Wrong value for captured value when calling boost resolve::async_resolve - c++11

Question

I can't understand the behaviour of the code below.
When defining the symbol BUG, the third print of the variable this is wrong.
I think there is something in the method resolver::async_resolve that breaks the code. I'd like to understand what :-)
Thanks
#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

Your bug is not due to passing to Connect by value vs by const reference, it's undefined behaviour due to calling a dangling reference to a lambda.
This is because you're capturing Connnected by reference in the lambda passed to 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
}
);
By the time Connected() is called, it's been popped off the stack and destroyed.
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
}
The call to handler() creates the "Connected" lambda on the stack and passes it to Connect, which in turn creates a lambda which captures Connected by reference, and starts an asynchronous operation.
handler() then returns, popping "Connected" off the stack, destructing it.
ios.run() prevents Test::Start() from returning as it waits for async_resolve to return.
async_resolve completes, and calls its lambda, which in return calls Connected(), which has been destroyed.
You can solve this by capturing Connected by-value
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();
}
);
}