for_each on a reverse_iterator of a custom bidirectional iterator requires OutputIterator
c++, c++11, c++14, reverse-iterator, stl
Solution
The iterator requirements for all iterators are listed in [iterator.iterators]:
`reference` refers to the typedef of `iterator_traits<my_iterator<..>>`:
In the following sections, `a` and `b` denote values of type `X` or `const X`, `difference_type` and `reference` refer to the types `iterator_traits<X>::difference_type` and `iterator_traits<X>::reference`, respectively, [..]
Since the primary template of `iterator_traits` just defaults the typedefs to the types defined in the template argument itself, we're talking about the `reference` typedef of `my_iterator` - and that one is inherited from the base `std::iterator<...>`, which defaults it to `T&`. Your `operator*` returns an `int` though, which is certainly not `int&`.
Uncommenting your line is fine for InputIterators since `int` is convertible to `int`:
It fails for ForwardIterators though - [forward.iterators]/1:
A class or pointer type `X` satisfies the requirements of a forward iterator if
— if `X` is a mutable iterator, `reference` is a reference to `T`; if `X` is a const iterator, `reference` is a reference to `const T`,
Problem
I created a simple immutable bidirectional iterator: ``` #include <iostream> #include <memory> #include <iterator> #include <vector> #include <algorithm> class my_iterator : public std::iterator<std::bidirectional_iterator_tag, int //, std::ptrdiff_t, int*, int > { int d_val; public: my_iterator() : d_val(0) {} my_iterator(int val) : d_val(val) {} my_iterator operator--(int) { d_val--; return my_iterator(d_val + 1); } my_iterator &operator--() { d_val--; return *this; } my_iterator operator++(int) { d_val++; return my_iterator(d_val - 1); } my_iterator &operator++() { d_val++; return *this; } int operator*() const { return d_val; } bool operator==(my_iterator const &o) { return d_val == o.d_val; } bool operator!=(my_iterator const &o) { return d_val != o.d_val ; } }; int main() { std::reverse_iterator<my_iterator> reverse_it_begin(25); std::reverse_iterator<my_iterator> reverse_it_end(12); std::for_each(reverse_it_begin, reverse_it_end, [](int e){ std::cout << e << ' '; }); std::cout << '\n'; } ``` The iterator is immutable, since operator*() returns an int instead of a int reference. To the best of my understanding this is possible, since whether an iterator meets the BidirectionalIterator concept or the OutputIterator concept is orthogonal (all 4 combinations are possible). However, the code below results in an compile time error, namely: ``` /usr/include/c++/4.9/bits/stl_iterator.h:164:9: error: invalid initialization of non-const reference of type 'std::reverse_iterator<my_iterator>::reference {aka int&}' from an rvalue of type 'int' ``` Full context: ``` In file included from /usr/include/c++/4.9/bits/stl_algobase.h:67:0, from /usr/include/c++/4.9/bits/char_traits.h:39, from /usr/include/c++/4.9/ios:40, from /usr/include/c++/4.9/ostream:38, from /usr/include/c++/4.9/iostream:39, from prog.cpp:1: /usr/include/c++/4.9/bits/stl_iterator.h: In instantiation of 'std::reverse_iterator<_Iterator>::reference std::reverse_iterator<_Iterator>::operator*() const [with _Iterator = my_iterator; std::reverse_iterator<_Iterator>::reference = int&]': /usr/include/c++/4.9/bits/stl_algo.h:3755:6: required from '_Funct std::for_each(_IIter, _IIter, _Funct) [with _IIter = std::reverse_iterator<my_iterator>; _Funct = main()::<lambda(int)>]' prog.cpp:30:86: required from here /usr/include/c++/4.9/bits/stl_iterator.h:164:9: error: invalid initialization of non-const reference of type 'std::reverse_iterator<my_iterator>::reference {aka int&}' from an rvalue of type 'int' return *--__tmp; ^ Success time: 0 mem ``` The pages on cppreference about the reverse_iterator and for_each state that both need a BidirectionalIterator and an InputIterator respectively. I think both requirements are met, however the stl still assigns a dereferenced value to a reference. Why does stl's for_each/reverse_iterator expect a T &operator*() on an iterator that needs not to be an OutputIterator? PS: The commented line can fix the problem by stating that references should be stored by value, very hacky of course.