输入迭代器+输出迭代器+ X =正向迭代器

Question

I want to solve the equation in the title and find X. As far as I understand, an input or output iterator, unlike a forward iterator, gets invalidated as soon as a copy of it is advanced. Is there any additional requirement in order for a type T which satisfies both input iterator concept and output iterator concept to satisfy the forward iterator concept?

In any case, can you provide a real-world example of an iterator which is both input and output but is not forward? Thanks

Answer 1

Iterator categories deal with operations allowed on an iterator. Dereferencing forward iterator must yield T& (C++03) while dereferencing input iterator must only yield a type convertible to T . As an example, vector<bool>::iterator is an input/output iterator, but not a forward iterator (because dereferencing it returns a proxy convertible to bool — because you can't return a reference to a bit).

Answer 2

并且根据此处的图表，可以默认构造正向迭代器，但不能构造输入或输出迭代器。