How to understand std::distance in C++

Question

The code is as follow:

int B[] = {3,5};
int C[] = {4,5};
cout << distance(B,C);

The output is:

-4

Can anyone explain why is this?

Answer 1

The distance(first, last) function tells you how many items are between the iterator at first and last. Note that pointers are iterators, random-access iterators to be specific. So the distance between one pointer and another is their difference, as defined by operator-.

So your question boils down to "How many ints are there between the int pointed to by B and the int pointed to by C?

distance dutifully subtracts the pointers and tells you.

The trick is that distance is supposed to be applied to iterators from the same container. Your code does not live up to that promise. The compiler is free to place the B and C arrays wherever it pleases, hence the result you see is meaningless. Like many things in C++, it's up to you to ensure that you're using distance properly. If you don't, you'll get undefined behavior, where the language makes no guarantees what will happen.

Answer 2

std::distance(__first, __last) is designed to generalize pointer arithmetic, it returns a value n such that __first + n = __last.
for your case, the arguments are pointers of int*, in terms of iteration, they are random accessed iterators. the implementation simply returns a value of __last - __first:
simply (int*)C - (int*)B.