base/cxx20_to_address.h

   1 // Copyright 2021 The Chromium Authors
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_CXX20_TO_ADDRESS_H_
   6 #define BASE_CXX20_TO_ADDRESS_H_
   7
   8 #include <memory>
   9 #include <type_traits>
  10
  11 namespace base {
  12
  13 namespace {
  14
  15 template <typename Ptr, typename = void>
  16 struct has_std_to_address : std::false_type {};
  17
  18 template <typename Ptr>
  19 struct has_std_to_address<
  20     Ptr,
  21     std::void_t<decltype(std::pointer_traits<Ptr>::to_address(
  22         std::declval<Ptr>()))>> : std::true_type {};
  23
  24 }  // namespace
  25
  26 // Implementation of C++20's std::to_address.
  27 // Note: This does consider specializations of pointer_traits<>::to_address,
  28 // even though it's a C++20 member function, because CheckedContiguousIterator
  29 // specializes pointer_traits<> with a to_address() member.
  30 //
  31 // Reference: https://wg21.link/pointer.conversion#lib:to_address
  32 template <typename T>
  33 constexpr T* to_address(T* p) noexcept {
  34   static_assert(!std::is_function_v<T>,
  35                 "Error: T must not be a function type.");
  36   return p;
  37 }
  38
  39 template <typename Ptr>
  40 constexpr auto to_address(const Ptr& p) noexcept {
  41   if constexpr (has_std_to_address<Ptr>::value) {
  42     return std::pointer_traits<Ptr>::to_address(p);
  43   } else {
  44     return base::to_address(p.operator->());
  45   }
  46 }
  47
  48 }  // namespace base
  49
  50 #endif  // BASE_CXX20_TO_ADDRESS_H_