类分配上的映射迭代器错误

Question

I have an error in map iterators. The problem is the following:

class JacobianCol
{
private:
...
    JacobianColData::iterator _L_begin;
public:
    JacobianColData::iterator L_begin();
...
};

In another module:

JacobianCol LUSolver::col_subtract(const JacobianColData::iterator &alpha, JacobianCol &X, JacobianCol &Y)
{
    JacobianCol result = Y;

    //alternate "result" variable

    return result;
}

Call:

...
J[*it] = col_subtract(friend_element, J[diag_index], J[*it]);
...

And when I assign result of col_subtract to J[*it] I get J[*it].L_begin pointing to a deallocated memory (pointer to a previous J[*it] ).

Answer 1

When you do:

JacobianCol result = Y;

result._L_begin keep pointing to Y._col_data.begin(). When the object which Y is a reference to is deleted, result._L_begin keep pointing to that location, which is now invalid.

Answer 2

JacobianCol has the _col_data member. ie, the input instance JacobianCol Y has a _col_data instance and _L_Begin of this is initialized with that map's iterator. When you return the result, a different _col_data member is created. Now, the _L_Begin iterator is copied from the original Y instance. That is why you get the observed behavior.

To resolve this implement the copy constructor, copy the map and the re-initialize the iterator by explicitly calling _col_data.begin() in the copy constructor

JacobianCol(const JacobianCol& aSrc_in)
{
    _coll_data = aSrc_in._coll_data;
    _L_Begin = _coll_data.begin();
}