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/*
* 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
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