How does weak_ptr prevent circular references in C++?

The method of weak_ptr in C++ to prevent circular references: use weak_ptr to hold a reference to an object, and use a normal pointer to hold another reference. When the object has no other strong references, weak_ptr automatically sets the reference to nullptr, thus preventing circular references.

Use weak_ptr in C++ to prevent circular references

What is a circular reference?

Circular references occur when two or more objects hold references to each other, forming a cycle. In C++, this causes a memory leak because the object cannot be released.

What is weak_ptr?

weak_ptr is a special smart pointer in C++ that points to an object but does not increase the reference count of the object. This means that the object held by weak_ptr can be destroyed without other strong references (i.e. ordinary pointers).

weak_ptr How to prevent circular references?

To prevent circular references, you can use weak_ptr to hold one reference to the object and use a normal pointer to hold the other reference. When the object has no other strong references, weak_ptr automatically sets the reference to nullptr, thus preventing circular references.

Practical Case

Consider the following example:

// 创建两个对象
class A {
public:
    std::weak_ptr<B> b;
};

class B {
public:
    std::shared_ptr<A> a;
};

int main() {
    // 创建循环引用
    auto a = std::make_shared<A>();
    auto b = std::make_shared<B>();
    a->b = b;
    b->a = a;

    // 释放 a，但 b 仍然持有 a 的引用
    a.reset();

    // 检查 b 是否仍持有 a 的引用
    if (b->a.expired()) {
        std::cout << "b no longer holds a reference to a" << std::endl;
    }
    return 0;
}

Output:

b no longer holds a reference to a

In this case , weak_ptr a->b Automatically sets the reference to nullptr after a is released, thus preventing circular references.

Conclusion

By using weak_ptr, you can prevent circular references in C++ and thus avoid memory leaks. When establishing references between objects, the use of weak_ptr and plain pointers should be carefully considered to ensure correct reference management.

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