From 84ef2c111745fcafd32747aeba7a5e83cfede8bd Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Tue, 13 Nov 2018 12:56:48 +1100
Subject: [PATCH] tuple/type: add 'nth' accessor for tuples

---
 test/tuple/type.cpp | 11 +++++++++++
 tuple/type.hpp      | 34 +++++++++++++++++++++++++++++-----
 2 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/test/tuple/type.cpp b/test/tuple/type.cpp
index cf295e78..57d2b1d3 100644
--- a/test/tuple/type.cpp
+++ b/test/tuple/type.cpp
@@ -21,6 +21,17 @@ main (void)
         tap.expect_eq (cruft::tuple::type::index<tuple_t,int>::value, 1u, "tuple index extraction");
     }
 
+    {
+        using tuple_t = std::tuple<float,int,void>;
+        tap.expect (
+            std::is_same_v<
+                cruft::tuple::type::nth_t<tuple_t,1>,
+                int
+            >,
+            "tuple type indexing with 'nth'"
+        );
+    }
+
     {
 #if !defined(NO_RTTI)
         auto tuple = std::make_tuple (1u, 1, 1.f, 1.);
diff --git a/tuple/type.hpp b/tuple/type.hpp
index 6b4a1e9d..517031be 100644
--- a/tuple/type.hpp
+++ b/tuple/type.hpp
@@ -172,8 +172,8 @@ namespace cruft::tuple::type {
 
 
     ///////////////////////////////////////////////////////////////////////////
-    // transform a tuple-like type (ie, something that responds to
-    // tuple_element) into a tuple
+    /// Transform a tuple-like type (ie, something that responds to
+    /// tuple_element) into a tuple
     template <
         typename TupleT,
         typename Indices = std::make_index_sequence<
@@ -202,7 +202,7 @@ namespace cruft::tuple::type {
 
 
     ///////////////////////////////////////////////////////////////////////////
-    // concatenate a variadic collection of tuple-like types
+    /// Concatenate a variadic collection of tuple-like types
     template <typename ...Args>
     struct cat;
 
@@ -234,7 +234,7 @@ namespace cruft::tuple::type {
 
 
     ///////////////////////////////////////////////////////////////////////////
-    // remove a type from a tuple-like type
+    /// Remove a type from a tuple-like type
     template <
         typename DoomedT,
         typename TupleT
@@ -280,6 +280,7 @@ namespace cruft::tuple::type {
 
 
     ///////////////////////////////////////////////////////////////////////////
+    /// Remove duplicate types from the list of held types in a tuple
     template <typename TupleT>
     struct unique;
 
@@ -309,4 +310,27 @@ namespace cruft::tuple::type {
     //-------------------------------------------------------------------------
     template <typename T>
     using unique_t = typename unique<T>::type;
-}
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// Convenience accessor for the 'nth' type of a tuple.
+    template <typename TupleT, std::size_t IndexV>
+    struct nth;
+
+
+    //-------------------------------------------------------------------------
+    template <typename HeadT, typename ...TailT>
+    struct nth<std::tuple<HeadT,TailT...>,0> {
+        using type = HeadT;
+    };
+
+
+    //-------------------------------------------------------------------------
+    template <typename HeadT, typename ...TailT, std::size_t IndexV>
+    struct nth<std::tuple<HeadT,TailT...>,IndexV> : nth<std::tuple<TailT...>,IndexV-1> { };
+
+
+    //-------------------------------------------------------------------------
+    template <typename TupleT, std::size_t IndexV>
+    using nth_t = typename nth<TupleT, IndexV>::type;
+};