src/connectivity/bluetooth/lib/cpp-type/member_pointer_traits.h

// Copyright 2020 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef SRC_CONNECTIVITY_BLUETOOTH_LIB_CPP_TYPE_MEMBER_POINTER_TRAITS_H_
#define SRC_CONNECTIVITY_BLUETOOTH_LIB_CPP_TYPE_MEMBER_POINTER_TRAITS_H_

#include <cstddef>
#include <memory>
#include <type_traits>

namespace bt_lib_cpp_type {

// Given a pointer-to-data-member, declare public types ClassType and MemberType deduced from the
// type of the pointer. Note that the template argument is a pointer, not a type.
// Example:
//   using MemberType = typename MemberPointerTraits<&Foo::bar>::MemberType;            // OK
//   using MemberType = typename MemberPointerTraits<decltype(&Foo::bar)>::MemberType;  // Wrong
template <auto>
class MemberPointerTraits;  // undefined base case for non-pointers-to-data-member

template <typename ClassT, typename MemberT, MemberT ClassT::*PointerToMember>
class MemberPointerTraits<PointerToMember> {
 public:
  // For class InnerClass declared within OuterClass, this will be deduced as InnerClass.
  using ClassType = std::remove_cv_t<ClassT>;
  using MemberType = MemberT;

  // Returns the offset of MemberT within the layout of ClassT.
  // This is necessary because pointers-to-data-member can not be used with the offsetof macro (see
  // http://wg21.link/p0908r0 for a description and proposed standard fix). Note this constructs and
  // value-initializes an object of ClassT, so care should be taken if that has side effects.
  static size_t constexpr offset() {
    // Pointer subtraction is only defined for array objects per ISO/IEC 14882:2017(E) § 8.7
    // [expr.add] ¶ 2.5, so we have to actually construct an object rather than use a nullptr
    // pointer or std::declval (whose rules disallow odr-use).
    static_assert(std::is_standard_layout_v<ClassType>);

    // This assumes that the layout of the ClassT temporary construct is the same as that of ClassT
    // objects elsewhere, an assumption that is likely UB as only the first member of ClassT is
    // guaranteed (by [class.mem] ¶ 17) to be pointer-interconvertible with ClassT. Subsequent
    // members are only guaranteed to have increasing addresses ([class.mem] ¶ 24), so it's not
    // impossible that a compiler may use different padding around members that are not accessed in
    // this function than it uses elsewhere. For now, it's safest to use this _only_ with
    // [[gnu::packed]] classes.
    const ClassType temporary{};  // Value initialization in case of const members
    return size_t(&(temporary.*PointerToMember)) - size_t(&temporary);
  }
};

}  // namespace bt_lib_cpp_type

#endif  // SRC_CONNECTIVITY_BLUETOOTH_LIB_CPP_TYPE_MEMBER_POINTER_TRAITS_H_