Fixed logic of difference_type selection, which was completely wack. In the

process, added digit_traits<> to compute the number of digits in intmax_t even
when not supplied by numeric_limits<>.


[SVN r8747]

include/boost/detail/numeric_traits.hpp

@@ -49,6 +49,10 @@
 // See http://www.boost.org for most recent version including documentation.
 
 // Revision History
+// 23 Jan 2001 - Fixed logic of difference_type selection, which was
+//               completely wack. In the process, added digit_traits<>
+//               to compute the number of digits in intmax_t even when
+//               not supplied by numeric_limits<>.
 // 21 Jan 2001 - Created
 
 #ifndef BOOST_NUMERIC_TRAITS_HPP_DWA20001901
@@ -93,6 +97,43 @@ namespace boost { namespace detail {
       enum { value = (Number(-1) < Number(0)) };
   };
 
+# ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS
+  // digit_traits - compute the number of digits in a built-in integer
+  // type. Needed for implementations on which numeric_limits is not specialized
+  // for intmax_t (e.g. VC6).
+  template <bool is_specialized> struct digit_traits_select;
+
+  // numeric_limits is specialized; just select that version of digits
+  template <> struct digit_traits_select<true>
+  {
+      template <class T> struct traits
+      {
+          enum { digits = std::numeric_limits<T>::digits };
+      };
+  };
+
+  // numeric_limits is not specialized; compute digits from sizeof(T)
+  template <> struct digit_traits_select<false>
+  {
+      template <class T> struct traits
+      {
+          enum { digits = sizeof(T) * std::numeric_limits<unsigned char>::digits
+                         - (is_signed<T>::value ? 1 : 0)
+          };
+      };
+  };
+
+  // here's the "usable" template
+  template <class T> struct digit_traits
+  {
+      enum {
+          digits = digit_traits_select<
+                std::numeric_limits<T>::is_specialized
+          >::template traits<T>::digits
+      };
+  };
+#endif  
+
   // Template class integer_traits<Integer> -- traits of various integer types
   // This should probably be rolled into boost::integer_traits one day, but I
 // need it to work without <limits>
@@ -104,24 +145,27 @@ namespace boost { namespace detail {
       typedef Integer integer_type;
       typedef std::numeric_limits<integer_type> x;
 #   ifdef BOOST_MSVC
-      enum { is_integer = x::is_integer };
+      // for some reason, MSVC asserts when it shouldn't unless we make these
+      // local definitions
+      enum {
+          is_integer = x::is_integer,
+          is_specialized = x::is_specialized
+      };
+      BOOST_STATIC_ASSERT(is_integer);
+      BOOST_STATIC_ASSERT(is_specialized);
 #   endif
    public:
-#  ifndef BOOST_MSVC // for some reason, this asserts when it shouldn't
-      BOOST_STATIC_ASSERT(x::is_integer);
-#  else
-      BOOST_STATIC_ASSERT(is_integer);
-#  endif
       typedef typename
-      if_true<x::is_signed
+      if_true<(x::is_signed
-              & !(x::is_bounded
+              && (!x::is_bounded
-                  | x::digits >= std::numeric_limits<boost::intmax_t>::digits)>::template then<
+                 // digits is the number of no-sign bits
+                  || (x::digits + 1 >= digit_traits<boost::intmax_t>::digits)))>::template then<
         Integer,
           
-      typename if_true<(x::digits < std::numeric_limits<signed int>::digits)>::template then<
+      typename if_true<(x::digits + 1 < digit_traits<signed int>::digits)>::template then<
         signed int,
 
-      typename if_true<(x::digits < std::numeric_limits<signed long>::digits)>::template then<
+      typename if_true<(x::digits + 1 < digit_traits<signed long>::digits)>::template then<
         signed long,
 
    // else