Metaprogramming using Boost.CallableTraits and Boost.MP11 (#444)

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: beojan

phlex/metaprogramming/CMakeLists.txt

@@ -12,15 +12,6 @@ cet_make_library(
 
 install(FILES delegate.hpp type_deduction.hpp DESTINATION include/phlex/metaprogramming)
 
-install(
-  FILES
-    detail/basic_concepts.hpp
-    detail/ctor_reflect_types.hpp
-    detail/number_output_objects.hpp
-    detail/number_parameters.hpp
-    detail/parameter_types.hpp
-    detail/return_type.hpp
-  DESTINATION include/phlex/metaprogramming/detail
-)
+install(FILES detail/ctor_reflect_types.hpp DESTINATION include/phlex/metaprogramming/detail)
 
 add_library(phlex::metaprogramming ALIAS phlex_metaprogramming_int)

phlex/metaprogramming/detail/basic_concepts.hpp

@@ -2,22 +2,28 @@
 #define PHLEX_METAPROGRAMMING_TYPE_DEDUCTION_HPP
 
 #include "phlex/metaprogramming/detail/ctor_reflect_types.hpp"
-#include "phlex/metaprogramming/detail/number_output_objects.hpp"
-#include "phlex/metaprogramming/detail/number_parameters.hpp"
-#include "phlex/metaprogramming/detail/parameter_types.hpp"
-#include "phlex/metaprogramming/detail/return_type.hpp"
+
+#include "boost/callable_traits.hpp"
+#include "boost/mp11/algorithm.hpp"
+#include "boost/mp11/list.hpp"
 
 #include <atomic>
 #include <iterator>
 #include <tuple>
 #include <type_traits>
 
 namespace phlex::experimental {
+  namespace ct = boost::callable_traits;
+  namespace mp11 = boost::mp11;
   template <typename T>
-  using return_type = decltype(detail::return_type_impl(std::declval<T>()));
+  using return_type = ct::return_type_t<T>;
 
+  // A simple mp_if doesn't work because both branches always need to be valid.
+  // With eval_if and eval_if_not the false branch doesn't need to be valid, but
+  // does need to be expressed in this F, Args... format.
   template <typename T>
-  using function_parameter_types = decltype(detail::parameter_types_impl(std::declval<T>()));
+  using function_parameter_types = mp11::
+    mp_eval_if_not<std::is_member_function_pointer<T>, ct::args_t<T>, mp11::mp_rest, ct::args_t<T>>;
 
   template <std::size_t I, typename T>
   using function_parameter_type = std::tuple_element_t<I, function_parameter_types<T>>;
@@ -26,37 +32,29 @@ namespace phlex::experimental {
   using constructor_parameter_types = typename refl::as_tuple<T>;
 
   template <typename T>
-  constexpr std::size_t number_parameters = detail::number_parameters_impl<T>;
+  constexpr std::size_t number_parameters = mp11::mp_size<function_parameter_types<T>>::value;
 
+  // Wrapping in a tuple then "flattening" (which just removes one nested inner layer of tuple)
+  // ensures a single T by itself becomes std::tuple<T> without changing anything that's already
+  // a tuple.
   template <typename T>
-  constexpr std::size_t number_output_objects = detail::number_output_objects_impl<T>;
-
-  using detail::number_types;
-
-  namespace detail {
-    template <typename Head, typename... Tail>
-    std::tuple<Tail...> skip_first_type_impl(std::tuple<Head, Tail...> const&);
+  constexpr std::size_t number_types = mp11::mp_size<mp11::mp_flatten<std::tuple<T>>>::value;
 
-    template <typename Head, typename... Tail>
-    std::tuple<Tail...> skip_first_type_impl(std::pair<Head, Tail...> const&);
-  }
+  template <typename T>
+  constexpr std::size_t number_output_objects =
+    std::same_as<void, return_type<T>> ? 0 : number_types<return_type<T>>;
 
+  // mp_apply<std::tuple, ..> converts other wrapper types (e.g. std::pair) to tuple
   template <typename Tuple>
-  using skip_first_type = decltype(detail::skip_first_type_impl(std::declval<Tuple>()));
+  using skip_first_type = mp11::mp_rest<mp11::mp_apply<std::tuple, Tuple>>;
 
   template <typename T, typename... Args>
   struct check_parameters {
     using input_parameters = function_parameter_types<T>;
     static_assert(std::tuple_size<input_parameters>{} >= sizeof...(Args));
-
-    template <std::size_t... Is>
-    static constexpr bool check_params_for(std::index_sequence<Is...>)
-    {
-      return std::conjunction_v<std::is_same<std::tuple_element_t<Is, input_parameters>, Args>...>;
-    }
-
+    static constexpr bool value =
+      mp11::mp_starts_with<input_parameters, std::tuple<Args...>>::value;
     constexpr operator bool() noexcept { return value; }
-    static constexpr bool value = check_params_for(std::index_sequence_for<Args...>{});
   };
 
   // ===================================================================
@@ -66,20 +64,10 @@ namespace phlex::experimental {
   template <typename T>
   struct is_non_const_lvalue_reference<T const&> : std::false_type {};
 
+  // mp_similar<std::atomic<int>, T> just checks if T is an atomic (of any type)
   template <typename T>
-  class remove_atomic {
-  public:
-    using type = T;
-  };
-
-  template <typename T>
-  class remove_atomic<std::atomic<T>> {
-  public:
-    using type = T;
-  };
-
-  template <typename T>
-  using remove_atomic_t = remove_atomic<T>::type;
+  using remove_atomic_t =
+    mp11::mp_eval_if_not<mp11::mp_similar<std::atomic<int>, T>, T, mp11::mp_front, T>;
 
   template <typename T>
   concept container = requires {