117 lines
4.0 KiB
C++
117 lines
4.0 KiB
C++
/*
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* Copyright 2022 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <cstdlib>
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#include <type_traits>
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#include <utility>
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namespace android::ftl {
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// Enforces and documents non-null pre/post-condition for (raw or smart) pointers.
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//
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// void get_length(const ftl::NonNull<std::shared_ptr<std::string>>& string_ptr,
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// ftl::NonNull<std::size_t*> length_ptr) {
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// // No need for `nullptr` checks.
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// *length_ptr = string_ptr->length();
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// }
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//
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// const auto string_ptr = ftl::as_non_null(std::make_shared<std::string>("android"));
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// std::size_t size;
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// get_length(string_ptr, ftl::as_non_null(&size));
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// assert(size == 7u);
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//
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// For compatibility with std::unique_ptr<T> and performance with std::shared_ptr<T>, move
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// operations are allowed despite breaking the invariant:
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//
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// using Pair = std::pair<ftl::NonNull<std::shared_ptr<int>>, std::shared_ptr<int>>;
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//
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// Pair dupe_if(ftl::NonNull<std::unique_ptr<int>> non_null_ptr, bool condition) {
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// // Move the underlying pointer out, so `non_null_ptr` must not be accessed after this point.
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// auto unique_ptr = std::move(non_null_ptr).take();
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//
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// auto non_null_shared_ptr = ftl::as_non_null(std::shared_ptr<int>(std::move(unique_ptr)));
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// auto nullable_shared_ptr = condition ? non_null_shared_ptr.get() : nullptr;
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//
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// return {std::move(non_null_shared_ptr), std::move(nullable_shared_ptr)};
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// }
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//
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// auto ptr = ftl::as_non_null(std::make_unique<int>(42));
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// const auto [ptr1, ptr2] = dupe_if(std::move(ptr), true);
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// assert(ptr1.get() == ptr2);
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//
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template <typename Pointer>
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class NonNull final {
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struct Passkey {};
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public:
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// Disallow `nullptr` explicitly for clear compilation errors.
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NonNull() = delete;
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NonNull(std::nullptr_t) = delete;
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// Copy operations.
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constexpr NonNull(const NonNull&) = default;
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constexpr NonNull& operator=(const NonNull&) = default;
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constexpr const Pointer& get() const { return pointer_; }
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constexpr explicit operator const Pointer&() const { return get(); }
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// Move operations. These break the invariant, so care must be taken to avoid subsequent access.
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constexpr NonNull(NonNull&&) = default;
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constexpr NonNull& operator=(NonNull&&) = default;
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constexpr Pointer take() && { return std::move(pointer_); }
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constexpr explicit operator Pointer() && { return take(); }
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// Dereferencing.
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constexpr decltype(auto) operator*() const { return *get(); }
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constexpr decltype(auto) operator->() const { return get(); }
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// Private constructor for ftl::as_non_null. Excluded from candidate constructors for conversions
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// through the passkey idiom, for clear compilation errors.
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template <typename P>
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constexpr NonNull(Passkey, P&& pointer) : pointer_(std::forward<P>(pointer)) {
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if (!pointer_) std::abort();
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}
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private:
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template <typename P>
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friend constexpr auto as_non_null(P&&) -> NonNull<std::decay_t<P>>;
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Pointer pointer_;
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};
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template <typename P>
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constexpr auto as_non_null(P&& pointer) -> NonNull<std::decay_t<P>> {
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using Passkey = typename NonNull<std::decay_t<P>>::Passkey;
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return {Passkey{}, std::forward<P>(pointer)};
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}
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template <typename P, typename Q>
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constexpr bool operator==(const NonNull<P>& lhs, const NonNull<Q>& rhs) {
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return lhs.get() == rhs.get();
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}
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template <typename P, typename Q>
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constexpr bool operator!=(const NonNull<P>& lhs, const NonNull<Q>& rhs) {
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return !operator==(lhs, rhs);
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}
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} // namespace android::ftl
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