Re: casting pointers/arrays to multidimensional arrays

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 04 Nov 2009 07:43:40 -0500

Message-ID:

<daniel_t-5D6C54.07434004112009@earthlink.us.supernews.com>

In article
<f3dab86c-491a-441c-a0ed-7acb69aec579@d21g2000yqn.googlegroups.com>,
Francesco <xtrigger303@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.