Question

Consider standard algorithms like, say, std::for_each.

template<class InputIterator, class Function>
Function for_each(InputIterator first, InputIterator last, Function f);

As far as I can tell, there is actually no requirement placed on the relative states of the two InputIterator arguments.

Does that mean that the following is technically valid? Or is it undefined? What can I realistically expect it to do?

std::vector<int> v{0,1,2,3,4};
std::for_each(
   v.begin()+3,  // range [3,0)
   v.begin(),
   [](int){}
);

geordi tells me:

error: function requires a valid iterator range [__first, __last). [+ 13 discarded lines]

but I can’t tell how compliant this debug diagnostic is.

---

I came up with this question when trying to pedantically determine how explicit is defined the behaviour of the following:

std::vector<int> v; // <-- empty
std::for_each(      // <-- total no-op? stated or just left to implication?
   v.begin(),
   v.end(),
   [](int){}
);

Answer 1

The standard explicitly requires the last iterator to be reachable from the first iterator. That means that by incrementing first one should be able to eventually hit last.

24.1 Iterator requirements

…

6 An iterator j is called reachable from an iterator i if and only if
there is a finite sequence of applications of the expression ++i that
makes i == j. If j is reachable from i, they refer to the same
container.

7 Most of the library’s algorithmic templates that operate
on data structures have interfaces that use ranges. A range is a pair
of iterators that designate the beginning and end of the computation.
A range [i, i) is an empty range; in general, a range [i, j) refers to
the elements in the data structure starting with the one pointed to by
i and up to but not including the one pointed to by j. Range [i, j) is
valid if and only if j is reachable from i. The result of the
application of functions in the library to invalid ranges is
undefined.