Is it defined to provide an inverted range to C++ standard algorithms?

Question

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

template
Function for_each(InputIterator first         


        

Answer 1

This is explained by section 24.1 of the standard, "Iterator Requirements":

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.

…

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.

So v.begin() + 3 is reachable from v.begin(), but not the reverse. So [v.begin()+3, v.begin()) is not a valid range, and your call to for_each is undefined.