included rich_cons

[SVN r11015]

include/boost/tuple/detail/tuple_basic.hpp

@@ -22,6 +22,9 @@
 // William Kempf, Vesa Karvonen, John Max Skaller, Ed Brey, Beman Davis,
 // David Abrahams.
 
+// Revision history:
+// 2001 08 30 David Abrahams
+//      Added default constructor for cons<>.
 // ----------------------------------------------------------------- 
 
 #ifndef BOOST_TUPLE_BASIC_HPP
@@ -79,6 +82,14 @@ struct default_arg {
 // from all temporaries. 
      static typename boost::remove_cv<T>::type f() { return T(); }
 };
+
+// This is just to produce a more informative error message
+// The code would fail in any case
+template<class T, int N>
+struct default_arg<T[N]> {
+  static T* f() { 
+    return generate_error<T[N]>::arrays_are_not_valid_tuple_elements; }
+};
    
 template <class T>
 struct default_arg<T&> {
@@ -235,6 +246,12 @@ struct cons {
   typename tuple_access_traits<tail_type>::const_type 
   get_tail() const { return tail; }  
 
+  cons() : head(detail::tuples::default_arg<HT>::f()), tail() {}
+  // the argument for head is not strictly needed, but it prevents 
+  // array type elements. This is good, since array type elements 
+  // cannot be supported properly in any case (no assignment, 
+  // copy works only if the tails are exactly the same type, ...)
+  
   cons(typename tuple_access_traits<head_type>::parameter_type h,
        const tail_type& t)
     : head (h), tail(t) {}  
@@ -302,7 +319,9 @@ struct cons<HT, null_type> {
   get_head() const { return head; }
   
   const null_type get_tail() const { return null_type(); }  
-  
+
+  cons() : head(detail::tuples::default_arg<HT>::f()) {}
+
   cons(typename tuple_access_traits<head_type>::parameter_type h,
        const null_type& = null_type())
     : head (h) {}