base/cxx20_to_address.h

// Copyright 2021 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_CXX20_TO_ADDRESS_H_
#define BASE_CXX20_TO_ADDRESS_H_

#include <memory>
#include <type_traits>

namespace gurl_base {

namespace {

template <typename Ptr, typename = void>
struct has_std_to_address : std::false_type {};

template <typename Ptr>
struct has_std_to_address<
    Ptr,
    std::void_t<decltype(std::pointer_traits<Ptr>::to_address(
        std::declval<Ptr>()))>> : std::true_type {};

}  // namespace

// Implementation of C++20's std::to_address.
// Note: This does consider specializations of pointer_traits<>::to_address,
// even though it's a C++20 member function, because CheckedContiguousIterator
// specializes pointer_traits<> with a to_address() member.
//
// Reference: https://wg21.link/pointer.conversion#lib:to_address
template <typename T>
constexpr T* to_address(T* p) noexcept {
  static_assert(!std::is_function<T>::value,
                "Error: T must not be a function type.");
  return p;
}

template <typename Ptr>
constexpr auto to_address(const Ptr& p) noexcept {
  if constexpr (has_std_to_address<Ptr>::value) {
    return std::pointer_traits<Ptr>::to_address(p);
  } else {
    return gurl_base::to_address(p.operator->());
  }
}

}  // namespace base

#endif  // BASE_CXX20_TO_ADDRESS_H_