How do the performance and overhead of different C++ smart pointer types compare?

C Comparison of smart pointer types: unique_ptr: exclusive ownership, low overhead (1 pointer size); shared_ptr: shared ownership, high overhead (reference counting, control block); weak_ptr: weak reference, low overhead (1 pointer size) ). Applicable scenarios: Frequent allocation/release: unique_ptr Shared ownership: shared_ptr or weak_ptr Management of memory by reference count: shared_ptr

Comparison of performance and overhead of different C smart pointer types

Smart pointers are class templates in C used to manage dynamically allocated memory. They provide the convenience and security of memory management, eliminating the need to manually manage pointers. Different smart pointer types offer different capabilities and overhead, and understanding these differences is critical to making the best choice in your application.

Types and Overhead

The most commonly used smart pointer types in C include:

• unique_ptr: Exclusive Ownership A pointer used to manage the life cycle of a single object. Overhead: Low (1 pointer size)
• shared_ptr: A pointer with shared ownership, allowing multiple pointers to reference the same object simultaneously. Overhead: High (reference count, control block)
• weak_ptr: Weak reference does not participate in the reference count of the object and will not prevent the object from being deleted. Overhead: Low (1 pointer size)

Performance comparison

The performance of different smart pointer types varies depending on usage scenarios. For operations that perform frequent pointer allocations and deallocations, the less expensive unique_ptr will result in better performance.

For shared ownership cases, shared_ptr is a robust and easy-to-use solution, but its reference counting mechanism introduces overhead. In this case, consider using weak_ptr to achieve non-ownership sharing.

Practical Case

Suppose we have a function that needs to manage a dynamically allocated string container. We can use different smart pointer types to manage the life cycle of the container:

// 使用 unique_ptr
void example_unique_ptr() {
  // 分配并初始化字符串容器
  auto container = std::make_unique<std::vector<std::string>>(100);

  // 对容器进行操作

  // 不再需要容器后，unique_ptr 自动释放它
}

// 使用 shared_ptr
void example_shared_ptr() {
  std::shared_ptr<std::vector<std::string>> container;

  {
    // 创建一个临时的 shared_ptr，指向动态分配的容器
    auto tmp = std::make_shared<std::vector<std::string>>(100);
    container = tmp;  // 将所有权转移到 container

    // 对容器进行操作
  }

  // 离开作用域时，tmp 失效，但 container 仍指向容器
  // 等到所有 shared_ptr 引用都被销毁后，容器才会被释放
}

// 使用 weak_ptr
void example_weak_ptr() {
  std::shared_ptr<std::vector<std::string>> container;

  {
    // 创建一个临时 shared_ptr，没有直接所有权
    auto tmp = std::make_shared<std::vector<std::string>>(100);
    std::weak_ptr<std::vector<std::string>> weak_container(tmp);

    // 对容器进行操作

    if (auto locked = weak_container.lock()) {
      // locked 现在是一个指向容器的 shared_ptr
    }
  }

  // 离开作用域时，tmp 失效，container 可能仍然存在
  // 如果没有其他 shared_ptr 引用容器，它会被释放
}

Selection Guide

Selecting the most appropriate smart pointer type depends on the specific needs of the application :

• Exclusive ownership and frequent allocation/free operations: unique_ptr
• Shared ownership and sensitivity to overhead: weak_ptr or shared_ptr
• Manage memory by reference count: shared_ptr

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