returning address or local variable or temporary C++ warning

Possible Duplicate:
c++ warning: address of local variable

Hi, When i write this code:

//Returns the transpose matrix of this one
SparseMatrix& SparseMatrix::transpose()const{
    vector<Element> result;
    size_t i;
    for(i=0;i<_matrix.size();++i){
        result.push_back(Element(_matrix.at(i)._col, _matrix.at(i)._row, _matrix.at(i)._val));
    }

    return SparseMatrix(numCol,numRow,result);
}

I get the warning "returning address or local variable or temporary". The last line calls the SparseMatrix constructor. I don't understand what's wrong with this code, and how can i fix it so i can return a SparseMatrix object as i want.