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Sender: David Brown <davidb@opus.davidb.org>
From: David Brown <cla@davidb.org>
Subject: Re: Ada's Slide To Oblivion ...
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Date: 2002-02-07T21:36:59+00:00
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Hyman Rosen <hyrosen@mail.com> wrote:
> Ray Blaak wrote:
>> You most certainly can:
>>        typedef void (*FUNC)();
>>        int i = 0;
>>        FUNC f = (FUNC) &i;
>> 
>> That this crashes with access violations on a sane OS is a good thing, but
>> nothing in the language is preventing the code/data conversions.
> 
> False. The typecast '(FUNC)&i' is not legal standard C or C++.
> It's not even a case of undefined results - it's just illegal.
> Some compilers permit it as an extension, though.

Legality aside, I have yet to find a compiler that doesn't accept this.
In fact, I have code that does this very thing, it creates a small
amount of data that happens to be instructions, casts it to a function
pointer and calls it.

There are a certain set of problems that would require hand-crafted
assembly to implement if this were not allowed.  These constructs are
usually (hopefully) only done in OS implementations, and runtime
libraries for languages.

Dave Brown