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From: cjrgreen@concentric.net (Christopher Green)
Subject: Re: Alternate to Unchecked_Conversion - Portable?
Date: 1999/02/23
Message-ID: <36d3ba85.713118@nntp.concentric.net>#1/1
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References: <36d05e39.0@news.pacifier.com>
 <36d2638e.6427631@nntp.concentric.net> <7avpi0$jke$1@nnrp1.dejanews.com>
Organization: Concentric Internet Services
Newsgroups: comp.lang.ada
Date: 1999-02-23T00:00:00+00:00
List-Id: <comp.lang.ada>

On Wed, 24 Feb 1999 02:52:52 GMT, robert_dewar@my-dejanews.com wrote:

>In article <36d2638e.6427631@nntp.concentric.net>,
>  cjrgreen@concentric.net (Christopher Green) wrote:
>> It is primarily useful in situations in which alternative
>> implementations end up causing a bitwise copy.  If
>> the object to be converted is large, or the conversion
>> must be done many times, this can be a win.
>
>This seems bogus to me. If you replace this by unchecked
>conversion of *pointers* (i.e. access values), NOT the
>items themselves, then there is no bit copying, and no
>inefficiency at all in the access.
>
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The situations I'm thinking of involve conversion of
arrays of integers or pointers, large record objects,
and the like. You are surely right in that there is no
inherent reason why Unchecked_Conversion on
scalars and pointers should be inefficient.

For example, in X Window System programming, 
there are many C functions that return arrays of
indefinite size (the size must be obtained through
another argument or another function call).  In
this case, it is natural enough, though not fully
portable, to declare an Ada array of the actual
size and map it onto the C array with an address
clause.

In Ada 95, the package Interfaces.C.Pointers is
a better solution to this kind of problem.

-- 
Chris Green
Advanced Technology Center
Laguna Hills, California