即使正确使用make_shared,shared_from_this()也会导致std :: bad_weak_ptr

Question

我正在使用独立的Asio和C++ 11创建一个C++服务器应用程序,并且收到错误,这就是我寻求帮助的原因.

错误

在类中worker_thread,在调用期间shared_from_this(),bad_weak_ptr会引发异常,导致程序崩溃.

布局

1. 该类在容器内connection_manager创建和存储类型的对象std::shared_ptr<worker_thread>std::vector
2. 该类worker_thread继承自std::enable_shared_from_this<worker_thread>.
3. 该类worker_thread创建类型的对象std::shared_ptr<connection>.
4. 该类connection需要一个指向该类的指针(它是一个共享指针)worker_thread,以便在可以调用的时候void handle_finish(std::shared_ptr<connection>)

程序流程

1. 该类worker_thread是通过其构造函数创建的,该类connection_manager使用std::make_shared<worker_thread>两个共享指针作为参数.
2. void init()从所谓worker_thread的connection_manager
3. 后来在节目中,connection_manager呼吁std::shared_ptr<connection> get_available_connection()从worker_thread
4. 在此方法执行期间,connection通过创建new std::make_shared<connection>,其中一个参数是指向当前 worker_thread获取的当前通道的共享指针shared_from_this()
5. 在shared_from_this()通话过程中,程序崩溃并出现bad_weak_ptr异常.

研究

根据我的研究,这个错误的最常见原因是:

1. 何时shared_from_this()在构造函数(或构造函数调用的函数)中调用
2. 当没有std::shared_ptr指向该对象时.

在我的计划中:

1. 对构造函数的调用和它们get_available_connection()是分开的,并且通过在终端中输出行,似乎worker_thread是在调用get_available_connection()发生时构造和初始化的.
2. 的connection_manager类包含一个共享指针的每一个worker_thread对象.

码

一切something_ptr都是std::shared_ptr<something>

头文件

connection_manager.hpp

typedef asio::executor_work_guard<asio::io_context::executor_type>
    io_context_work;
std::vector<worker_thread_ptr> workers;
std::vector<io_context_ptr> io_contexts;
std::vector<io_context_work> work;

worker_thread.hpp

class worker_thread : std::enable_shared_from_this<worker_thread> {
public:

/// Create a worker thread.
explicit worker_thread(io_context_ptr io, config_ptr vars_global);

void init();
void join();

connection_ptr get_available_connection();
//...

connection.hpp

explicit connection(std::shared_ptr<worker_thread> worker,
            std::shared_ptr<asio::io_context> io, 
            config_ptr vars_parent);

源文件

connection_manager.cpp

connection_manager::connection_manager(config_ptr vars) {
    std::size_t number_of_threads = vars->worker_threads;
    while(number_of_threads > 0) {
        io_context_ptr io_context(new asio::io_context);
        io_contexts.push_back(io_context);
        work.push_back(asio::make_work_guard(*io_context));

        worker_thread_ptr worker =
            std::make_shared<worker_thread>(io_context, vars);
        workers.push_back(worker);

        worker->init();

        --number_of_threads;
    }   
} 

connection_ptr connection_manager::get_available_connection() {
    std::size_t index_of_min_thread = 0;
    std::size_t worker_count = workers.size();
    for(std::size_t i = 1; i < worker_count; ++i) {
        if(workers[i]->active_connection_count() <
                workers[index_of_min_thread]->active_connection_count())
            index_of_min_thread = i;
    }
    return workers[index_of_min_thread]->get_available_connection();
}

worker_thread.cpp

worker_thread::worker_thread(io_context_ptr io, 
        config_ptr vars_global)
    :io_context(io), active_conn_count(0), vars(vars_global),
    worker(
        [this]() {
            if(io_context)
                io_context->run();
        }   
    ) {}

void worker_thread::init() {
    //Additional initialisation, this is called by connection_manager
    //after this thread's construction
}   

connection_ptr worker_thread::get_available_connection() {
    connection_ptr conn;
    if(!available_connections.empty()) {
        conn = available_connections.front();
        available_connections.pop();
        active_connections.insert(conn);
        return conn;
    } else {
        conn = std::make_shared<connection>(shared_from_this(), io_context, vars);
        active_connections.insert(conn);
        return conn;
    }
}

如果这个问题之前得到了回答,我很抱歉,但我试图解决这个问题,经过一段时间的努力,我觉得最好寻求帮助.

编辑 这是一个最低限度的测试,但失败了.它需要CMake,您可能需要更改所需的最低版本.

Google云端硬盘链接

Answer 1

我认为您的问题可能是您使用默认private继承.

这是一个崩溃的程序的简单示例:

class GoodUsage : public std::enable_shared_from_this<GoodUsage>
{
public:
    void DoSomething()
    {
        auto good = shared_from_this();
    }
};

class BadUsage : std::enable_shared_from_this<BadUsage> // private inheritence
{
public:
    void DoSomething()
    {
        auto bad = shared_from_this();
    }
};


int main()
{
    auto good = std::make_shared<GoodUsage>();
    auto bad = std::make_shared<BadUsage>();
    good->DoSomething(); // ok
    bad->DoSomething(); // throws std::bad_weak_ptr    
}