourth, have you experienced or can you think of any cases
> where pointer arithmetic has helped to solve a problem that
> would have been more difficult with Ada access types alone?

Not that I can recall.

-- 
Nick Roberts