Generate an nxtcpp_traits.h with metaprogramming helpers

The files currently has Builder and BuiltObject template type aliases to
go from builder type to built object type or vice-versa.

generator/main.py

@@ -397,6 +397,7 @@ def main():
         additional_params = {'native_methods': lambda typ: cpp_native_methods(api_params['types'], typ)}
         renders.append(FileRender('apicpp.h', 'nxt/nxtcpp.h', [base_params, api_params, additional_params]))
         renders.append(FileRender('apicpp.cpp', 'nxt/nxtcpp.cpp', [base_params, api_params, additional_params]))
+        renders.append(FileRender('apicpp_traits.h', 'nxt/nxtcpp_traits.h', [base_params, api_params, additional_params]))
 
     if 'mock_nxt' in targets:
         renders.append(FileRender('mock_api.h', 'mock/mock_nxt.h', [base_params, api_params, c_params]))

generator/templates/apicpp_traits.h

@@ -0,0 +1,43 @@
+//* Copyright 2017 The NXT Authors
+//*
+//* Licensed under the Apache License, Version 2.0 (the "License");
+//* you may not use this file except in compliance with the License.
+//* You may obtain a copy of the License at
+//*
+//*     http://www.apache.org/licenses/LICENSE-2.0
+//*
+//* Unless required by applicable law or agreed to in writing, software
+//* distributed under the License is distributed on an "AS IS" BASIS,
+//* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//* See the License for the specific language governing permissions and
+//* limitations under the License.
+
+#ifndef NXTCPP_TRAITS_H
+#define NXTCPP_TRAITS_H
+
+#include "nxtcpp.h"
+
+#include <type_traits>
+
+namespace nxt {
+
+    template<typename Builder>
+    using BuiltObject = decltype(std::declval<Builder>().GetResult());
+
+    template<typename BuiltObject>
+    struct BuiltObjectTrait {
+    };
+
+    {% for type in by_category["object"] if type.is_builder %}
+        template<>
+        struct BuiltObjectTrait<BuiltObject<{{as_cppType(type.name)}}>> {
+            using Builder = {{as_cppType(type.name)}};
+        };
+    {% endfor %}
+
+    template<typename BuiltObject>
+    using Builder = typename BuiltObjectTrait<BuiltObject>::Builder;
+
+};
+
+#endif // NXTCPP_TRAITS_H