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Commit 04534e26 authored by Dominik Laskowski's avatar Dominik Laskowski
Browse files

FTL: Add invariant for non-null pointers 

Upcasting is not supported for now.

Bug: 185536303
Test: ftl_test
Change-Id: Id37d3e9a4a794291417405eb2a8a300a9d2bfe72
parent b7d92d9f

include/ftl/non_null.h

0 → 100644
+116 −0
Original line number Diff line number Diff line 
/*

 * 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

libs/ftl/Android.bp

+1 −0
Original line number Diff line number Diff line
@@ -22,6 +22,7 @@ cc_test { 
        "flags_test.cpp",

        "future_test.cpp",

        "match_test.cpp",

        "non_null_test.cpp",

        "optional_test.cpp",

        "small_map_test.cpp",

        "small_vector_test.cpp",

libs/ftl/non_null_test.cpp

0 → 100644
+75 −0
Original line number Diff line number Diff line 
/*

 * 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.

 */



#include <ftl/non_null.h>

#include <gtest/gtest.h>



#include <memory>

#include <string>

#include <string_view>



namespace android::test {

namespace {



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();

}



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)};

}



}  // namespace



// Keep in sync with example usage in header file.

TEST(NonNull, Example) {

  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));

  EXPECT_EQ(size, 7u);



  auto ptr = ftl::as_non_null(std::make_unique<int>(42));

  const auto [ptr1, ptr2] = dupe_if(std::move(ptr), true);

  EXPECT_EQ(ptr1.get(), ptr2);

}



namespace {



constexpr std::string_view kApple = "apple";

constexpr std::string_view kOrange = "orange";



using StringViewPtr = ftl::NonNull<const std::string_view*>;

constexpr StringViewPtr kApplePtr = ftl::as_non_null(&kApple);

constexpr StringViewPtr kOrangePtr = ftl::as_non_null(&kOrange);



constexpr StringViewPtr longest(StringViewPtr ptr1, StringViewPtr ptr2) {

  return ptr1->length() > ptr2->length() ? ptr1 : ptr2;

}



static_assert(longest(kApplePtr, kOrangePtr) == kOrangePtr);



}  // namespace

}  // namespace android::test