Run-time error - Vector iterator not dereferencable? - vector

Question

For class, Im making a program that manages a hotel. Im getting a run-time error when my program gets to this function:Vector iterator not dereferencable. I used the debugger to find the problem area, but I cant figure out what is wrong with it. Any Suggestions?
Customer & ListOfCustomers::getByID(int id)
{
if(!sortedByID)sortByID();
vector<Customer>::iterator iter;
Customer cus;
cus.customerID=id;
iter = lower_bound(customers.begin(),customers.end(),cus,compareCustomersByID);
if( (*iter).customerID == id ) // <---DEBUGGER SAYS ERROR HERE IN THIS LINE
{
return *iter;
}
else
{
return NullCustomer();
}
}
Here is the lower_bound function. It is Inside #include algorithm
template<class _FwdIt,
class _Ty,
class _Pr> inline
_FwdIt lower_bound(_FwdIt _First, _FwdIt _Last,
const _Ty& _Val, _Pr _Pred)
{// find first element not before _Val, using _Pred
// _DEBUG_ORDER_PRED(_First, _Last, _Pred);
return (_Rechecked(_First,
_Lower_bound(_Unchecked(_First), _Unchecked(_Last), _Val, _Pred,
_Dist_type(_First))));
}
EDIT: added a space so that the lower_bound function would be formatted correctly as code.

Answer 1

You are using the lower_bound function for searching. Its purpose is a little different than that. This is what lower_bound does:
Returns an iterator pointing to the first element in the sorted range [first,last) which does not compare less than value.
And another definition from here:
Specifically, it returns the first position where value could be inserted without violating the ordering.
So for example, if the thing you are looking for is not in the vector, it will return an iterator that points after the last item in the vector, and that iterator can't be dereferenced because it does not exist.
Take a look at this example:
int myints[] = {10,20,30,30,20,10,10,20};
vector<int> v(myints,myints+8); // 10 20 30 30 20 10 10 20
vector<int>::iterator low;
sort (v.begin(), v.end()); // 10 10 10 20 20 20 30 30
low=lower_bound (v.begin(), v.end(), 60); // ^it will point here
cout << "lower_bound at position " << int(low- v.begin()) << endl;
As you can see from the output, the iterator will point to the 9th element in the vector (index 8). But the vector only has 8 elements (indexed 0-7). The explanation for this is that you can insert the new item in the vector at index 8 without violating the ordering.
I think that what you really want is the find function. Here is an example:
int myints[] = {10,20,30,30,20,10,10,20};
vector<int> v(myints,myints+8); // 10 20 30 30 20 10 10 20
vector<int>::iterator find_it1 = find(v.begin(), v.end(), 30);
vector<int>::iterator find_it2 = find(v.begin(), v.end(), 80);
if(find_it1 == v.end())
cout << "30 not found" << endl;
else
cout << "30 found at position " << int(find_it1 - v.begin()) << endl;
if(find_it2 == v.end())
cout << "80 not found" << endl;
else
cout << "80 found at position " << int(find_it2 - v.begin()) << endl;
Here is the output:
30 found at position 2
80 not found