how portable is end iterator decrement?

Question

Just encountered decrement of end() iterator in my company source codes and it looks strange for me. As far as I remember this was working on some platforms, bu

Answer 1

from the documentation for std::prev

Although the expression --c.end() often compiles, it is not guaranteed to do so: c.end() is an rvalue expression, and there is no iterator requirement that specifies that decrement of an rvalue is guaranteed to work. In particular, when iterators are implemented as pointers, --c.end() does not compile, while std::prev(c.end()) does.

which means the implementation for the prefix decrement operation may not be the inside class form iterator iterator::operator--(int) but overloaded outside class form iterator operator--(iterator&, int).

so you should either prefer std::prev or do the following: { auto end = a.end(); --end; };