🚀 go-pugleaf

In article 14306@nlm.nih.gov, bkline@cortex.nlm.nih.gov (Bob Kline) writes:
>
>I have found (on more than one occasion) published code which
>appears to rely on an assumption about c++ for which I haven't 
>been able to find any justification in the ARM or the draft
>standard itself.  The assumption is that a static array of
>char is guaranteed to be suitably aligned to hold objects of
>any types overlaid on the array's space.  Here is an example
>from James Coplien's "Advanced C++ Programming Styles and
>Idioms" (page 149, ff.):
>
>    unsigned char bigBuf[4096], *bigBufPtr = bigBuf;
>
>    class LAPD {
>    public:
>        void *operator new(size_t) { return (void *)bigBufPtr; }
>        ...
>    };
>
>The details of the class are elided here, but several pages later
>the data members are shown to include a couple of pointers, which
>(at least on some machines with which I am familiar) would cause
>things to blow up if they were not placed on a suitably aligned
>memory boundary.
>
>So my question is: does the draft standard require that all arrays
>of char be placed by the compiler at an address which would meet
>the alignment requirements for any object which might be placed
>there?  If so, where should I look in the draft to find this
>requirement?

An array of char must be suitably aligned for access to the char elements.
I don't believe there is any requirement that it be suitably aligned for
any other type. For this kind of general usage, I think you have to use an
array element type corresponding to worst-case alignment requirements.

For example, if long double has the severest alignment requirements (as on
the Sparc), you should write for example
	long double bigBuf[4096/sizeof(long double)];
	void* bigBufPtr = bigBuf;

It is a requirement that the space returned by operator new be suitably
aligned for any object of the requested size. The sample code probably works
on many systems, but I don't think the standard allows you to assume so.
---
Steve Clamage, stephen.clamage@eng.sun.com



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