C++智能指针详解与最佳实践

智能指针的概念与作用

智能指针是C++标准库提供的模板类，用于自动管理动态分配的内存资源。其核心目标是解决原生指针易导致的内存泄漏、悬垂指针等问题，通过RAII（Resource Acquisition Is Initialization）机制实现资源的自动释放。

智能指针的分类与特性

unique_ptr

独占所有权的智能指针，同一时刻只能有一个unique_ptr指向特定内存。禁止复制构造和赋值操作，但支持移动语义。适用于明确资源唯一所有权的场景。

std::unique_ptr<int> ptr1(new int(42));
std::unique_ptr<int> ptr2 = std::move(ptr1); // 所有权转移

shared_ptr

通过引用计数实现共享所有权，多个shared_ptr可指向同一对象。当最后一个持有者被销毁时，资源自动释放。适用于需要共享资源的场景。

std::shared_ptr<int> ptr3 = std::make_shared<int>(100);
auto ptr4 = ptr3; // 引用计数增加

weak_ptr

配合shared_ptr使用，解决循环引用问题。不增加引用计数，需通过lock()方法获取临时shared_ptr访问资源。

std::weak_ptr<int> weak(ptr3);
if (auto temp = weak.lock()) {
    // 安全使用资源
}

智能指针的最佳实践

优先使用std::make_unique和std::make_shared而非直接new操作。这两个工厂函数提供异常安全保证，且make_shared能合并内存分配优化性能。

auto ptr5 = std::make_unique<std::string>("Hello");
auto ptr6 = std::make_shared<std::vector<int>>(10, 1);

自定义删除器扩展功能。智能指针允许指定自定义删除逻辑，适用于管理非内存资源（如文件句柄）。

auto fileDeleter = [](FILE* f) { fclose(f); };
std::unique_ptr<FILE, decltype(fileDeleter)> filePtr(fopen("data.txt", "r"), fileDeleter);

性能与内存考量

shared_ptr的引用计数通常采用原子操作保证线程安全，但会带来额外开销。循环引用会导致内存泄漏，需用weak_ptr打破循环。make_shared会一次性分配对象和控制块内存，减少内存碎片。

与原生指针的互操作

通过get()获取原生指针，但需确保智能指针生命周期覆盖使用时段。避免将原生指针同时交给多个智能指针管理，会导致重复释放。release()方法用于转移unique_ptr所有权并返回原生指针。

int* raw = ptr1.release(); // ptr1放弃所有权
delete raw; // 需手动释放

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