initializing a vector based on a 2d array

Question

I have a Visual Studio 2008 C++03 application where I would like to initialize a std::vector based on a two dimensional array.

For example:

#define DATA_SIZE 6

struct Data
{
    UINT a;
    BYTE b;
    BYTE c;
    BYTE d;

    Data()
        : /* initialize all members to 0*/
    {
    };

    explicit Data( const BYTE data[ DATA_SIZE ] )
        : a( data[ 0 ] << 16 | data[ 1 ] << 8 | data[ 2 ] ),
          b( data[ 3 ] ),
          c( data[ 4 ] ),
          d( data[ 5 ] )
    {
    };
};

inline bool operator==( const Data& lhs, const Data& rhs )
{
    return /* true if all members are equal */
};

int main( int argc, char* argv[] )
{
    const BYTE source[][ DATA_SIZE ] = 
    {
        { 0x01, 0xfe, 0xaa, 0x01, 0xcc, 0x13 },
        { 0x02, 0xa1, 0x02, 0xbb, 0x02, 0xdd }
    }

    // how should this be done?
    std::vector< Data > data_list( source[ 0 ], source[ countof( source) - 1 ] );

    ASSERT( data_list[ 0 ] == Data( source[ 0 ] ) );
    ASSERT( data_list[ 1 ] == Data( source[ 1 ] ) );
    return 0;
}

Is there a way to do this without a for loop iterating through each item in the data array and calling push_back?

If you removed the explicit keyword would that not allow the compiler to implicitly convert the BYTE arrays to Data objects? — Dennis
– Dennis, Commented May 21, 2012 at 16:45
If you used std::tr1::array<BYTE, N> instead of raw C-arrays, this would be completely trivial. — ildjarn
– ildjarn, Commented May 21, 2012 at 16:50
@PaulH: No, absolutely not. It would be far better to have the constructor take a first/last range and assert in the constructor that the size of the range is the correct size, or have it take an array<BYTE, DATA_SIZE>, possibly by const&. The use of DATA_SIZE in the declaration gives a false sense of security, as there is no requirement that the provided pointer point to an array of that size. — James McNellis
– James McNellis, Commented May 21, 2012 at 16:59

Robᵩ · Accepted Answer · 2012-05-21 17:07:34Z

3

The least change you could make is:

std::vector< Data > data_list(
  &source[ 0 ],
  &source[ countof( source) ] );

answered May 21, 2012 at 17:07

Robᵩ

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Comments

ildjarn · Accepted Answer · 2012-05-21 17:09:27Z

Here's an approach using Boost.Array:

#include <cstddef>
#include <cassert>
#include <vector>
#include <boost/array.hpp>

typedef unsigned UINT;
typedef unsigned char BYTE;

std::size_t const DATA_SIZE = 6;

struct Data
{
    UINT a;
    BYTE b, c, d;

    Data() : a(), b(), c(), d() { }

    Data(boost::array<BYTE, DATA_SIZE> const& data)
      : a(data[0] << 16 | data[1] << 8 | data[2]),
        b(data[3]),
        c(data[4]),
        d(data[5])
    { }
};

inline bool operator ==(Data const& lhs, Data const& rhs)
{
    return lhs.a == rhs.a
        && lhs.b == rhs.b
        && lhs.c == rhs.c
        && lhs.d == rhs.d;
}

int main()
{
    boost::array<boost::array<BYTE, DATA_SIZE>, 2> const source =
    {{
        {{ 0x01, 0xfe, 0xaa, 0x01, 0xcc, 0x13 }},
        {{ 0x02, 0xa1, 0x02, 0xbb, 0x02, 0xdd }}
    }};

    std::vector<Data> data_list(source.begin(), source.end());

    assert(data_list[0] == Data(source[0]));
    assert(data_list[1] == Data(source[1]));
}

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