Added thomas' interoperability idea

[SVN r21762]

test/interoperable.cpp

@@ -0,0 +1,59 @@
+// Copyright David Abrahams 2004. Use, modification and distribution is
+// subject to the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/iterator/iterator_adaptor.hpp>
+#include <boost/pending/iterator_tests.hpp>
+#include <cassert>
+
+struct mutable_it : boost::iterator_adaptor<mutable_it,int*>
+{
+    typedef boost::iterator_adaptor<mutable_it,int*> super_t;
+    
+    mutable_it();
+    explicit mutable_it(int* p) : super_t(p) {}
+    
+    bool equal(mutable_it const& rhs) const
+    {
+        return this->base() == rhs.base();
+    }
+};
+
+struct constant_it : boost::iterator_adaptor<constant_it,int const*>
+{
+    typedef boost::iterator_adaptor<constant_it,int const*> super_t;
+    
+    constant_it();
+    explicit constant_it(int* p) : super_t(p) {}
+    constant_it(mutable_it const& x) : super_t(x.base()) {}
+
+    bool equal(constant_it const& rhs) const
+    {
+        return this->base() == rhs.base();
+    }
+};
+
+int main()
+{
+    int data[] = { 49, 77 };
+    
+    mutable_it i(data);
+    constant_it j(data + 1);
+    assert(i < j);
+    assert(j > i);
+    assert(i <= j);
+    assert(j >= i);
+    assert(j - i == 1);
+    assert(i - j == -1);
+    
+    constant_it k = i;
+
+    assert(!(i < k));
+    assert(!(k > i));
+    assert(i <= k);
+    assert(k >= i);
+    assert(k - i == 0);
+    assert(i - k == 0);
+    
+    return 0;
+}