Investigation: add a solution for default initialisation of missing arguments

...still somewhat unsatisfactory, because
- no clear compile error message when invoking pickArg with insufficient arguments
- the default initialisation case in SelectVararg is duplicated and messy

research/try.cpp

@@ -95,21 +95,21 @@ using lib::meta::BuildIndexSeq;
 using lib::meta::BuildIdxIter;
 
 
-template<size_t i>
-struct Pick
+template<size_t i, typename INIT=void>
+struct SelectVararg
   { 
     template<typename ARG, typename...ARGS>
     static auto
     get (ARG, ARGS ...args)
       {
-        return Pick<i-1>::get (args...);
+        return SelectVararg<i-1,INIT>::get (args...);
       }
     
-    static auto get() { return nullptr; }
+    static INIT get() { return INIT{}; }
   };
 
-template<>
-struct Pick<0>
+template<typename INIT>
+struct SelectVararg<0, INIT>
   { 
     template<typename ARG, typename...ARGS>
     static ARG
@@ -118,9 +118,23 @@ struct Pick<0>
         return a;
       }
     
-    static auto get() { return nullptr; }
+    static INIT get() { return INIT{}; }
   };
 
+template<size_t idx, typename...ARGS>
+constexpr inline auto
+pickArg (ARGS... args)
+  {
+    return SelectVararg<idx>::get (args...);
+  }
+
+template<size_t idx, typename INIT, typename...ARGS>
+constexpr inline auto
+pickInit (ARGS... args)
+  {
+    return SelectVararg<idx, INIT>::get (args...);
+  }
+
 
 
 using Arr = std::array<int, 3>;
@@ -130,8 +144,8 @@ template<size_t...idx1, size_t...idx2, typename...ARGS>
 Arr
 dispatch_ (IndexSeq<idx1...>,IndexSeq<idx2...>, ARGS...args)
   {
-    Arr arr{Pick<idx1>::get(args...) ...};
-    fun2 (Pick<idx2>::get(args...) ...);
+    Arr arr{pickInit<idx1,int> (args...) ...};
+    fun2 (pickArg<idx2> (args...) ...);
     return arr;
   }
 
@@ -158,6 +172,9 @@ main (int, char**)
     auto arr = dispatch (2,3,4,5,6,7,8);
     cout << util::join(arr) << "| " << showSizeof(arr) <<endl;
     
+    arr = dispatch (7,8);
+    cout << util::join(arr);
+    
     cout <<  "\n.gulp.\n";
     
     return 0;

src/lib/meta/variadic-helper.hpp

@@ -171,7 +171,7 @@ namespace meta {
       using First = typename BuildIndexSeq<min(c,n)>::Ascending;
       
       template<size_t c>
-      using After = typename BuildIndexSeq<max(size_t(0),n-c)>::template OffsetBy<c>;
+      using After = typename BuildIndexSeq< (n>c)? n-c : 0>::template OffsetBy<c>;
     };
   
   template<>