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

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

bkietz commented on a change in pull request #10551:
URL: https://github.com/apache/arrow/pull/10551#discussion_r653976569



##########
File path: cpp/src/arrow/util/reflection_test.cc
##########
@@ -0,0 +1,121 @@
+// 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.
+
+#include <gtest/gtest.h>
+
+#include "arrow/util/reflection_internal.h"
+
+namespace arrow {
+namespace internal {
+
+// unmodified structure which we wish to reflect on:
+struct Person {
+  std::string name;
+  int age;
+};
+
+// enumeration of properties:
+template <>
+struct ReflectionTraits<Person> {
+  using Properties = std::tuple<DataMember<Person, int, &Person::age>,

Review comment:
       It was intentional; I wanted to make clear that the ordering follows the trait and not the declaration in the struct. But in practice we'd probably always order identically. I'll switch them

##########
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:
       Agreed

##########
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:
       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66639
   Older GCC doesn't think __PRETTY_FUNCTION__ is constexpr. Shame, too: with constexpr it's possible to ensure the sliced-away portions of __PRETTY_FUNCTION__ don't make it into the object file

##########
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:
       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66639
   Older GCC doesn't think `__PRETTY_FUNCTION__` is constexpr. Shame, too: with constexpr it's possible to ensure the sliced-away portions of `__PRETTY_FUNCTION__` don't make it into the object file

##########
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:
       Sorry, the comment is trying to communicate the algebra of the situation. Pretty function can be thought of as string interpolation, like `"const char* raw<T>() [with T = {}]".format(typename)`. Compilers' format strings differ so we have to compute the length of the extraneous content by using a known typename, for which I chose "double". I know the length of `dbl_len = "const char* raw<T>() [with T = {}]".format('double')` is 6 more than the number of extraneous chars, so `extra_len = dbl_len - 6` which I can subtract from `len("const char* raw<T>() [with T = {}]".format(typename))` to get the length of the type name

##########
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:
       Again, would be much easier to read of I weren't trying to shoehorn this into something c++11 constexpr compatible

##########
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:
       I'll add more comments.




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