Add type Required

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 0.2.35
+
+* Added `mbo::types::Required` which is similar to `RefWrap` but stores the actual type (and unlike `std::optional` cannot be reset).
+
 # 0.2.34
 
 * Added `mbo::testing::WhenTransformedBy` which allows to compare containers after transforming them.

README.md

@@ -145,6 +145,9 @@ The C++ library is organized in functional groups each residing in their own dir
         * Mainly, this prevents calling the destructor and thus prevents termination issues (initialization order fiasco).
     * mbo/types:ref_wrap_cc, mbo/types/ref_wrap.h
         * template-type `RefWrap<T>`: similar to `std::reference_wrapper` but supports operators `->` and `*`.
+    * mbo/types:required_cc, mbo/types/required.h
+        * template-type `Required<T>`: similar to `RefWrap` but stores the actual type (and unlike `std::optional` cannot be reset).
+
     * mbo/types:template_search_cc, mbo/types/template_search.h:
         * template struct `BinarySearch` implements templated binary search algorithm.
         * template struct `LinearSearch` implements templated linear search algorithm.

mbo/testing/matchers.h

@@ -16,7 +16,7 @@
 #ifndef MBO_TESTING_MATCHERS_H_
 #define MBO_TESTING_MATCHERS_H_
 
-#include <concepts>
+#include <concepts>  // IWYU pragma: keep
 #include <type_traits>
 
 #include "gmock/gmock-matchers.h"

mbo/types/BUILD.bazel

@@ -137,6 +137,21 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "required_cc",
+    hdrs = ["required.h"],
+    visibility = ["//visibility:public"],
+)
+
+cc_test(
+    name = "required_test",
+    srcs = ["required_test.cc"],
+    deps = [
+        ":required_cc",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_library(
     name = "stringify_cc",
     srcs = ["stringify.cc"],

mbo/types/required.h

@@ -0,0 +1,108 @@
+// SPDX-FileCopyrightText: Copyright (c) The helly25/mbo authors (helly25.com)
+// SPDX-License-Identifier: Apache-2.0
+//
+// 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.
+
+#ifndef MBO_TYPES_REQUIRED_H_
+#define MBO_TYPES_REQUIRED_H_
+
+#include <compare>   // IWYU pragma: keep
+#include <concepts>  // IWYU pragma: keep
+#include <utility>
+
+namespace mbo::types {
+
+// Template class `Required<T>` is a wrapper around a type `T`. The value can be
+// replaced without using assign or move-assign operators. Instead the wrapper
+// will call the destructor and then in-place construct the wrapper value using
+// the type's move constructor.
+//
+// In other words the reson for this type is he rare case where a type `T` that
+// follows the rule of 0/3/5 and of the 4 ctor/assignments only allows for
+// move-construction all while (say a test) needs to be implemented with assign.
+//
+// The type is similar to `std::optional` only it cannot be `std::nullopt`.
+// The type is however closer to `RefWrap`.
+//
+// The wrapper is not implemented itself as a wrapper or specialization of STL
+// type `std::optional` due to its overhead.
+template<typename T>
+class Required {
+ public:
+  using type = T;
+
+  Required() = default;
+
+  constexpr explicit Required(T v) : value_(std::move(v)) {}
+
+  template<typename U>
+  requires(std::constructible_from<T, U> && !std::same_as<T, U>)
+  constexpr explicit Required(U&& v) : value_(std::forward<U>(v)) {}
+
+  template<typename... Args>
+  requires(std::constructible_from<T, Args...>)
+  constexpr explicit Required(std::in_place_t, Args&&... args) : value_(std::forward<Args>(args)...) {}
+
+  constexpr Required& emplace(T v) {  // NOLINT(readability-identifier-naming)
+    value_.~T();
+    new (&value_) T(std::move(v));
+    return *this;
+  }
+
+  template<typename... Args>
+  requires(std::constructible_from<T, Args...>)
+  constexpr Required& emplace(Args&&... args) {  // NOLINT(readability-identifier-naming)
+    value_.~T();
+    new (&value_) T(std::forward<Args>(args)...);
+    return *this;
+  }
+
+  constexpr T& get() noexcept { return value_; }  // NOLINT(*-identifier-naming)
+
+  constexpr const T& get() const noexcept { return value_; }  // NOLINT(*-identifier-naming)
+
+  constexpr T* operator->() noexcept __attribute__((returns_nonnull)) { return &value_; }
+
+  constexpr const T* operator->() const noexcept __attribute__((returns_nonnull)) { return &value_; }
+
+  constexpr T& operator*() noexcept { return value_; }
+
+  constexpr const T& operator*() const noexcept { return value_; }
+
+  constexpr operator T&() noexcept { return value_; }  // NOLINT(*-explicit-*)
+
+  constexpr operator const T &() const noexcept { return value_; }  // NOLINT(*-explicit-*)
+
+  template<std::three_way_comparable_with<T> U>
+  constexpr auto operator<=>(const Required<U>& other) const noexcept {
+    if (value_ == other) {
+      return decltype(value_ <=> other)::equivalent;
+    }
+    return value_ <=> other.value_;
+  }
+
+  template<std::three_way_comparable_with<T> U>
+  constexpr auto operator<=>(const U& other) const noexcept {
+    if (value_ == other) {
+      return decltype(value_ <=> other)::equivalent;
+    }
+    return value_ <=> other;
+  }
+
+ private:
+  T value_;
+};
+
+}  // namespace mbo::types
+
+#endif  // MBO_TYPES_REQUIRED_H_

mbo/types/required_test.cc

@@ -0,0 +1,134 @@
+// SPDX-FileCopyrightText: Copyright (c) The helly25/mbo authors (helly25.com)
+// SPDX-License-Identifier: Apache-2.0
+//
+// 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 "mbo/types/required.h"
+
+#include <string>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace mbo::types {
+namespace {
+
+// NOLINTBEGIN(*-magic-numbers)
+
+using ::testing::Ge;
+using ::testing::IsEmpty;
+using ::testing::Le;
+using ::testing::Not;
+using ::testing::Pair;
+
+struct RequiredTest : ::testing::Test {};
+
+TEST_F(RequiredTest, Basics) {
+  Required<int> req(25);
+  ASSERT_THAT(req, 25);
+  req.emplace(33);
+  EXPECT_THAT(req, 33);
+}
+
+TEST_F(RequiredTest, Compare) {
+  Required<const int> req(25);
+  EXPECT_THAT(req, 25);
+  EXPECT_THAT(req <= 55, true);
+  EXPECT_THAT(req < 55, true);
+  EXPECT_THAT(req <= 11, false);
+  EXPECT_THAT(req < 11, false);
+  EXPECT_THAT(req == 55, false);
+  EXPECT_THAT(req != 55, true);
+  EXPECT_THAT(req == 25, true);
+  EXPECT_THAT(req != 25, false);
+  EXPECT_THAT(55 >= req, true);
+  EXPECT_THAT(55 > req, true);
+  EXPECT_THAT(11 >= req, false);
+  EXPECT_THAT(11 > req, false);
+  EXPECT_THAT(req, Le(55));
+  EXPECT_THAT(req, Le(25));
+  EXPECT_THAT(req, Not(Le(11)));
+  EXPECT_THAT(req, Ge(11));
+  EXPECT_THAT(req, Ge(25));
+  EXPECT_THAT(req, Not(Ge(33)));
+  int val = 25;
+  Required<int> cmp(val);
+  EXPECT_THAT(req == val, true);
+  EXPECT_THAT(req != val, false);
+  EXPECT_THAT(req <= val, true);
+  EXPECT_THAT(req >= val, true);
+  EXPECT_THAT(req < val, false);
+  EXPECT_THAT(req > val, false);
+  val = 11;
+  EXPECT_THAT(req == val, false);
+  EXPECT_THAT(req != val, true);
+  EXPECT_THAT(req <= val, false);
+  EXPECT_THAT(req >= val, true);
+  EXPECT_THAT(req < val, false);
+  EXPECT_THAT(req > val, true);
+  val = 33;
+  EXPECT_THAT(req == val, false);
+  EXPECT_THAT(req != val, true);
+  EXPECT_THAT(req <= val, true);
+  EXPECT_THAT(req >= val, false);
+  EXPECT_THAT(req < val, true);
+  EXPECT_THAT(req > val, false);
+}
+
+TEST_F(RequiredTest, Pair) {
+  Required<std::pair<int, int>> req({25, 33});
+  EXPECT_THAT(*req, Pair(25, 33));
+  EXPECT_THAT(req->first, 25);
+  EXPECT_THAT(req->second, 33);
+  req.emplace({42, 99});
+  EXPECT_THAT(*req, Pair(42, 99));
+  EXPECT_THAT(req->first, 42);
+  EXPECT_THAT(req->second, 99);
+}
+
+TEST_F(RequiredTest, PairByArgs) {
+  Required<std::pair<int, int>> req(std::in_place, 25, 33);
+  EXPECT_THAT(*req, Pair(25, 33));
+  EXPECT_THAT(req->first, 25);
+  EXPECT_THAT(req->second, 33);
+  req.emplace(42, 99);
+  EXPECT_THAT(*req, Pair(42, 99));
+  EXPECT_THAT(req->first, 42);
+  EXPECT_THAT(req->second, 99);
+}
+
+struct NoDefCtor {
+  NoDefCtor() = delete;
+
+  explicit NoDefCtor(int v) : value(v) {}
+
+  bool operator==(int v) const { return v == value; }
+
+  int value;
+};
+
+TEST_F(RequiredTest, NoDefCtor) {
+  Required<NoDefCtor> req(25);
+  EXPECT_THAT(*req, 25);
+}
+
+TEST_F(RequiredTest, DefCtor) {
+  Required<std::string> req;
+  EXPECT_THAT(*req, IsEmpty());
+}
+
+// NOLINTEND(*-magic-numbers)
+
+}  // namespace
+}  // namespace mbo::types