C++11智能指针终极指南

C++11 智能指针详解（下）

shared_ptr：共享所有权智能指针

shared_ptr 通过引用计数机制管理资源，多个指针可以共享同一对象的所有权。当最后一个 shared_ptr 离开作用域时，资源自动释放。

构造方式：

std::shared_ptr<int> p1(new int(10));
std::shared_ptr<int> p2 = std::make_shared<int>(20); // 更高效

引用计数操作：

p1.use_count(); // 获取当前引用计数
p1.reset();     // 减少引用计数（可能释放资源）

循环引用问题：

struct Node {
    std::shared_ptr<Node> next;
};
auto node1 = std::make_shared<Node>();
auto node2 = std::make_shared<Node>();
node1->next = node2;
node2->next = node1; // 循环引用导致内存泄漏

weak_ptr：解决循环引用的助手

weak_ptr 是 shared_ptr 的观察者，不增加引用计数，可检测资源是否有效。

基本用法：

std::weak_ptr<int> wp(p1);
if (auto sp = wp.lock()) { // 尝试提升为shared_ptr
    // 资源仍存在
}

解决循环引用：

struct SafeNode {
    std::weak_ptr<SafeNode> next; // 使用weak_ptr替代
};

unique_ptr 与 shared_ptr 的转换

unique_ptr 可以转移所有权到 shared_ptr，反之则不行：

std::unique_ptr<int> up(new int(42));
std::shared_ptr<int> sp = std::move(up); // 合法

自定义删除器

两种智能指针都支持自定义删除逻辑：

// 文件指针自定义删除
std::shared_ptr<FILE> filePtr(fopen("test.txt","r"), [](FILE* f){
    if(f) fclose(f);
});

// 数组删除器
std::unique_ptr<int[]> arr(new int[10]);

性能与安全建议

优先使用 make_shared 而非 new（单次内存分配）
避免原始指针与智能指针混用
按需选择指针类型：
- 独占所有权用 unique_ptr
- 共享所有权用 shared_ptr
- 观察资源用 weak_ptr

典型应用场景

资源自动管理（文件、网络连接）
工厂模式返回对象
容器存储动态对象
多线程共享只读数据

通过合理组合这三种智能指针，可以显著降低C++内存管理复杂度，同时保持代码的高效性。现代C++项目应尽量避免直接使用 new/delete。

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