Re: casting pointers/arrays to multidimensional arrays

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 3 Nov 2009 08:10:20 -0800 (PST)
Message-ID:
<f384f139-cf9b-4c55-83a9-68c273525363@j4g2000yqe.googlegroups.com>
On Nov 3, 3:41 pm, Francesco <xtrigger...@gmail.com> wrote:

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.

//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


They're reinterpret_cast. Formally, I think using the results
of them here is undefined behavior. If you're implementation
says it's OK, they should work. And in practice, most
implementations don't say anything, but they work anyway.

Of course, they're not something you'd want to see in production
code.

--
James Kanze

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