调用boost resolve::async_resolve时捕获值的错误值

Question

我无法理解下面代码的行为。定义符号BUG时，变量this的第三次打印是错误的。

我认为resolver::async_resolve方法中有些东西破坏了代码。我想了解什么:-)

谢谢

#include <boost/asio.hpp>
#include <iostream>

using namespace std;

template <typename F>
#ifdef BUG
void Connect( boost::asio::ip::tcp::resolver& resolver, F Connected )
#else
void Connect( boost::asio::ip::tcp::resolver& resolver, const F& Connected )
#endif
{
    resolver.async_resolve(
        boost::asio::ip::tcp::resolver::query{ "localhost", "8088" },
        [&Connected]( const boost::system::error_code& ec, boost::asio::ip::tcp::resolver::iterator i )
        {
            Connected();
        }
    );
}

struct Test
{
    void Start()
    {
        cout << "this1 " << hex << this << dec << endl;
        auto handler = [this]()
        {
            cout << "this2 " << hex << this << dec << endl;
            boost::asio::ip::tcp::resolver resolver{ ios };
            Connect( resolver, [this]()
                {
                    cout << "this3 " << hex << this << dec << std::endl;
                }
            );
        };
        handler();
        ios.run();
    }

    boost::asio::io_service ios;
};

int main()
{
    Test t;
    t.Start();
}

Answer 1

您的错误不是通过值传递给Connect而不是通过const引用传递，而是由于调用对lambda的悬空引用而导致的未定义行为。

这是因为在传递给async_resolve的lambda中通过引用捕获async_resolve。

resolver.async_resolve(
    boost::asio::ip::tcp::resolver::query{ "localhost", "8088" },
    [&Connected]( const boost::system::error_code& ec, boost::asio::ip::tcp::resolver::iterator i )
    {
        Connected(); // Connected is captured by reference
    }
);

当调用Connected()时，它已经从堆栈中弹出并销毁。

void Start()
{
    cout << "this1 " << hex << this << dec << endl;
    auto handler = [this]()
    {
        cout << "this2 " << hex << this << dec << endl;
        boost::asio::ip::tcp::resolver resolver{ ios };
        Connect( resolver, [this]()
            {
                cout << "this3 " << hex << this << dec << std::endl;
            }
        );
    };
    handler(); // after this function returns Connected will be destructed
    ios.run(); // the thread is blocked in ios.run until the resolve returns
}

• 对handler()的调用在堆栈上创建"Connected“lambda并将其传递给Connect，后者将创建一个lambda ，该通过引用捕获，并启动一个异步操作。
• 然后handler()返回，从堆栈中弹出"Connected“，销毁它。
• ios.run()在等待async_resolve返回时阻止Test::Start()返回。
• async_resolve完成了，并调用了它的lambda，作为回报，它调用了已被销毁的Connected()。

您可以通过捕获 by value来解决这一问题。

void Connect( boost::asio::ip::tcp::resolver& resolver, F Connected )
{
    resolver.async_resolve(
        boost::asio::ip::tcp::resolver::query{ "localhost", "8088" },
        [Connected]( const boost::system::error_code& ec, boost::asio::ip::tcp::resolver::iterator i )
        {
            Connected();
        }
    );
}