What are the underlying implementation principles of C++ smart pointers?

C++ smart pointers implement automatic memory management through pointer counting, destructors, and virtual function tables. The pointer count keeps track of the reference count, and when the reference count drops to 0, the destructor releases the original pointer. Virtual function tables enable polymorphism, allowing specific behaviors to be implemented for different types of smart pointers.

The underlying implementation principle of C++ smart pointers

C++ smart pointer is a class template used to manage the life cycle of raw pointers and provide automatic memory management functions , thereby avoiding problems caused by manual memory management, such as memory leaks and wild pointers.

The underlying implementation mechanism of smart pointers mainly includes:

Pointer counting

There is a reference count inside the smart pointer, which is used to track how many objects refer to the original object it manages. pointer. Each object that references a smart pointer increments the reference count, and each object that dereferences a smart pointer decrements the reference count. When the reference count drops to 0, the smart pointer automatically releases the original pointer it manages.

Destructor

The destructor of a smart pointer is responsible for releasing the original pointer it manages when the object is destroyed. When the smart pointer's reference count drops to 0, the destructor is called, freeing the original pointer and pointing it to nullptr.

Virtual function table

Smart pointers use virtual function tables to achieve polymorphism. By inheriting a base class, you can implement different behaviors for different types of smart pointers (such as shared_ptr, unique_ptr), such as how the reference count is increased and decreased.

Practical case

The following is an example of using unique_ptr to manage raw pointers:

#include <memory>

class Example {
public:
    Example() {
        std::cout << "Example constructor called" << std::endl;
    }

    ~Example() {
        std::cout << "Example destructor called" << std::endl;
    }
};

int main() {
    // 使用 unique_ptr 创建智能指针
    std::unique_ptr<Example> example(new Example);

    // 使用智能指针访问原始指针
    example->Example();

    // 离开 scope 时，unique_ptr 将自动释放原始指针
    return 0;
}

In the above example, unique_ptr Ensure that the Example object is automatically released when the main function exits, regardless of whether an exception occurs.

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