TITLE: stl initialization of multidim array

(Newsgroups: comp.std.c++, 18 Oct 96)

PROBLEM: "Ole Petter Brunstad" <brunstad@online.no>

I am new to STL, and I am having problems initializing a multidimentional
array.
I thought this was the right way: 

    int data[] = { 1, 2, 3, 4, 5, 6, 11, 12, 13, 14, 15, 16, 
		21, 22, 23, 24,25, 26 };
    vector<vector<unsigned int> > &values(3);
    for (int i = 0; i < 3; i++)
    copy(data +  i*6, data  + i*6 + 6, values[i].begin());

but each values[i] only gets 4 items long (except for the last one 
that is 6 items long).

Can anyone tell me how this should be done?


RESPONSE: kanze@gabi-soft.fr (J. Kanze)

There are several ways.  First, however, what you have written is
undefined behavior; I suspect that there are systems or implementations
on which it will core dump.  (A tip: you really need Cay Horstman's safe
STL for experimenting.  Unless I'm badly mistaken, it would have
generated an error on each of the calls to copy.)

As written, the contained vectors never actually contain any elements.
Accessing beyond the end of a vector using the iterator (which is what
copy will do in your code) does NOT extend the iterator; it is simply
undefined behavior.  You need something to extend the vector.

The simplest solution, or at least the closest to your code, would be to
replace the current invocation of copy with:

		copy( &data[ 6 * i ] ,
		      &data[ 6 * i + 6 ] ,
		      back_inserter( values[ i ] ) ) ;

The back_inserter function returns an iterator which inserts: operator++
is a no-op, and dereferencing causes an additional element to be
appended at the end of the container.

I would probably tend to reflect the actual data structure in the
initialization data as well, using something like:

    static int const    data[][ 6 ] =
    {
        {    1 ,  2 ,  3 ,  4 ,  5 ,  6 } ,
        {   11 , 12 , 13 , 14 , 15 , 16 } ,
        {   21 , 22 , 23 , 24 , 25 , 26 } ,
    } ;

    vector< vector< int > >
                        values ;
    for ( int i = 0; i < sizeof( data ) / sizeof( data[ 0 ] ) ; i ++ )
        values.push_back( vector< int >( data[ i ] , data[ i + 1 ] ) ) ;

(While I'm at it: can anyone quote a passage from the standard that
guarantees that "data[ i + 1 ]" will be a valide end iterator for the
begin iterator "data[ i ]"?  This works on all implementations I can
possibly think of, and I think that there are enough restrictions on the
data layout in the C standard to guarantee it, but I'd feel better if
someone could confirm its correctness.)

For more information concerning this problem, you probably want to read
Andy Koenig's article "Copying and Insertion in STL Algorithms", in the
Sept. '96 issue of C++ Report.

One last point: the code you have written shouldn't compiler under the
standard.  You didn't really mean for values to be a reference, did you?