bind与shared_ptr

bind : 传递值. 会导致shared_ptr的引用计数++
muduo中TCPConnectionPtr的生命周期介绍

bind 只传递值而非引用

在函数传参时，可能发生shared_ptr的拷贝，造成引用计数++
- pass by value : 如果传入的是个左值，会发生拷贝构造，引用计数++；如果传入的是个右值(即临时构造的shared_ptr)，那么发生移动构造，引用计数不变
- pass by reference 如const& : 不拷贝，引用计数不变
- pass by value : 即函数的形参是一个非引用或者指针。pass by reference : 即函数的形参是一个引用&。
那么，bind class的成员函数时，由于要传入一个class对象的指针
- 这是一个pass by value的过程
- 我们绑定上去的指针可以是shared_ptr/裸指针，但不能是weak_ptr
- 如果是以左值传入shared_ptr对象，那么引用计数++
- 如果是以右值传入shared_ptr临时量 / 或者传入裸指针，那么引用计数当然不变
bind –> tuple
- 只能pass by value，而不能pass by reference!!!!
- pass by value : 如果传入的是个左值，会发生拷贝构造 ; 如果传入的是个右值，那么发生移动构造
- 哪怕这个函数的形参就是个引用&，但是我们如果绑定的话，绑定上去的对象也是会进行一个拷贝然后赋值给形参！！（与直觉不同，因为我们正常形参是引用的话那么传入的也应当是一个引用，但是这里bind绑定的其实是一个值）（见f6 f7）

testCode.cpp

shared_from_this() 返回的是一个this的shared_ptr的临时量
class TestClass : public std::enable_shared_from_this<TestClass>
{
public:
    using call_back = function<void(void)>;
    void hello()
    {
        cout<<"hello"<<endl;
    }
    void testFunc()
    {
        cout<<"========testFunc start========"<<endl;
        //  shared_from_this() return a 临时的 shared_ptr，引用计数++。如果不用变量保存下来的话，这条语句过了之后就--。也就是只有这一条语句的use_count变了+1
        cout<<"+ shared_from_this() " << shared_from_this().use_count() <<endl;
        cout<<"========testFunc end========"<<endl;
    }
    void testBind()
    {
        cout<<"=======testBind start=========="<<endl;
        cb_ = std::bind(&TestClass::hello,shared_from_this());          //  ++
        cb_();
        cout<<"bind + shared_from_this() = "<<shared_from_this().use_count()<<endl;         // ++ --
        cout<<"=======testBind end=========="<<endl;
    }

    void testPassAsValue(shared_ptr<TestClass> p)                       // ++
    {
        cout<<"============testPassAsValue start============"<<endl;
        cout<<p.use_count()<<endl;
        cout<<"============testPassAsValue end============"<<endl;
    }

    void testPassAsRef(const shared_ptr<TestClass>& p)
    {
        cout<<"==========testPassAsRef start=============="<<endl;
        cout<<p.use_count()<<endl;
        cout<<"==========testPassAsRef end================"<<endl;
    }

    ~TestClass(){
        cout<<"~TestClass()"<<endl;
    }
private:
    call_back cb_;
};

int main()
{
    std::shared_ptr<TestClass> sp(new TestClass());
    cout<<sp.use_count()<<endl;                         //  1
    std::weak_ptr<TestClass> wp(sp);
    cout<<sp.use_count()<<endl;                         //  1


    cout<<"********************************"<<endl;
    //  bind 绑定指针 是传值 ，会导致引用计数 ++
    std::bind(&TestClass::testFunc,sp)();             // 3 = 1(sp) + 1(bind) + 1 (shared_from_this())
    cout<<sp.use_count()<<endl;                         //  1

    cout<<"********************************"<<endl;
    function<void(void)> f1 = std::bind(&TestClass::testBind,sp);
    cout<<sp.use_count()<<endl;                         //  2 = 1(sp) + 1(f1 bind)
    f1();                                               //  4 = 1(sp) + 1(bind) + 1(bind) + 1(shared_from_this())
    cout<<sp.use_count()<<endl;                         //  3 = 1(sp) + 1(f1 bind) + 1(cb_ bind)

    cout<<"********************************"<<endl;
    function<void(shared_ptr<TestClass>)> f2 = std::bind(&TestClass::testPassAsValue,sp,std::placeholders::_1);    //  4 = 1(sp) + 1(f1 bind) + 1(cb_ bind) + 1(f2 bind)
    cout<<sp.use_count()<<endl;          //  4

    f1();                                //  5

    f2(sp);                         //  5

    cout<<"********************************"<<endl;

    function<void(const shared_ptr<TestClass>& p)> f3 = std::bind(&TestClass::testPassAsRef,sp,std::placeholders::_1);
    cout<<sp.use_count()<<endl;     //  5 = 4 + 1(f3 bind)
    f3(sp);                    // 5 = 4 + 1(f3 bind)  传引用不是传值 不会导致引用计数++

    cout<<"********************************"<<endl;

    function<void(void)> f4 = std::bind(&TestClass::testPassAsValue,sp,sp);
    cout<<sp.use_count()<<endl;     //  7 = 5 + 1(f4 bind) +1(f4 bind)

    // pass by value but 传入右值
    cout<<"********************************"<<endl;
    function<void(void)> f5 = std::bind(&TestClass::testFunc, shared_ptr<TestClass>(new TestClass()));          //  移动构造
    f5();                           //  2 = bind (临时量移动构造给别人) + shared_from_this()


    cout<<"*******************************************"<<endl;

    //  下面证明了bind只能传递值
    **哪怕这个函数的形参就是个 引用&，但是我们如果绑定的话，绑定上去的对象也是会进行一个拷贝 然后赋值给形参！！**
    
    shared_ptr<TestClass> p(new TestClass());
    function<void(void)> f6 = std::bind(&TestClass::testPassAsRef,p,p);
    cout<<p.use_count()<<endl;       //  3 = 1(define) + 1(bind) + 1(bind  bind只能传值 )
    f6();                            //  3
    cout<<"*******************************************"<<endl;
    shared_ptr<TestClass> p2(new TestClass());
    function<void(const shared_ptr<TestClass> & )> f7 = std::bind(&TestClass::testPassAsRef,p2,std::placeholders::_1);
    cout<<p2.use_count()<<endl;      //  2 = 1(define) + 1(bind)
    f7(p2);                          //  2 = 1(define) + 1(bind) 与上面的区别在于 bind 只能传值 而 这个形参 我们在调用的时候 是可以直接填引用进去的
}

1
1
********************************
========testFunc start========
+ shared_from_this() 3
========testFunc end========
1
********************************
2
=======testBind start==========
hello
bind + shared_from_this() = 4
=======testBind end==========
3
********************************
4
=======testBind start==========
hello
bind + shared_from_this() = 5
=======testBind end==========
============testPassAsValue start============
5
============testPassAsValue end============
********************************
5
==========testPassAsRef start==============
5
==========testPassAsRef end================
********************************
7
********************************
========testFunc start========
+ shared_from_this() 2
========testFunc end========
*******************************************
3
==========testPassAsRef start==============
3
==========testPassAsRef end================
*******************************************
2
==========testPassAsRef start==============
2
==========testPassAsRef end================
~TestClass()
~TestClass()
~TestClass()

Process finished with exit code 0

还有，易知，weak_ptr的复制不会导致shared_ptr的引用计数改变

TcpConnectionPtr的生命周期

TcpConnection的引用存在于
- TcpServer中的connections_<name,TcpConnectionPtr>。shared_ptr。引用计数 = 1
- channel 中的weak_ptr 当handleEvent时会lock为shared_ptr(即TcpConnectionPtr) 。引用计数++ 保证不在handleEvent时析构TcpConnection
- 还有就是在连接断开时的局部变量TcpConnectionPtr 以及 bind参数时候的拷贝TcpConnectionPtr。
一个TcpConnection何时析构？
- 断开连接后，TcpConnectionPtr = 0
- 即整个handleEvent(handleClose)结束之后(最后一个成员函数是connetionDestroyed），TcpConnectionPtr = 0，此时TcpConnection析构（channel也就跟着析构）
将一条连接断开都需要做什么操作
- 大概来讲就是
  - 取消对该fd的监听：channel_->disableAll() — TcpConnection内做(因为这是TcpConnection封装的成员)
  - 执行user设置的callback：connectionCallback_(connPtr); — TcpConnection内做
  - 将TcpConnection从TcpServer中的记录移除：connections_.erase(conn->name()); — TcpServer内做（因为这是TcpServer封装的成员）
  - channel从poller中删除掉 — TcpConnection内做

TcpConnection析构流程 / 断开连接流程
poller -> channel::closeCallback -> TcpConnection::handleClose -> TcpServer::removeConnection -> TcpConnection::connectionDestoyed
在进入handleClose之后，不必担心TcpConnection会在执行完成前就被销毁，因为进入之前就已经用一个weak_ptr做了lock提升，保证了引用计数至少为1
去掉一块错误注释其余不变

相关代码

//  poller ->  channel::closeCallback -> TcpConnection::handleClose -> TcpServer::removeConnection -> TcpConnection::connectionDestoyed
//  之前注册给channel的关于关闭连接的回调操作都已经完成 
    //  且TcpConnection也一直保持着没有析构。
//  该函数结束之后，TcpConnection就会几乎马上被析构!
void TcpConnection::handleClose()
{
    TcpConnectionPtr connPtr(shared_from_this());    

    LOG_INFO("shared_ptr cnt %ld",connPtr.use_count());
    //  3  = 1(channel::tie_.lock) + 1(TcpServer::connections_[connName] = conn;) + 1(connPtr)

    //  TcpServer::removeConnection
    //  **connections_.erase(TcpConnectionPtr)**
    closeCallback_(connPtr);            

    LOG_INFO("shared_ptr cnt %ld tcpconnection %p point to %p",connPtr.use_count(),connPtr.get(),this);
    //  2 = 1(channel::tie_.lock) + 1(connPtr)
}

--------------------------------------------------------------
//  removeConnection -> removeConnectionInLoop
void TcpServer::removeConnection(const TcpConnectionPtr &conn)
{
    //  这里也证明了 即便形参是一个引用 即便正常调用函数传递的是个引用，可是，只要bind了，传递的就是一个值 会拷贝
    LOG_INFO("shared_ptr cnt %ld\n",conn.use_count());              //  3 没变
    loop_->runInLoop(
        std::bind(&TcpServer::removeConnectionInLoop, this, conn));
    LOG_INFO("shared_ptr cnt %ld\n",conn.use_count());              //  2
}

//  handleRead n = 0 -> handleClose - >removeConnection -> removeConnectionInLoop -> TcpConnection::connectionDestroyed
void TcpServer::removeConnectionInLoop(const TcpConnectionPtr &conn)
{
    LOG_INFO("shared_ptr cnt %ld",conn.use_count());                //  4 = 3+1 因为bind是拷贝值 会导致引用计数++
    
    //  tcpconnection use count -- : tcpserver 从记录中删除该连接
    connections_.erase(conn->name());                   

    LOG_INFO("after erase shared_ptr cnt %ld",conn.use_count());    //  3 = 1(channel::tie_.lock) + 1(connPtr) + 1(bind拷贝conn)

    (conn->getLoop())->runInLoop(
        std::bind(&TcpConnection::connectionDestroyed,conn.get())); //  根据以上分析 传递裸指针也可 反正在这个函数里TcpConnectionPtr的引用计数一定不是0
//  connectionDestroyed : TcpConnection析构前调用的最后一个成员函数
}


// 连接销毁
void TcpConnection::connectionDestroyed()
{
    channel_->remove(); // 把channel从poller中删除掉
}

连接断开时的相关日志如下

[INFO] 2022-10-27-22-07-43.011788 : /home/shc/Muduo/src/TcpConnection.cpp (135) <handleClose>   TcpConnection::handleClose fd = 6 state = 3

[INFO] 2022-10-27-22-07-43.011795 : /home/shc/Muduo/src/EpollPoller.cpp (37) <updateChannel>   fd = 6 , events = 0 , index = 1

[INFO] 2022-10-27-22-07-43.011807 : /home/shc/Muduo/src/EpollPoller.cpp (90) <update>   fd = 6 is to be epolled or deleted in loop 0x7ffcce544ab0 on thread 8121
[INFO] 2022-10-27-22-07-43.011833 : /home/shc/Muduo/src/TcpConnection.cpp (146) <handleClose>   shared_ptr cnt 3
[INFO] 2022-10-27-22-07-43.011856 : /home/shc/Muduo/src/TcpConnection.cpp (148) <handleClose>   shared_ptr cnt 3
[INFO] 2022-10-27-22-07-43.011883 : testserver.cpp (40) <onConnection>   connection down : from 127.0.0.1:50216 to 127.0.0.1:6667!

[INFO] 2022-10-27-22-07-43.011944 : /home/shc/Muduo/src/TcpConnection.cpp (153) <handleClose>   shared_ptr cnt 3
[INFO] 2022-10-27-22-07-43.011981 : /home/shc/Muduo/src/TcpServer.cpp (120) <removeConnection>   shared_ptr cnt 3

[INFO] 2022-10-27-22-07-43.012043 : /home/shc/Muduo/src/EventLoop.cpp (149) <runInLoop>   cb runInLoop in loop 0x7ffcce544ab0 on thread 8121
[INFO] 2022-10-27-22-07-43.012071 : /home/shc/Muduo/src/EventLoop.cpp (155) <runInLoop>   直接调用cb on loop 0x7ffcce544ab0 in thread 8121

[INFO] 2022-10-27-22-07-43.012101 : /home/shc/Muduo/src/TcpServer.cpp (130) <removeConnectionInLoop>   shared_ptr cnt 4
[INFO] 2022-10-27-22-07-43.012126 : /home/shc/Muduo/src/TcpServer.cpp (131) <removeConnectionInLoop>   TcpServer::removeConnectionInLoop [EchoServer] - connection EchoServer-127.0.0.1:6667#1

[INFO] 2022-10-27-22-07-43.012169 : /home/shc/Muduo/src/TcpServer.cpp (137) <removeConnectionInLoop>   after erase shared_ptr cnt 3
[INFO] 2022-10-27-22-07-43.012195 : /home/shc/Muduo/src/EventLoop.cpp (149) <runInLoop>   cb runInLoop in loop 0x7ffcce544ab0 on thread 8121
[INFO] 2022-10-27-22-07-43.012220 : /home/shc/Muduo/src/EventLoop.cpp (155) <runInLoop>   直接调用cb on loop 0x7ffcce544ab0 in thread 8121

[INFO] 2022-10-27-22-07-43.012228 : /home/shc/Muduo/src/TcpConnection.cpp (332) <connectionDestroyed>   state_ = 0
[INFO] 2022-10-27-22-07-43.012252 : /home/shc/Muduo/src/EpollPoller.cpp (109) <removeChannel>   fd = 6 , events = 0 , index = 2 in loop 0x7ffcce544ab0 on thread 8121

[INFO] 2022-10-27-22-07-43.012274 : /home/shc/Muduo/src/EpollPoller.cpp (90) <update>   fd = 6 is to be epolled or deleted in loop 0x7ffcce544ab0 on thread 8121
[INFO] 2022-10-27-22-07-43.012311 : /home/shc/Muduo/src/TcpServer.cpp (148) <removeConnectionInLoop>   after removeConnectionInLoop
[INFO] 2022-10-27-22-07-43.012344 : /home/shc/Muduo/src/TcpServer.cpp (124) <removeConnection>   shared_ptr cnt 2

[INFO] 2022-10-27-22-07-43.012376 : /home/shc/Muduo/src/TcpConnection.cpp (160) <handleClose>   shared_ptr cnt 2 tcpconnection 0x56195873ad90 point to 0x56195873ad90
[INFO] 2022-10-27-22-07-43.012405 : /home/shc/Muduo/src/TcpConnection.cpp (45) <~TcpConnection>   TcpConnection::ctor (0x56195873ad90) [EchoServer-127.0.0.1:6667#1] on socket 6 with state 0

bind 只传递值 而非 引用

TcpConnectionPtr的生命周期

bind 只传递值而非引用