Re: Validity of pointer conversions

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Sat, 5 Jan 2008 12:34:38 -0800 (PST)
Message-ID:
<7325b4dd-0427-43b6-a32c-f391f57f1bb8@21g2000hsj.googlegroups.com>
On Jan 5, 4:02 pm, Ioannis Vranos <j...@no.spam> wrote:

Are the following codes guaranteed to work always?

1.

#include <iostream>

inline void some_func(int *p, const std::size_t SIZE)
{
     using namespace std;
     for(size_t i=0; i<SIZE; ++i)
        cout<< p[i]<< " ";
}

int main()
{
     int array[10][5]= {0};

     some_func(array[0], sizeof(array)/sizeof(**array));
         std::cout<< std::endl;
}

The above prints 50 zeros. I think it is guaranteed to work,
since all arrays are sequences of their elements.


And? I don't see any relationship between what you just said
and any guarantee of working. You have an array bounds
violation, which is undeefined behavior. And there have been
(and maybe still are) implementations which detect it, and
treat it as an error condition.

2.

#include <iostream>

int main()
{
     using namespace std;

     int array[50]= {0};

     int (*p)[5]= reinterpret_cast<int (*)[5]> (&array[0]);

     for (size_t i= 0; i< 10; ++i)
          for(size_t j=0; j<5; ++j)
              cout<< p[i][j]<<" ";
      cout<< endl;
}

Here p behaves as a 2-dimensional matrix, that is a 10x5
matrix.


Almost nothing involving reinterpret_cast is guaranteed to work.
About the only thing that the standard guarantees is that if you
cast the value back to its original type, and you haven't
violated any alignment restrictions in the intermediate types,
the value will compare equal to the original value (and thus,
designate the same object designated by the original pointer).

=46rom a quality of implementation point of view: the standard
does say that the conversion is expected to be "unsurprising"
for someone familiar with the addressing architecture of the
processor, so I would expect this to work on most
implementations.

I think it is guaranteed to work for the same reason
as the first one,


It is totally unrelated to the first. reinterpret_cast is quite
different from other conversions.

that is we can treat an array (sequence) of integers as
various types of integer arrays.


If by that you mean that you can play games with the dimensions,
as long as the total number of elements is unchanged, that is
simply false.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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