From fcbe835f4bfa4bdb791769ab276880f20cd8a55d Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@pdimov.com>
Date: Sat, 1 Apr 2017 17:39:02 +0300
Subject: [PATCH] Add a variadic mp_transform implementation

---
 include/boost/mp11/algorithm.hpp | 32 ++++++++++++++++++++++++++++++--
 test/mp_transform.cpp            | 20 ++++++++++++++++++++
 2 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index cda0eb7..f7d033d 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -44,6 +44,16 @@ template<class L1, class L2> using mp_assign = typename detail::mp_assign_impl<L
 // mp_clear<L>
 template<class L> using mp_clear = mp_assign<L, mp_list<>>;
 
+// mp_fold<L, V, F> forward declaration
+namespace detail
+{
+
+template<class L, class V, template<class...> class F> struct mp_fold_impl;
+
+} // namespace detail
+
+template<class L, class V, template<class...> class F> using mp_fold = typename detail::mp_fold_impl<L, V, F>::type;
+
 // mp_transform<F, L...>
 namespace detail
 {
@@ -71,6 +81,26 @@ template<template<class...> class F, template<class...> class L1, class... T1, t
 
 template<template<class...> class F, class... L> using mp_transform = typename detail::mp_transform_impl<F, L...>::type;
 
+namespace detail
+{
+
+template<template<class...> class F, template<class...> class L1, class... T1, template<class...> class L2, class... T2, template<class...> class L3, class... T3, template<class...> class L4, class... T4, class... L> struct mp_transform_impl<F, L1<T1...>, L2<T2...>, L3<T3...>, L4<T4...>, L...>
+{
+    static_assert( sizeof...(T1) == sizeof...(T2) && sizeof...(T1) == sizeof...(T3) && sizeof...(T1) == sizeof...(T4), "The arguments of mp_transform should be of the same size" );
+
+    using A1 = L1<mp_list<T1, T2, T3, T4>...>;
+
+    template<class V, class T> using _f = mp_transform<mp_push_back, V, T>;
+
+    using A2 = mp_fold<mp_list<L...>, A1, _f>;
+
+    template<class T> using _g = mp_apply<F, T>;
+
+    using type = mp_transform<_g, A2>;
+};
+
+} // namespace detail
+
 // mp_transform_if<P, F, L...>
 namespace detail
 {
@@ -753,8 +783,6 @@ template<template<class...> class L, class T1, class T2, class T3, class T4, cla
 
 } // namespace detail
 
-template<class L, class V, template<class...> class F> using mp_fold = typename detail::mp_fold_impl<L, V, F>::type;
-
 // mp_reverse_fold<L, V, F>
 namespace detail
 {
diff --git a/test/mp_transform.cpp b/test/mp_transform.cpp
index 6782278..ed6f1dd 100644
--- a/test/mp_transform.cpp
+++ b/test/mp_transform.cpp
@@ -29,6 +29,15 @@ struct Z2 {};
 struct Z3 {};
 struct Z4 {};
 
+struct U1 {};
+struct U2 {};
+
+struct V1 {};
+struct V2 {};
+
+struct W1 {};
+struct W2 {};
+
 template<class T> using add_pointer = typename std::add_pointer<T>::type;
 template<class T, class U> using is_same = typename std::is_same<T, U>::type;
 
@@ -81,5 +90,16 @@ int main()
 
     //
 
+    using L8 = std::pair<Z1, Z2>;
+    using L9 = std::pair<U1, U2>;
+    using L10 = std::pair<V1, V2>;
+    using L11 = std::pair<W1, W2>;
+
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform<std::tuple, L5, L6, L8, L9>, std::pair<std::tuple<X1, Y1, Z1, U1>, std::tuple<X2, Y2, Z2, U2>>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform<std::tuple, L5, L6, L8, L9, L10>, std::pair<std::tuple<X1, Y1, Z1, U1, V1>, std::tuple<X2, Y2, Z2, U2, V2>>>));
+    BOOST_TEST_TRAIT_TRUE((std::is_same<mp_transform<std::tuple, L5, L6, L8, L9, L10, L11>, std::pair<std::tuple<X1, Y1, Z1, U1, V1, W1>, std::tuple<X2, Y2, Z2, U2, V2, W2>>>));
+
+    //
+
     return boost::report_errors();
 }