删除函数std::unique_ptr<_Tp, _Dp>& std::unique_ptr<_Tp, _Dp>::operator=(const std::unique_ptr<_Tp, _Dp>&)

Question

我目前正在尝试建立一个非常基本的实体组件系统。 为此，我定义了以下类：

class Component
{
public:
    virtual ~Component() {};  // Ensure polymorphic behavior with a virtual destructor
};

class TransformComponent : public Component
{
    glm::vec2 position = {0.0f, 0.0f};
    float rotation = 0.0f;
    float scale = 1.0f;
};

using identifier =  uuids::uuid;
class Entity {
private:
    identifier id;
    bool isActive = true;
    std::string name = "new entity";

    std::vector<std::unique_ptr<Component>> components;
public:
    Entity() : isActive(true), id( UUID::get()), name("NEW") {};
    ~Entity() = default;

    // Disable copying
    Entity(const Entity&) = delete;
    Entity& operator=(const Entity&) = delete;

    // Define move constructor and move assignment
    Entity(Entity&& other) noexcept;
    Entity& operator=(Entity&& other) noexcept;

    template<typename T>
    T* GetOrAdd() {
        static_assert(std::is_base_of<Component, T>::value, "T must be derived from Component");
        for (auto& comp : components) {
            if (auto result = dynamic_cast<T*>(comp.get())) {
                return result;
            }
        }
        auto newComp = std::make_unique<T>();
        T* result = newComp.get();
        components.push_back(std::move(newComp));
        return result;
    }

当我尝试将实体作为 std::unique_ptr 添加到另一个类时，如下所示：

class Scene {
        public:
            Scene(std::string sceneName, bool isEditorMode);
            ~Scene();
        private: 
            std::unique_ptr<Entity> ent = std::make_unique<Entity>();
}

我收到以下错误：

D:\Projects\searchengineproject\vocabulary\game\src\rendering\HierarchyPanel.cpp: In constructor 'gl3::HierarchyPanel::HierarchyPanel()':
D:\Projects\searchengineproject\vocabulary\game\src\rendering\HierarchyPanel.cpp:9:52: error: use of deleted function 'gl3::Scene& gl3::Scene::operator=(const gl3::Scene&)'
    9 |         currentScene = *GlobalHelper::GetScenes()[0];
      |                                                    ^
In file included from D:\Projects\searchengineproject\vocabulary\game\src\rendering\HierarchyPanel.h:4,
                 from D:\Projects\searchengineproject\vocabulary\game\src\rendering\HierarchyPanel.cpp:1:
D:/Projects/searchengineproject/vocabulary/game/src/entities/Scene.h:21:11: note: 'gl3::Scene& gl3::Scene::operator=(const gl3::Scene&)' is implicitly deleted because the default definition would be ill-formed:
   21 |     class Scene {
      |           ^~~~~
D:/Projects/searchengineproject/vocabulary/game/src/entities/Scene.h:21:11: error: use of deleted function 'std::unique_ptr<_Tp, _Dp>& std::unique_ptr<_Tp, _Dp>::operator=(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = gl3::Entity; _Dp = std::default_delete<gl3::Entity>]'
In file included from D:/Program Files/CLion 2022.3.3/bin/mingw/lib/gcc/x86_64-w64-mingw32/11.2.0/include/c++/bits/locale_conv.h:41,
                 from D:/Program Files/CLion 2022.3.3/bin/mingw/lib/gcc/x86_64-w64-mingw32/11.2.0/include/c++/locale:43,
                 from D:/Program Files/CLion 2022.3.3/bin/mingw/lib/gcc/x86_64-w64-mingw32/11.2.0/include/c++/bits/fs_path.h:37,
                 from D:/Program Files/CLion 2022.3.3/bin/mingw/lib/gcc/x86_64-w64-mingw32/11.2.0/include/c++/filesystem:45,
                 from D:/Projects/searchengineproject/vocabulary/game/src/rendering/Texture.h:6,
                 from D:/Projects/searchengineproject/VokabelTrainer/game/src/GlobalHelper.h:3,
                 from D:\Projects\searchengineproject\vocabulary\game\src\rendering\HierarchyPanel.h:3,
                 from D:\Projects\searchengineproject\vocabulary\game\src\rendering\HierarchyPanel.cpp:1:
D:/Program Files/CLion 2022.3.3/bin/mingw/lib/gcc/x86_64-w64-mingw32/11.2.0/include/c++/bits/unique_ptr.h:469:19: note: declared here
  469 |       unique_ptr& operator=(const unique_ptr&) = delete;
      |                   ^~~~~~~~
mingw32-make[3]: *** [game\CMakeFiles\game_lab_3.dir\build.make:316: game/CMakeFiles/game_lab_3.dir/src/rendering/HierarchyPanel.cpp.obj] Error 1
mingw32-make[3]: *** Waiting for unfinished jobs....
mingw32-make[2]: *** [CMakeFiles\Makefile2:2095: game/CMakeFiles/game_lab_3.dir/all] Error 2
mingw32-make[1]: *** [CMakeFiles\Makefile2:2102: game/CMakeFiles/game_lab_3.dir/rule] Error 2
mingw32-make: *** [Makefile:1047: game_lab_3] Error 2

我在互联网上读到过类似的问题，但解决方案都是关于避免使用复制构造函数，这应该在我的情况下定义。

我可能会错过什么？

Answer 1

std::unique_ptr 不能分配给另一个（右值）std::unique_ptr。这是因为当原始的 unique_ptr 删除其数据时，它的剩余副本将是一个悬空指针（不好）。

如果您想删除之前分配的数据并与新分配的数据交换，请使用重置方法。

如果您想移动所有权（本质上是更改哪个 unique_ptr 应该负责删除和包含数据），您可以使用 std::move 将一个 unique_ptr 分配给另一个 unique_ptr。

例如：

class Integer
{
  public:
    int data;
    Integer(int _data) : data(_data)
    {std::cout << “made integer ” << _data << “\n”;
    ~Integer(){std::cout << “destructed integer ”
        << data << “\n”;}
}
int main()
{
  std::unique_ptr<Integer> iptr = 
          std::make_unique<Integer>(5);
  std::cout << *iptr << “\n;
  std::unique_ptr<Integer> new_owner = 
          std::move(iptr);
  new_owner.reset(new Integer(10));
  std::cout << *new_owner << “\n”;
}

输出

made integer 5
5
destructed integer 5
made integer 10
10