Question

I have a Visual Studio 2008 C++ project where I need to decode a 6-byte array of data in to a structure.

My thought was to do something like this:

#pragma pack( push, 1 )
struct Map 
{
    BYTE alpha    : 8;
    BYTE bravo    : 8;
    UINT charlie  : 24;
    BYTE delta    : 8;
    BYTE padding0 : 8;  // ensure correct byte boundary
    BYTE padding1 : 8;
};
#pragma pack( pop )
// sizeof( Map ) == 8 bytes

BYTE test_data[ 6 ] = { 0x07,     // alpha
                        0x0B,     // bravo
                        0x00,     // charlie high byte
                        0x00,     // charlie med byte
                        0x01,     // charlie low byte
                        0x33 };   // delta
Map map = { };
memcpy( &map, test_data, sizeof( test_data ) );
ASSERT( map.alpha == 0x07 );
ASSERT( map.bravo == 0x0B );
ASSERT( map.charlie == 0x01 );
ASSERT( map.delta == 0x33 );

But, the data in charlie and delta is never correct.

Is there a way to structure my struct to get this correct or am I stuck setting each field individually?

Thanks

Answer 1

To do this in a portable way:

struct Map 
{
    BYTE alpha;
    BYTE bravo;
    UINT charlie;
    BYTE delta;

    Map(const BYTE data[6])
     : alpha(data[0])
     , bravo(data[1])
     , charlie((data[2] << 16) | (data[3] << 8) | data[4])
     , delta(data[5])
    {}
};

int main()
{
    BYTE test_data[ 6 ] = { 0x07,     // alpha
                0x0B,     // bravo
                0x00,     // charlie high byte
                0x00,     // charlie med byte
                0x01,     // charlie low byte
                0x33 };   // delta
    Map map(test_data);
    ASSERT( map.alpha == 0x07 );
    ASSERT( map.bravo == 0x0B );
    ASSERT( map.charlie == 0x01 );
    ASSERT( map.delta == 0x33 );
}