Question

Thought it was pretty straight forward.

But I get a “iterator not dereferencable” errro when running the below code.

What’s wrong?

template<typename T>
struct SumsTo : public std::binary_function<T, T, bool>
{
    int myInt;
    SumsTo(int a)
    {
        myInt = a;
    }
    bool operator()(const T& l, const T& r)
    {   
        cout << l << " + " << r;
        if ((l + r) == myInt)
        {
            cout << " does add to " << myInt;
        }
        else 
        {
            cout << " DOES NOT add to " << myInt;
        }
        return true;
    }
};

void main()
{
    list<int> l1;
    l1.push_back(1);
    l1.push_back(2);
    l1.push_back(3);
    l1.push_back(4);

    list<int> l2;
    l2.push_back(9);
    l2.push_back(8);
    l2.push_back(7);
    l2.push_back(6);

    transform(l1.begin(), l1.end(), l2.begin(), l2.end(), SumsTo<int>(10) );
}

Answer 1

Your functor is fine. The problem is in the call to transform.

Transform has the prototype

transform(_InputIterator1 __first1, _InputIterator1 __last1,
          _InputIterator2 __first2, _OutputIterator __result,
          _BinaryOperation __binary_op)

your call is

transform(l1.begin(), l1.end(), l2.begin(), l2.end(), SumsTo<int>(10) );

instead of l2.end(), the fourth iterator argument needs to be the beginning of the result sequence. It should reference a sequence of objects that you can construct from bool.

If you want to save the results into l2, then you want

transform(l1.begin(), l1.end(), l2.begin(), l2.begin(), SumsTo<int>(10) );

As GMan suggests, another approach is std::back_inserter from <iterator>:

vector<bool> sums10; // vector<bool> is Good Enough for Me
transform(l1.begin(), l1.end(), l2.begin(), back_inserter(sums10), SumsTo<int>(10) );