Merge pull request #15 from boostorg/next_prior_fix_iterator_backtracking

Reworked next() and prior() taking the distance arguments.

include/boost/next_prior.hpp

@@ -13,6 +13,17 @@
 #define BOOST_NEXT_PRIOR_HPP_INCLUDED
 
 #include <iterator>
+#if defined(_MSC_VER) && _MSC_VER <= 1310
+#include <boost/mpl/and.hpp>
+#include <boost/type_traits/is_integral.hpp>
+#endif
+#include <boost/type_traits/is_unsigned.hpp>
+#include <boost/type_traits/integral_promotion.hpp>
+#include <boost/type_traits/make_signed.hpp>
+#include <boost/type_traits/has_plus.hpp>
+#include <boost/type_traits/has_plus_assign.hpp>
+#include <boost/type_traits/has_minus.hpp>
+#include <boost/type_traits/has_minus_assign.hpp>
 
 namespace boost {
 
@@ -26,14 +37,118 @@ namespace boost {
 
 //  Contributed by Dave Abrahams
 
+namespace next_prior_detail {
+
+template< typename T, typename Distance, bool HasPlus = has_plus< T, Distance >::value >
+struct next_impl2
+{
+    static T call(T x, Distance n)
+    {
+        std::advance(x, n);
+        return x;
+    }
+};
+
+template< typename T, typename Distance >
+struct next_impl2< T, Distance, true >
+{
+    static T call(T x, Distance n)
+    {
+        return x + n;
+    }
+};
+
+
+template< typename T, typename Distance, bool HasPlusAssign = has_plus_assign< T, Distance >::value >
+struct next_impl1 :
+    public next_impl2< T, Distance >
+{
+};
+
+template< typename T, typename Distance >
+struct next_impl1< T, Distance, true >
+{
+    static T call(T x, Distance n)
+    {
+        x += n;
+        return x;
+    }
+};
+
+
+template<
+    typename T,
+    typename Distance,
+    typename PromotedDistance = typename integral_promotion< Distance >::type,
+#if !defined(_MSC_VER) || _MSC_VER > 1310
+    bool IsUInt = is_unsigned< PromotedDistance >::value
+#else
+    // MSVC 7.1 has problems with applying is_unsigned to non-integral types
+    bool IsUInt = mpl::and_< is_integral< PromotedDistance >, is_unsigned< PromotedDistance > >::value
+#endif
+>
+struct prior_impl3
+{
+    static T call(T x, Distance n)
+    {
+        std::advance(x, -n);
+        return x;
+    }
+};
+
+template< typename T, typename Distance, typename PromotedDistance >
+struct prior_impl3< T, Distance, PromotedDistance, true >
+{
+    static T call(T x, Distance n)
+    {
+        typedef typename make_signed< PromotedDistance >::type signed_distance;
+        std::advance(x, -static_cast< signed_distance >(static_cast< PromotedDistance >(n)));
+        return x;
+    }
+};
+
+
+template< typename T, typename Distance, bool HasMinus = has_minus< T, Distance >::value >
+struct prior_impl2 :
+    public prior_impl3< T, Distance >
+{
+};
+
+template< typename T, typename Distance >
+struct prior_impl2< T, Distance, true >
+{
+    static T call(T x, Distance n)
+    {
+        return x - n;
+    }
+};
+
+
+template< typename T, typename Distance, bool HasMinusAssign = has_minus_assign< T, Distance >::value >
+struct prior_impl1 :
+    public prior_impl2< T, Distance >
+{
+};
+
+template< typename T, typename Distance >
+struct prior_impl1< T, Distance, true >
+{
+    static T call(T x, Distance n)
+    {
+        x -= n;
+        return x;
+    }
+};
+
+} // namespace next_prior_detail
+
 template <class T>
 inline T next(T x) { return ++x; }
 
 template <class T, class Distance>
 inline T next(T x, Distance n)
 {
-    std::advance(x, n);
+    return next_prior_detail::next_impl1< T, Distance >::call(x, n);
-    return x;
 }
 
 template <class T>
@@ -42,8 +157,7 @@ inline T prior(T x) { return --x; }
 template <class T, class Distance>
 inline T prior(T x, Distance n)
 {
-    std::advance(x, -n);
+    return next_prior_detail::prior_impl1< T, Distance >::call(x, n);
-    return x;
 }
 
 } // namespace boost

test/next_prior_test.cpp

@@ -66,14 +66,32 @@ bool minus_n_test(RandomAccessIterator first, RandomAccessIterator last, Bidirec
     return std::distance(i, last) == std::distance(j, last2);
 }
 
+template<class Iterator, class Distance>
+bool minus_n_unsigned_test(Iterator first, Iterator last, Distance size)
+{
+    Iterator i = boost::prior(last, size);
+    return i == first;
+}
+
 int test_main(int, char*[])
 {
     std::vector<int> x(8);
     std::list<int> y(x.begin(), x.end());
 
+    // Tests with iterators
     BOOST_REQUIRE(plus_one_test(x.begin(), x.end(), y.begin()));
     BOOST_REQUIRE(plus_n_test(x.begin(), x.end(), y.begin()));
     BOOST_REQUIRE(minus_one_test(x.begin(), x.end(), y.end()));
     BOOST_REQUIRE(minus_n_test(x.begin(), x.end(), y.end()));
+    BOOST_REQUIRE(minus_n_unsigned_test(x.begin(), x.end(), x.size()));
+    BOOST_REQUIRE(minus_n_unsigned_test(y.begin(), y.end(), y.size()));
+
+    // Tests with integers
+    BOOST_REQUIRE(boost::next(5) == 6);
+    BOOST_REQUIRE(boost::next(5, 7) == 12);
+    BOOST_REQUIRE(boost::prior(5) == 4);
+    BOOST_REQUIRE(boost::prior(5, 7) == -2);
+    BOOST_REQUIRE(boost::prior(5, 7u) == -2);
+
     return 0;
 }