Re: casting pointers/arrays to multidimensional arrays

From:
Francesco <xtrigger303@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 6 Nov 2009 01:04:08 -0800 (PST)
Message-ID:
<8af0c844-0479-4027-aeed-823c58f39963@m16g2000yqc.googlegroups.com>
On 4 Nov, 13:43, "Daniel T." <danie...@earthlink.net> wrote:

In article
<f3dab86c-491a-441c-a0ed-7acb69aec...@d21g2000yqn.googlegroups.com>,

 Francesco <xtrigger...@gmail.com> wrote:

Hi to all,

would any of you gentlemen please comment on the reinterpret_casts in
the code below?
Do you think they're ok or not? Any reference to the standard would be
greatly appreciated.

Thank in advance,
FB

//CODE
#include <iostream>
#include <iomanip>

typedef float ( * tMtx )[ 4 ];

void Fill( tMtx inMtx )
{
    for( int c = 0; c < 16; ++c )
        inMtx[ c / 4 ][ c % 4 ] = c;
}

void Print( tMtx inMtx )
{
    std::cout << "n->" << inMtx << std::endl;
    for( int c1 = 0; c1 < 4; ++c1 )
    {
        for( int c2 = 0; c2 < 4; ++c2 )
            std::cout << std::setw( 5 ) << inMtx[ c1 ][ c2 =

];

        std::cout << std::endl;
    }
    std::cout << std::endl;
}

int main()
{
    float mtx1[ 16 ] = { 0 };
    float mtx2[ 4 ][ 4 ];
    float * mtx3 = new float[ 16 ];
    float * mtx4 = reinterpret_cast< float * >( new float[ 4 ][ 4=

 ] );

    // are these casts OK?
    Fill( reinterpret_cast< tMtx >( mtx1 ) );
    Fill( mtx2 );
    Fill( reinterpret_cast< tMtx >( mtx3 ) );
    Fill( reinterpret_cast< tMtx >( mtx4 ) );

    Print( reinterpret_cast< tMtx >( mtx1 ) );
    Print( mtx2 );
    Print( reinterpret_cast< tMtx >( mtx3 ) );
    Print( reinterpret_cast< tMtx >( mtx4 ) );

    delete[] mtx3;
    delete[] mtx4;

    std::cin.get();
}
//ENDCODE


It's doable, but I would do it differently:

// begin code
typedef float m16[16];
typedef float m44[4][4];

void Fill(m44& v) {
   for(int c = 0; c < 16; ++c)
      v[c / 4][c % 4] = c;

}

void Fill(m16& v) {
   Fill(reinterpret_cast<m44&>(v));

}

void Print(const m44& v) {
   cout << "n->" << v << endl;
   for (int c1 = 0; c1 < 4; ++c1) {
      for (int c2 = 0; c2 < 4; ++c2)
         cout << setw(5) << v[c1][c2];
      cout << endl;
   }
   cout << endl;

}

void Print(const m16& v) {
   Print(reinterpret_cast<const m44&>(v));

}

int main()
{
   float mtx1[16] = { 0 };
   float mtx2[4][4];

   Fill(mtx1);
   Fill(mtx2);

   Print(mtx1);
   Print(mtx2);

   cin.get();}

// end code

Even if you are stuck with the Fill and Print you have, I would still
wrap the casts in functions that take the appropriate types. That way
you don't have (1) reinterpret_cast all over your code and (2) it is
less likely that someone will reinterpret_cast the wrong sort of
variable.- Nascondi testo citato

- Mostra testo citato -


Thanks for the answers and hints.
I was just interested in knowing if casting from a "flat" pointer to a
multidimensional array (or backwards) is formally defined, undefined
behavior or implementation defined.
For the record: I've found an old thread in comp.lang.c where people
had mixed feelings about the issue and it seems that both standards (C
and C++) are not that clear on the subject.

http://groups.google.it/group/comp.lang.c/browse_thread/thread/367141614ade=
e786/

Summary: the memory layout of something like float [16] should be the
same as float[4][4] but since the implementation might "magically" do
different bounds checking (??), the following code might cause trouble
on some (??) implementations:

//CODE
#include <cassert>

int main()
{
    float array44[4][4];
    // DEFINED, UB OR IMPLEM DEFINED ?
    float * ptr = reinterpret_cast<float*>( array44 );
    ptr[6] = 123.123;
    assert( (void*)&array44[1][2] == (void*)&ptr[1*4+2] );

    // OPPOSITE CASE
    float array16[16] = {0};
    // DEFINED, UB OR IMPLEM DEFINED ?
    float (*ptr44)[4] = reinterpret_cast<float(*)[4]>( array16 );
    ptr44[1][2] = 123.123;
    assert( (void*)&array16[1*4+2] == (void*)&ptr44[1][2] );
}

//ENDCODE

Again for the record: I'm interested in using the flat to
multidimensional cast because I'd prefer to use the [][][] in some
functions that have as input flat arrays (created by somebody else)...
Thanks a lot again,
Francesco

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