[GitHub] [arrow] dsunder commented on a change in pull request #10551: ARROW-13097: [C++] Provide simple reflection utility

[GitBox <gi...@apache.org>]

dsunder commented on a change in pull request #10551:
URL: https://github.com/apache/arrow/pull/10551#discussion_r654014167



##########
File path: cpp/src/arrow/util/reflection_internal.h
##########
@@ -0,0 +1,192 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you 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.
+
+#pragma once
+
+#include <array>
+#include <string>
+#include <tuple>
+#include <utility>
+
+namespace arrow {
+namespace internal {
+
+namespace detail {
+
+#ifdef _MSC_VER
+#define ARROW_PRETTY_FUNCTION __FUNCSIG__
+#else
+#define ARROW_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#endif
+
+template <typename T>
+const char* raw() {
+  return ARROW_PRETTY_FUNCTION;
+}
+
+template <typename T>
+size_t raw_sizeof() {
+  return sizeof(ARROW_PRETTY_FUNCTION);
+}
+
+#undef ARROW_PRETTY_FUNCTION
+
+constexpr bool starts_with(char const* haystack, char const* needle) {
+  return needle[0] == '\0' ||

Review comment:
       Too bad we can't use c++17.  The use of recursion to define these simple `constexpr` functions makes me sad :(.

##########
File path: cpp/src/arrow/util/reflection_internal.h
##########
@@ -0,0 +1,192 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you 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.
+
+#pragma once
+
+#include <array>
+#include <string>
+#include <tuple>
+#include <utility>
+
+namespace arrow {
+namespace internal {
+
+namespace detail {
+
+#ifdef _MSC_VER
+#define ARROW_PRETTY_FUNCTION __FUNCSIG__
+#else
+#define ARROW_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#endif
+
+template <typename T>
+const char* raw() {

Review comment:
       `raw()` and `raw_sizeof()` are undescriptive names.  Consider renaming to `pretty_function_name()` and `prettty_function_name_size()`.  
   
   Is there a reason why these functions are not `constexpr`?  

##########
File path: cpp/src/arrow/util/reflection_internal.h
##########
@@ -0,0 +1,192 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you 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.
+
+#pragma once
+
+#include <array>
+#include <string>
+#include <tuple>
+#include <utility>
+
+namespace arrow {
+namespace internal {
+
+namespace detail {
+
+#ifdef _MSC_VER
+#define ARROW_PRETTY_FUNCTION __FUNCSIG__
+#else
+#define ARROW_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#endif
+
+template <typename T>
+const char* raw() {
+  return ARROW_PRETTY_FUNCTION;
+}
+
+template <typename T>
+size_t raw_sizeof() {
+  return sizeof(ARROW_PRETTY_FUNCTION);
+}
+
+#undef ARROW_PRETTY_FUNCTION
+
+constexpr bool starts_with(char const* haystack, char const* needle) {
+  return needle[0] == '\0' ||
+         (haystack[0] == needle[0] && starts_with(haystack + 1, needle + 1));
+}
+
+constexpr size_t search(char const* haystack, char const* needle) {
+  return haystack[0] == '\0' || starts_with(haystack, needle)
+             ? 0
+             : search(haystack + 1, needle) + 1;
+}
+
+const size_t typename_prefix = search(raw<double>(), "double");
+
+template <typename T>
+size_t struct_class_prefix() {
+#ifdef _MSC_VER
+  return starts_with(raw<T>() + typename_prefix, "struct ")
+             ? 7
+             : starts_with(raw<T>() + typename_prefix, "class ") ? 6 : 0;
+#else
+  return 0;
+#endif
+}
+
+template <typename T>
+size_t typename_length() {
+  // raw_sizeof<T>() - raw_sizeof<double>() ==
+  //     (length of T's name) - strlen("double")
+  // (length of T's name) ==
+  //     raw_sizeof<T>() - raw_sizeof<double>() + strlen("double")
+  return raw_sizeof<T>() - struct_class_prefix<T>() - raw_sizeof<double>() + 6;

Review comment:
       Why does this return value need ```- raw_sizeof<double>() +6```?




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