`shared_ptr`

destroy of shard_ptr对象、destroy of resource。

术语

Managed Object

由shared pointer 管理的object，这个名称源自:

1、nextptr shared_ptr - basics and internals with examples。

Managed pointer

指向managed object的pointer，这个名称源自:

1、cppreference std::shared_ptr

Stored pointer

shared_ptr object的pointer；

cppreference `std::shared_ptr`

std::shared_ptr is a smart pointer that retains(保持) shared ownership of an object through a pointer. Several shared_ptr objects may own the same object. The object is destroyed and its memory deallocated when either of the following happens:

1、the last remaining shared_ptr owning the object is destroyed;

2、the last remaining shared_ptr owning the object is assigned another pointer via operator= or reset().

A shared_ptr can share ownership of an object while storing a pointer to another object. This feature can be used to point to member objects while owning the object they belong to. The stored pointer is the one accessed by get(), the dereference and the comparison operators. The managed pointer is the one passed to the deleter when use count reaches zero.

NOTE:

1、在 nextptr shared_ptr - basics and internals with examples 中，列举了这样的例子；

2、这其实就是 aliasing constructor

Empty status

A shared_ptr may also own no objects, in which case it is called empty (an empty shared_ptr may have a non-null stored pointer if the aliasing constructor was used to create it).

NOTE: 没有搞懂

Data race

NOTE:

这部分内容放到了shared_ptr-and-multithread章节。

Example

Subtyping polymorphism

下面这个例子展示了std::shared_ptr + subtyping polymorphism:

#include <iostream>
#include <memory>
#include <thread>
#include <chrono>
#include <mutex>

struct Base
{
    Base()
    {
        std::cout << "  Base::Base()\n";
    }
    // Note: non-virtual destructor is OK here
    ~Base()
    {
        std::cout << "  Base::~Base()\n";
    }
};

struct Derived: public Base
{
    Derived()
    {
        std::cout << "  Derived::Derived()\n";
    }
    ~Derived()
    {
        std::cout << "  Derived::~Derived()\n";
    }
};

void thr(std::shared_ptr<Base> p)
{
    std::this_thread::sleep_for(std::chrono::seconds(3));
    std::shared_ptr<Base> lp = p; // thread-safe, even though the
                                  // shared use_count is incremented
    {
        static std::mutex io_mutex;
        std::lock_guard<std::mutex> lk(io_mutex);
        std::cout << "local pointer in a thread:\n" << "  lp.get() = " << lp.get() << ", lp.use_count() = " << lp.use_count() << '\n';
    }
}

int main()
{
    std::shared_ptr<Base> p = std::make_shared<Derived>();

    std::cout << "Created a shared Derived (as a pointer to Base)\n" << "  p.get() = " << p.get() << ", p.use_count() = " << p.use_count() << '\n';
    std::thread t1(thr, p), t2(thr, p), t3(thr, p);
    p.reset(); // release ownership from main
    std::cout << "Shared ownership between 3 threads and released\n" << "ownership from main:\n" << "  p.get() = " << p.get() << ", p.use_count() = " << p.use_count() << '\n';
    t1.join();
    t2.join();
    t3.join();
    std::cout << "All threads completed, the last one deleted Derived\n";
}
// g++ --std=c++11 test.cpp

如何维护shared ownership?

对于shared ownership，可以acquire、release；

Acquire shared ownership

本节标题的含义是: 获取对某个object/resource的shared ownership，在下面的文章中，涉及了这个topic:

cppreference std::shared_ptr # Notes

The ownership of an object can only be shared with another shared_ptr by

1、copy constructing or

2、copy assigning its value to another shared_ptr.

Constructing a new shared_ptr using the raw underlying pointer owned by another shared_ptr leads to undefined behavior.

NOTE: 关于这段话中描述的undefined behavior的场景，参见:

1、learncpp std::shared_ptr，在 Guide\learncpp-std-shared_ptr 中收录了这篇文章

`enable_shared_from_this` 和 `shared_from_this`

这也是实现"Acquire shared ownership"的一种方式，参见 ./shared_ptr-and-this。

Release shared ownership

本节标题的含义是: 释放对某个object/resource的shared ownership。

cppreference shared_ptr::reset

如何release shared ownership？在 "cppreference std::shared_ptr # Notes # Example "中展示的例子，演示了用法；

`std::shared_ptr` circular reference(循环引用)

一、nextptr-Using-weak_ptr-for-circular-references

二、spdlog章节

shared_ptr

术语