编辑“︁C++ weak ptr 详解”︁（章节）

== 实际应用场景 ==

=== 缓存系统 ===
weak_ptr 非常适合实现对象缓存，当外部不再需要对象时，缓存可以自动释放：

<syntaxhighlight lang="cpp">
#include <memory>
#include <unordered_map>
#include <iostream>

class Cache {
    std::unordered_map<int, std::weak_ptr<std::string>> cache;
    
public:
    std::shared_ptr<std::string> get(int id) {
        auto it = cache.find(id);
        if (it != cache.end()) {
            return it->second.lock();  // 返回shared_ptr或nullptr
        }
        return nullptr;
    }
    
    void store(int id, std::shared_ptr<std::string> value) {
        cache[id] = value;
    }
};

int main() {
    Cache cache;
    
    {
        auto data = std::make_shared<std::string>("Important Data");
        cache.store(1, data);
        
        if (auto cached = cache.get(1)) {
            std::cout << *cached << "\n";  // 输出: Important Data
        }
    }  // data离开作用域，被销毁
    
    if (auto cached = cache.get(1)) {
        std::cout << "Data still exists\n";
    } else {
        std::cout << "Data has been released\n";  // 会执行这一行
    }
    
    return 0;
}
</syntaxhighlight>

=== 观察者模式 ===
weak_ptr 可用于实现观察者模式，避免观察者意外延长被观察对象的生命周期：

<syntaxhighlight lang="cpp">
#include <memory>
#include <vector>
#include <iostream>

class Subject;

class Observer {
public:
    virtual void update() = 0;
    virtual ~Observer() = default;
};

class Subject {
    std::vector<std::weak_ptr<Observer>> observers;
    
public:
    void attach(std::weak_ptr<Observer> obs) {
        observers.push_back(obs);
    }
    
    void notify() {
        for (auto& weak_obs : observers) {
            if (auto obs = weak_obs.lock()) {
                obs->update();
            }
        }
    }
};

class ConcreteObserver : public Observer {
public:
    void update() override {
        std::cout << "Observer notified!\n";
    }
};

int main() {
    auto subject = std::make_shared<Subject>();
    auto observer = std::make_shared<ConcreteObserver>();
    
    subject->attach(observer);
    subject->notify();  // 输出: Observer notified!
    
    return 0;
}
</syntaxhighlight>