include/ftl/non_null.h

/*
 * Copyright 2022 The Android Open Source Project
 *
 * Licensed 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 <cstdlib>
#include <type_traits>
#include <utility>

namespace android::ftl {

// Enforces and documents non-null pre/post-condition for (raw or smart) pointers.
//
//   void get_length(const ftl::NonNull<std::shared_ptr<std::string>>& string_ptr,
//                   ftl::NonNull<std::size_t*> length_ptr) {
//     // No need for `nullptr` checks.
//     *length_ptr = string_ptr->length();
//   }
//
//   const auto string_ptr = ftl::as_non_null(std::make_shared<std::string>("android"));
//   std::size_t size;
//   get_length(string_ptr, ftl::as_non_null(&size));
//   assert(size == 7u);
//
// For compatibility with std::unique_ptr<T> and performance with std::shared_ptr<T>, move
// operations are allowed despite breaking the invariant:
//
//   using Pair = std::pair<ftl::NonNull<std::shared_ptr<int>>, std::shared_ptr<int>>;
//
//   Pair dupe_if(ftl::NonNull<std::unique_ptr<int>> non_null_ptr, bool condition) {
//     // Move the underlying pointer out, so `non_null_ptr` must not be accessed after this point.
//     auto unique_ptr = std::move(non_null_ptr).take();
//
//     auto non_null_shared_ptr = ftl::as_non_null(std::shared_ptr<int>(std::move(unique_ptr)));
//     auto nullable_shared_ptr = condition ? non_null_shared_ptr.get() : nullptr;
//
//     return {std::move(non_null_shared_ptr), std::move(nullable_shared_ptr)};
//   }
//
//   auto ptr = ftl::as_non_null(std::make_unique<int>(42));
//   const auto [ptr1, ptr2] = dupe_if(std::move(ptr), true);
//   assert(ptr1.get() == ptr2);
//
template <typename Pointer>
class NonNull final {
  struct Passkey {};

 public:
  // Disallow `nullptr` explicitly for clear compilation errors.
  NonNull() = delete;
  NonNull(std::nullptr_t) = delete;

  // Copy operations.

  constexpr NonNull(const NonNull&) = default;
  constexpr NonNull& operator=(const NonNull&) = default;

  constexpr const Pointer& get() const { return pointer_; }
  constexpr explicit operator const Pointer&() const { return get(); }

  // Move operations. These break the invariant, so care must be taken to avoid subsequent access.

  constexpr NonNull(NonNull&&) = default;
  constexpr NonNull& operator=(NonNull&&) = default;

  constexpr Pointer take() && { return std::move(pointer_); }
  constexpr explicit operator Pointer() && { return take(); }

  // Dereferencing.
  constexpr decltype(auto) operator*() const { return *get(); }
  constexpr decltype(auto) operator->() const { return get(); }

  // Private constructor for ftl::as_non_null. Excluded from candidate constructors for conversions
  // through the passkey idiom, for clear compilation errors.
  template <typename P>
  constexpr NonNull(Passkey, P&& pointer) : pointer_(std::forward<P>(pointer)) {
    if (!pointer_) std::abort();
  }

 private:
  template <typename P>
  friend constexpr auto as_non_null(P&&) -> NonNull<std::decay_t<P>>;

  Pointer pointer_;
};

template <typename P>
constexpr auto as_non_null(P&& pointer) -> NonNull<std::decay_t<P>> {
  using Passkey = typename NonNull<std::decay_t<P>>::Passkey;
  return {Passkey{}, std::forward<P>(pointer)};
}

template <typename P, typename Q>
constexpr bool operator==(const NonNull<P>& lhs, const NonNull<Q>& rhs) {
  return lhs.get() == rhs.get();
}

template <typename P, typename Q>
constexpr bool operator!=(const NonNull<P>& lhs, const NonNull<Q>& rhs) {
  return !operator==(lhs, rhs);
}

}  // namespace android::ftl