Backward iteration with unsigned index like size_t

In C++11 the first tool for iterating a string or vector should be range-based for-loop. But sometimes we really need the index of the element and we need the elements in reverse order. Index is usually needed when dealing with std::string. Iterating with a signed integer is easy in both directions. But if the vector size exceeds INT_MAXyou have a logic error if you try to iterate with signed index. Thus, the most correct way is to use unsigned integer, size_t, as the type of the index. Iterating forward with size_t is same as the signed variant, but iterating backward can lead to infinite loop if you are not careful. The trick is to use the not equal comparison.

int main()
{
	auto v = vector<int>{1, 2, 3, 5, 7, 11};
	
	// backwards with greater than or equal is ERROR with unsigned
	//for(size_t i = v.size() - 1; i >= 0; --i)
	//	cout << v[i] << ' ';
	
	// backwards with not equal, OK, the right way
	for(size_t i = v.size() - 1; i != -1; --i)
		cout << v[i] << ' ';
		
	// trying to look smart. Correct, but may confuse.
	for(size_t i = v.size(); i --> 0;)
		cout << v[i] << ' ';
}

You may be wondering how can you compare unsigned number with a negative number like -1. If you take into account the wrapping nature of unsigned integers and the implicit casting, then you will see that the comparison is correct. It has the same effect as comparing i != numeric_limits<size_t>::max().