Metaprogramming: integrate the new facilities into the library

research/try.cpp

@@ -93,81 +93,8 @@ fun2 ()
 using lib::meta::IndexSeq;
 using lib::meta::BuildIndexSeq;
 using lib::meta::BuildIdxIter;
-
+using lib::meta::pickArg;
-
+using lib::meta::pickInit;
-namespace {
-/**
- * @internal pick a single argument from a variadic parameter pack
- * @tparam i the index number (zero based) of the argument to select
- * @warning i must be smaller than the number of arguments available
- */
-template<size_t i>
-struct SelectVararg
-  { 
-    template<typename ARG, typename...ARGS>
-    static auto
-    get (ARG, ARGS&& ...args)
-      {
-        return SelectVararg<i-1>::get (std::forward<ARGS> (args)...);
-      }
-  };
-
-template<>
-struct SelectVararg<0>
-  { 
-    template<typename ARG, typename...ARGS>
-    static auto
-    get (ARG&& a, ARGS...)
-      {
-        return std::forward<ARG>(a);
-      }
-  };
-}
-
-
-/**
- * Helper to single out one argument from a variadic argument pack.
- * @tparam idx the index number (zero based) of the argument to select
- * @remark typically this function is used "driven" by an likewise variadic index sequence,
- *         where the index sequence itself is picked up by a pattern match; this usage pattern
- *         allows arbitrarily to handle some of the arguments of a variable argument list,
- *         as determined by the index sequence passed in.
- */
-template<size_t idx, typename...ARGS>
-constexpr inline auto
-pickArg (ARGS&&... args)
-  {
-    static_assert (idx < sizeof...(args), "insufficient number of arguments");
-    
-    return SelectVararg<idx>::get (std::forward<ARGS> (args)...);
-  }
-
-/**
- * Helper to pick one initialisation argument from a variadic argument pack,
- * falling back to a default constructed element of type `DEFAULT` in case of
- * insufficient number of variadic arguments.
- * @tparam idx the index number (zero based) of the argument to select
- * @tparam DEFALUT type of the default element to construct as fallback
- */
-template<size_t idx, typename DEFAULT, typename...ARGS>
-constexpr inline auto
-pickInit (ARGS&&... args)
-  {
-    enum{
-      SIZ = sizeof...(args),
-      IDX = idx < SIZ? idx : 0
-    };
-    
-    return (idx < SIZ)? SelectVararg<IDX>::get (std::forward<ARGS> (args)...)
-                      : DEFAULT{};
-  }
-
-template<size_t, typename DEFAULT>
-constexpr inline DEFAULT
-pickInit ()
-  {
-    return DEFAULT{};
-  }
 
 
 using Arr = std::array<int, 3>;

src/lib/meta/variadic-helper.hpp

@@ -245,5 +245,83 @@ namespace meta {
   
   
   
+  /* ==== Manipulation of variadic arguments ==== **/
+  
+  namespace { // Implementation delegate template...
+    /**
+     * @internal pick a single argument from a variadic parameter pack
+     * @tparam i the index number (zero based) of the argument to select
+     * @warning i must be smaller than the number of arguments available
+     */
+    template<size_t i>
+    struct SelectVararg
+      {
+        template<typename ARG, typename...ARGS>
+        static auto
+        get (ARG, ARGS&& ...args)
+          {
+            return SelectVararg<i-1>::get (std::forward<ARGS> (args)...);
+          }
+      };
+    
+    template<>
+    struct SelectVararg<0>
+      {
+        template<typename ARG, typename...ARGS>
+        static auto
+        get (ARG&& a, ARGS...)
+          {
+            return std::forward<ARG>(a);
+          }
+      };
+  }//(End)Implementation
+  
+  
+  /**
+   * Helper to single out one argument from a variadic argument pack.
+   * @tparam idx the index number (zero based) of the argument to select
+   * @remark typically this function is used "driven" by an likewise variadic index sequence,
+   *         where the index sequence itself is picked up by a pattern match; this usage pattern
+   *         allows arbitrarily to handle some of the arguments of a variable argument list,
+   *         as determined by the index sequence passed in.
+   */
+  template<size_t idx, typename...ARGS>
+  constexpr inline auto
+  pickArg (ARGS&&... args)
+    {
+      static_assert (idx < sizeof...(args), "insufficient number of arguments");
+      
+      return SelectVararg<idx>::get (std::forward<ARGS> (args)...);
+    }
+  
+  /**
+   * Helper to pick one initialisation argument from a variadic argument pack,
+   * falling back to a default constructed element of type `DEFAULT` in case of
+   * insufficient number of variadic arguments.
+   * @tparam idx the index number (zero based) of the argument to select
+   * @tparam DEFALUT type of the default element to construct as fallback
+   */
+  template<size_t idx, typename DEFAULT, typename...ARGS>
+  constexpr inline auto
+  pickInit (ARGS&&... args)
+    {
+      enum{
+        SIZ = sizeof...(args),
+        IDX = idx < SIZ? idx : 0
+      };
+      
+      return (idx < SIZ)? SelectVararg<IDX>::get (std::forward<ARGS> (args)...)
+                        : DEFAULT{};
+    }
+  
+  template<size_t, typename DEFAULT>
+  constexpr inline DEFAULT
+  pickInit ()
+    {
+      return DEFAULT{};
+    }
+  
+  
+  
 }} // namespace lib::meta
 #endif /*LIB_META_VARIADIC_HELPER_H*/

tests/library/meta/variadic-argument-picker-test.cpp

@@ -182,8 +182,9 @@ namespace test {
           CHECK (n1 == pickArg<0> (n1,n2,n3));
           CHECK (n2 == pickArg<1> (n1,n2,n3));
           CHECK (n3 == pickArg<2> (n1,n2,n3));
-          
+
-          pickArg<3> (n1,n2,n3);
+          // does not compile...
+//        pickArg<3> (n1,n2,n3);
         }