Can one make move_iterator from istream_iterator?

Question

Consider following code:

typedef istream_iterator<char> char_itr ;
char_itr eos;

string ll("some text here");

istringstream line_in(ll);
char_itr start(line_in);

move_iterator<char_itr> mstart(start); // !!!
move_iterator<char_itr> meos(eos);
vector<char> vc(mstart, meos);

Above code will not compile because of line (!!!):

error C2440: 'return' : cannot convert from 'const char' to 'char &&'

But if you replace mstart and meos with start and eos, respectively (regular iterators), the code will compile. Why I can't make move_iterators?

EDIT: For those wondering why I would like to move a character from a stream/string. Actual problem involves more complex data type than char which copying from a string should be avoided. char was used just for the sake of simplicity, to present the mechanism causing error.

Answer 1

Looking at istream_iterator::reference, we see that it's T const &. Dereferencing the iterator gives us such a reference. But to be able to move from something, that something needs to be modifiable. That's what the error message is trying to tell you.

You could make a move_iterator from an istream_iterator, by putting some custom iterator that keeps an internal (non const) storage inbetween.

But why would you want to move from an istream_iterator? Those are single pass iterators, and thus likely have nearly no internal storage.

Answer 2

There was a discussion of this on the std-discussion newsgroup earlier this year: https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/h7jGY95j1oc

The consensus appears to be that istream_iterator::reference is T const& to enforce the InputIterator contract; that is, to prevent users writing *it = value;. Unfortunately, this also prevents moving from the cached value.

As T.C. mentioned above, post the resolution to LWG2106 the code will compile; unfortunately because move_iterator::reference will be T const&& it will silently do the wrong thing, most likely invoking the copy constructor of your type.

Since istream_iterator modifies the cached value when incremented, it is (from a language POV) legal to const_cast the returned reference to T&. Unfortunately (again) that doesn't help here as there's no easy way to interpose a const_cast between the istream_iterator and move_iterator.

Possible workaround solutions:

• write your own istream_iterator, with a non-const reference typedef;
• write your own move_iterator performing const_cast;
• write an interposing const_cast iterator;
• use a mutable wrapper around the value type.

The latter option is surprisingly easy:

template<class T>
  struct mutable_wrapper {
    T mutable value;
    operator T&() const { return value; }
  };
// ...
using itr = std::istream_iterator<mutable_wrapper<MyType>>;

Example.