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From: ncm@nospam.cantrip.org (Nathan Myers)
Subject: Re: Minimum Size of Array in Standard (solution)
Date: 1998/11/12
Message-ID: <72e9ql$mgp$1@shell7.ba.best.com>#1/1
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Approved: stephen.clamage@sun.com (comp.std.c++)
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Hans Olsson<Email_To::.Hans.Olsson@dna.lth.se> wrote:
>Nathan Myers <ncm@nospam.cantrip.org> wrote:
>>Hans Olsson<Email_To::.Hans.Olsson@dna.lth.se> wrote:
>>>Nathan Myers <ncm@nospam.cantrip.org> wrote:
>>>>There is a standard-conforming way to get a similar effect.
>>>>
>>>>  template <class T>
>>>>    struct varstruct 
>>>>  {
>>>>    union {
>>>>      unsigned int num_elements;
>>>>      T dummy[(sizeof(unsigned int)+sizeof(T)-1)/sizeof(T)];
>>>>    };
>>>>    T* elements() { return (T*)(char*)(this+1); }
>>>>  };
>>>>
>>>>The dummy array ensures that (this+1) is aligned properly for a T.
>>>
>>> why not plain T dummy ?
>>
>>Because a single T might not be big enough.
>
>I understand that T might be smaller than unsigned int, but
>I fail to see the problem.
>
>A union cannot contain padding before its members (can it? would it 
>matter?).  Since (this+1) is suitably aligned for an object of the 
>union (or arrays of unions wouldn't work) it must thus also be suitably 
>aligned for an object of type T.

Maybe Hans is right.  I think a union cannot contain padding before
its first member, at least.  Then this

>>>>    union {
>>>>      T dummy;
>>>>      unsigned int num_elements;
>>>>    };

would be sufficient.

>>>Why not have a template-union instead of hiding the union inside
>>>a struct?
>>
>>The point is that the varstruct probably has useful things in it.
>
>But I though those other items should also be inside the union
>(preferably in a struct):
>
>struct header {...}
>template <class T> union varunion {
>  struct header;
>  T dummy;
>  T* elements() {return (T*)(char*)(this+1);}
>};
>varunion<T>* x = 
>   (varunion<T>*) operator new(sizeof(varunion<T>)+sizeof(T)*n);

This is close to what I meant. That a single T is enough is 
a useful insight.

-- 
Nathan Myers
ncm@nospam.cantrip.org  http://www.cantrip.org/



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