STL const_iterator cast -- compiler differences

c++, const-iterator, gcc, stl, visual-studio-2013

Solution

There is no portable way to do what you're attempting because there is no requirement that a `(const_)iterator` be constructible from a pointer to the underlying value type. libstdc++ happens to provide such a constructor but the VS standard library implementation doesn't. Instead, its `(const_)iterator` constructor takes a pointer to the underlying value type and a pointer to the container itself, which it uses to perform additional validation during debug builds.

The easiest solution is to replace `std::vector<unsigned char>::const_iterator` with `unsigned char const *`. A raw pointer falls in the RandomAccessIterator category, which is the same as `vector::(const_)iterator`s.

unsigned char const *foo::begin() {
    return value;
}

unsigned char const *foo::end() {
    return value + length;
}

If you needs the iterator to be a class type, then you'll need to create a custom iterator. While this can be done from scratch, it's a lot easier to use Boost.IteratorFacade, which will provide a bunch of the necessary boilerplate that goes into constructing a custom iterator.

#include <boost/iterator/iterator_facade.hpp>

struct const_foo_iterator : boost::iterator_facade<const_foo_iterator,
                                                    unsigned char const,
                                                    boost::random_access_traversal_tag>
{
  const_foo_iterator() = default;
  const_foo_iterator(unsigned char const *iter) : iter(iter) {}
private:
    friend class boost::iterator_core_access;

    void increment() { ++iter; }
    void decrement() { --iter; }
    void advance(std::ptrdiff_t n) { iter += n; }

    std::ptrdiff_t distance_to(const_foo_iterator const& other) const
    { return iter - other.iter; }

    bool equal(const_foo_iterator const& other) const
    { return this->iter == other.iter; }

    unsigned char const& dereference() const { return *iter; }
    unsigned char const* iter = nullptr;
};

const_foo_iterator foo::begin() {
    return value;
}

const_foo_iterator foo::end() {
    return value + length;
}

static_assert(std::is_same<std::iterator_traits<const_foo_iterator>::value_type,
                           unsigned char>::value, "value_type");
static_assert(std::is_same<std::iterator_traits<const_foo_iterator>::pointer,
                           unsigned char const *>::value, "pointer");
static_assert(std::is_same<std::iterator_traits<const_foo_iterator>::iterator_category,
                           std::random_access_iterator_tag>::value, "iterator_category");

Live demo

Problem

Original source