mirror of
https://github.com/kidfromjupiter/nearby.git
synced 2026-09-15 07:06:11 -04:00
Alexey Polyudov
de31c27947
Signed-off-by: Alexey Polyudov <apolyudov@google.com> Change-Id: I85d490e448375aa4eb7b09680757c02fff1420b4
86 lines
3.2 KiB
C++
86 lines
3.2 KiB
C++
// Copyright 2020 Google LLC
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef PLATFORM_CONTAINER_OF_H_
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#define PLATFORM_CONTAINER_OF_H_
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#include <cstddef>
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#include <type_traits>
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namespace location::nearby {
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// Similar to offsetof() macro, but implemented in a type-safe way,
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// OffsetOf(<pointer-to-member>) returns the byte offset of a given data
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// member in the ClassType.
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// Behavior is undefined if member is not a direct, non-static data member of
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// type ClassType.
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// usage example:
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// struct S { int x; double y; };
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// size_t y_offset = OffsetOf(&S::y);
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// CHECK(y_offset >= sizeof(int));
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//
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// the following is not guaranteed to work:
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// struct S1 { int x; };
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// struct S2 { double y; };
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// struct S : public S1, S2 { char t; };
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// size_t y_offset_bad = OffsetOf(&S::y);
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// because S::y is not a direct member of S; it is a member by inheritance.
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// To make sure OffsetOf works with inherited members, it must be called
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// with explicitly defined template parameters, as follows:
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// size_t y_offset_ok = OffsetOf<double,S>(&S::y);
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//
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// However, the following is guaranteed to work:
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// struct S1 { int x; };
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// struct S2 { double y; };
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// struct S3 { double z; };
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// struct S : public S1, S2 { S3 s3; char t; };
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// size_t s3_offset = OffsetOf(&S::s3);
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template <typename ValueType, typename ClassType>
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constexpr size_t OffsetOf(const ValueType ClassType::*member) {
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std::aligned_storage_t<sizeof(ClassType), alignof(ClassType)> obj_memory;
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ClassType* obj = reinterpret_cast<ClassType*>(&obj_memory);
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return reinterpret_cast<size_t>(&(obj->*member)) -
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reinterpret_cast<size_t>(obj);
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}
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// Similar to Linux containerof() macro, this function returns pointer to
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// the type instance that contains the specified member;
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// ContainerOf(<pointer to variable instance of type ValueType, which is member
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// of ClassType>, <pointer-to-ClassType-member>);
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// usage example:
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// struct S { int x; double y; } a;
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// S *b = ContainerOf(&a.y, &S::y);
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// CHECK(b == &a);
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template <typename ValueType, typename ClassType>
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ClassType* ContainerOf(ValueType* ptr, ValueType ClassType::*member) {
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using BaseValueType = std::remove_volatile_t<ValueType>;
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return reinterpret_cast<ClassType*>(
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reinterpret_cast<char*>(const_cast<BaseValueType*>(ptr)) -
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OffsetOf(member));
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}
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template <typename ValueType, typename ClassType>
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const ClassType* ContainerOf(const ValueType* ptr,
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ValueType ClassType::*member) {
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using BaseValueType = std::remove_volatile_t<ValueType>;
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return reinterpret_cast<const ClassType*>(
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reinterpret_cast<const char*>(const_cast<BaseValueType*>(ptr)) -
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OffsetOf(member));
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}
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} // namespace location::nearby
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#endif // PLATFORM_CONTAINER_OF_H_
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