checking if iterator is at the end of my list?

[Halo Fan]

I am working on an operator+ method so that I can add two lists of chars together and overall create a class that can manage "integers" of any size. It works beautifully (as in x + y actually works) when x and y have the same number of items in the list, just not when they are different in size. I added this piece of code to ensure that I wouldn't have the smaller value's iterator trying to dereference the end of the list:

			if (xCurr == tempX.val.end() )
				tempX.val.push_back('0');
			if (yCurr == tempY.val.end() )
				tempY.val.push_back('0');

and it only gets that far if at least one of the two (x and y) still has digits left. When I debug and input "11" for x and "111" for y and try to add them I actually get two results. One is that the debugger just crashes and says:

"list iterator not dereferencable"

The other result is that the function will run most of the way and then crash after seeming to miss the piece of code I mentioned above, as if it didn't exist but just had to try to execute this statement:

tempSumInt = (*xCurr - '0') + (*yCurr - '0') + (*cCurr - '0');

while xCurr (my iterator for my tempXobject) was at the end of the list.

Here is the function call I used:

cout << "x + y = " << x + y << ".\n";

and here is the full code so far for the function itself:
(Note: "val" is the identifier of the list<char> member in the private section of the LongInt class).

	LongInt operator+(const LongInt& x, const LongInt& y) //friend function
	{
		LongInt tempX = x, tempY = y, carry, sum;
		list<char>::const_iterator xCurr, yCurr, cCurr;
		int tempSumInt;
		char tempSumChar;

		tempX.val.reverse();
		tempY.val.reverse();

		xCurr = tempX.val.begin();
		yCurr = tempY.val.begin();
		cCurr = carry.val.begin();
 
		while (! (xCurr == tempX.val.end() && yCurr == tempY.val.end() ) )
		{
			
			if (xCurr == tempX.val.end() )
				tempX.val.push_back('0');
			if (yCurr == tempY.val.end() )
				tempY.val.push_back('0');
			
			tempSumInt = (*xCurr - '0') + (*yCurr - '0') + (*cCurr - '0');
			if (tempSumInt > 9)
			{
				tempSumInt = tempSumInt % 10;
				tempSumChar = static_cast<char>(tempSumInt + '0');
				sum.val.push_back(tempSumChar);
				carry.val.push_back('1');
				xCurr++;
				yCurr++;
				cCurr++;
			}
			else
			{
				tempSumChar = static_cast<char>(tempSumInt + '0');
				sum.val.push_back(tempSumChar);
				carry.val.push_back('0');
				xCurr++;
				yCurr++;
				cCurr++;
			}
			
		}
		sum.val.pop_front();
		sum.val.reverse();
				
		return sum;
	}

Thank you very much for your time in reading this! If you have any questions please ask. Hopefully the identifiers are clear enough :)

Ok so I figured out that, if it reached the end of the list and I wanted to add the extra digit to the end of the list to let it continue until both had finished, I also had to decrement the iterator (since the iterator would remain pointing at the end of the list even while I had put another value in front). I assumed the iterator would point to the '0' automatically but this fixed it:

			if (xCurr == tempX.val.end() )
			{
				tempX.val.push_back('0');
				--xCurr;
			}	
			if (yCurr == tempY.val.end() )
			{
				tempY.val.push_back('0');
				--yCurr;
			}