Re: references traps and pitfalls.

On 19 Dec, 00:07, mpho <tjab...@gmail.com> wrote:


[code snipped]

As it stands, your code exhibits undefined behaviour. The following is
a simplified and compilable version of what you are trying to do:

#include <vector>
#include <iterator>
using namespace std;

void func( vector <vector <int> > & mat ) {
    vector <vector <int> > tmp;
    for ( unsigned int i = 0; i < mat.size(); i++ ) {
        // **** problem is here - there is no such thing as tmp[i]
****
        copy( mat[i].begin(), mat[i].end(), back_inserter( tmp[i]));
    }
}

int main() {
    vector <vector <int> > m;
    vector <int> t;
    t.push_back( 1 );
    t.push_back( 2 );
    m.push_back( t );
    func( m );
    return 0;
}

At the point marked by the *** comment, the vector tmp has no i'th
element, but you try to pass it to the back inserter. One quick and
dirty solution is to create the element just before you use it:

void func( vector <vector <int> > & mat ) {
    vector <vector <int> > tmp;
    for ( unsigned int i = 0; i < mat.size(); i++ ) {
        vector <int> t;
        tmp.push_back( t ); // now tmp[i] exists
        copy( mat[i].begin(), mat[i].end(), back_inserter( tmp[i]));
    }
}

IMHO, a better solution would be to create (or find existing) a matrix
class which has an easier interface to use than that supplied by a
vector of vectors.

Neil Butterworth

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