Re: [boost] Dereferencing End Iterators (Was: Performance Tuning?)

Subject: Re: [boost] Dereferencing End Iterators (Was: Performance Tuning?)
From: Edward Grace (ej.grace_at_[hidden])
Date: 2009-07-22 13:52:58

On 22 Jul 2009, at 18:02, Christopher Jefferson wrote:

> On 22 Jul 2009, at 16:23, Stewart, Robert wrote:
>>
>>
>>> Ultimately are random access iterators *supposed* to be homeomorphic
>>> to the integers in the same (apparent) way C indices are?
>
> I'm sure that:
>
> int a[3]
> int* b = a + 4;
>
> Is illegal C code, as you have gone outside the bounds of your array,
> even if you never dereference the pointer.

Following your minimalistic example, I'm thinking about:

typdef int index;
int a[3];

index i = 0;  // Index idx is homeomorphic to the integers 'cause it  
is one.

i -= 1000;
i += 2002;

int b=a[i];

That's clearly ok as idx is 2 and the last line is b=*(a + 2).

Whereas.

int *p = a;

p = p - 1000; // Undefined?
p = p + 1002;  // Also undefined?

int b=*p;

Is presumably undefined.

That implies to me that pointer arithmetic is *not* like integer  
arithmetic since the first example is not identical to the second.

Baring in mind the old adage of Mark Twain,

   http://www.quotationspage.com/quote/369.html

it seems to me absurd that they are not the same.  I guess that's  
just the way it is.

-ed

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