c++ - 有人能给我提供一个设置和检索 unique_ptr 的简单示例吗

Question

我将如何使用智能指针编写以下内容:

class App
{
public:
    Project* GetProject();
    SetProject( Project* project );
private:
    Project* project;
}

Project* project = new Project();
App* app = new App();

app.SetProject( project );
Project* retrievedProject = app.GetProject();

我最感兴趣的是方法参数和返回技术...这样正确吗？

class App
{
public:
    void App::SetProject( unique_ptr<Project> project )
    {
        this->project.swap( project );
    }

    Project* App::GetProject() const
    {
        return this->project.get();
    }
private:
    unique_ptr< Project > project;
}

...

unique_ptr< Project > project( new Project() );
unique_ptr< App > app( new App() );

app.SetProject( project );
unique_ptr< Project > retrievedProject = app.GetProject();

这与这个问题有关:Should i stop using * pointers? - A few entry questions

我对现在的规则有点困惑...提前致谢!

编辑小问题:

Galik，您曾提到您很少想在函数调用之外声明 Project，然后将其传递给发布...我有以下内容(例如):

unique_ptr<Project>project( new Project() );
project.Title = "Bob";
project.Type = PROJECTYPE.DISTRIBUTION;
project.Manager = manager;

app.SetProject( project.release() );

这完全正常吗？我的问题是，我实际上是在为游戏创建工具，我担心随着垃圾收集的启动，不断标记删除会导致微卡顿……我只是担心而已吗？

Answer 1

这个问题的答案实际上取决于您想要做什么。您应该使用智能指针来表达所有权，并且您不应该真正传递关于您是否可以帮助它的智能指针。

如果例如您的App 类应该处理各种Project 对象并且只提供访问 将 Project 发送给外部人员，那么您应该通过返回对该项目的 reference 来表达这一点。

通过这种方式，多个调用者可以访问 App 拥有的内部 Project。

就像这样:

class Project
{
public:
    void do_stuff() {}
};

// This class is the Project Owner
// Meaning it is responsible for
// managing its life-time
class App
{
public:
    // return a reference expressing the caller
    // does not gain ownership merely access
    Project& GetProject() { return *project; }

    // replace configured Project
    // accepting a std::unique_ptr expresses the fact that
    // this function takes ownership of the Project
    void SetProject(std::unique_ptr<Project> project) { this->project = std::move(project); }

private:
    std::unique_ptr<Project> project;
};

int main()
{
    App app; // don't make pointers if we don't have to

    // project has ownership of the raw pointer
    std::unique_ptr<Project> project(new Project());

    // std::move() transfers ownership to the function's parameter
    app.SetProject(std::move(project));

    // get a reference to the internal Project
    Project& retrievedProject = app.GetProject();

    // work with configured Project
    retrievedProject.do_stuff();
}