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如何使用weak_ptr构造一个保存对父级的引用的对象？

发布于 2025-01-04 18:36:00 字数 108 浏览 0 评论 0原文

假设我有一个对象，其中包含子对象的shared_ptr。

我希望子对象对父对象有一个weak_ptr，子对象的构造函数应该是什么样子以及如何从父对象构造子对象？

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‖放下 2025-01-11 18:36:00

由于您拥有子对象的唯一所有权，因此可以保证子对象的寿命不会比其父对象的寿命长。你可以有一个像这样的模型。

struct Parent;

struct Child {
        Child( Parent& p ) : p_(&p) {}
        Parent* p_;
};

#include <memory>

struct Parent {
        std::unique_ptr<Child> c_;
        void AddChild() {
                c_.reset(new Child(*this));
        }
};

当然，子级在析构函数中对父级所做的任何事情都应该小心，它可能会因为其父级超出范围而被销毁。这是子级对其父级拥有 weak_ptr 的唯一优势（它仍然无法从析构函数中对父级执行任何操作，但至少它可以安全地告诉我们这一点），但是这个依赖于其父级由 shared_ptr 拥有，这对于子级来说是一种更加不灵活的设计。

这将是 weak_ptr 解决方案：

// NOT RECOMMENDED
#include <memory>

struct Parent;

struct Child {
        Child( const std::shared_ptr<Parent>& p ) : p_(p) {}
        std::weak_ptr<Parent> p_;
};

struct Parent : std::enable_shared_from_this<Parent> {
        std::unique_ptr<Child> c_;
        void AddChild() {
                // Warning, undefined behaviour if this Parent
                // isn't owner by shared_ptr
                c_.reset(new Child(shared_from_this()));
        }
};

As you have unique owner ship of child objects the child is guaranteed not to outlive it's parent. You could have a model something like this.

struct Parent;

struct Child {
        Child( Parent& p ) : p_(&p) {}
        Parent* p_;
};

#include <memory>

struct Parent {
        std::unique_ptr<Child> c_;
        void AddChild() {
                c_.reset(new Child(*this));
        }
};

Of course, the child should be careful with anything that it does with the parent in the destructor, it might be being destroyed because its parent is going out of scope. This is about the only advantage of a child having a weak_ptr to its parent (it still won't be able to do anything with the parent from its destructor but at least it can safely tell that) but this relies on its parent being owned by a shared_ptr which is a much more inflexible design for the child.

This would be the weak_ptr solution:

// NOT RECOMMENDED
#include <memory>

struct Parent;

struct Child {
        Child( const std::shared_ptr<Parent>& p ) : p_(p) {}
        std::weak_ptr<Parent> p_;
};

struct Parent : std::enable_shared_from_this<Parent> {
        std::unique_ptr<Child> c_;
        void AddChild() {
                // Warning, undefined behaviour if this Parent
                // isn't owner by shared_ptr
                c_.reset(new Child(shared_from_this()));
        }
};

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