How to use enable_shared_from_this in template functions?

Question

I am trying to use std::enabled_shared_from_this function in a template function but I keep getting message bad_weak_ptr error in calling shared_from_this() in a class.

template<T>
class A: public std::enable_shared_from_this <A<T>>
{

    /* some parts in the class */
    void Run()
    {
        resolver_.async_resolve(host_, port_, boost::bind_front_handler(&A:on_resolve, this->shared_from_this());
    }
    /* ... */
};

I have tried std::enable_shared_from_this>::shared_from_this() and this->shared_from_this() and I did not call shared_from_this() function in a class constructor. If there are any ways to solve this error, please let me know.

Answer 1

Your query is wrong, you're passing too many arguments. Instead, pass a single query parameter:

    boost::asio::ip::tcp::resolver::query q{host_, port_};
    resolver_.async_resolve(q,

Next up, you can use either bind method you prefer:

    boost::bind(&A::on_resolve, this->shared_from_this(),
        boost::asio::placeholders::error(),
        boost::asio::placeholders::results())

Or not using bind at all:

    resolver_.async_resolve(q,
        [self=this->shared_from_this()](boost::system::error_code ec, boost::asio::ip::tcp::resolver::results_type eps) {
            self->on_resolve(ec, eps);
        }
    );

Or using the convenience helper from Beast:

    boost::beast::bind_front_handler(&A::on_resolve, this->shared_from_this())

Live Demo

Live On Coliru

#include <memory> // for std::shared_from_this etc
#include <iostream>
#include <boost/asio.hpp>
#include <boost/bind.hpp>                    // for boost::bind
#include <boost/beast/core/bind_handler.hpp> // for bind_front_handler

template<typename T>
class A: public std::enable_shared_from_this<A<T>>
{
  public:
    A(boost::asio::io_context& io, std::string host, std::string port)
        : resolver_(io), host_(host), port_(port) {}

    void Run()
    {
        boost::asio::ip::tcp::resolver::query q{host_, port_};
        resolver_.async_resolve(q,
            boost::beast::bind_front_handler(&A::on_resolve, this->shared_from_this())
        );
        resolver_.async_resolve(q,
            boost::bind(&A::on_resolve, this->shared_from_this(),
                boost::asio::placeholders::error(),
                boost::asio::placeholders::results())
        );
        resolver_.async_resolve(q,
            [self=this->shared_from_this()](boost::system::error_code ec, boost::asio::ip::tcp::resolver::results_type eps) {
                self->on_resolve(ec, eps);
            }
        );
    }

  private:
    void on_resolve(boost::system::error_code ec, boost::asio::ip::tcp::resolver::results_type eps) {
        std::cout << "Resolved: (" << ec.message() << "):";
        for (auto& ep : eps) {
            std::cout << " " << ep.endpoint();
        }
        std::cout << "\n";
    }

    boost::asio::ip::tcp::resolver resolver_;
    std::string host_, port_;
};

int main() {
    boost::asio::io_context io;
    auto instance = std::make_shared<A<int> >(io, "localhost", "1234");
    instance->Run();

    io.run();
}

Prints (not on Coliru due to networking restrictions):

Resolved: (Success): 127.0.0.1:1234
Resolved: (Success): 127.0.0.1:1234
Resolved: (Success): 127.0.0.1:1234