如何在模板函数中使用enable_shared_from_this？

Question

我正在尝试在模板函数中使用 std::enabled_shared_from_this 函数，但我在 class 中调用 shared_from_this() 时不断收到消息 bad_weak_ptr 错误。

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());
    }
    /* ... */
};

我试过 std::enable_shared_from_this>::shared_from_this() 和 this->shared_from_this() 但我没有在 shared_from_this() 中调用函数 class构造函数。如果有任何方法可以解决此错误，请告诉我。

Answer 1

您的查询有误，you're passing too many arguments。相反，传递单个查询参数：

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

接下来，您可以使用您喜欢的任一绑定方法：

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

或者根本不使用绑定：

    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);
        }
    );

或者使用 Beast 的便捷助手：

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

现场演示

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();
}

打印（由于网络限制，不在 Coliru 上）：

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

如何在模板函数中使用enable_shared_from_this？

How to use enable_shared_from_this in template functions?

c++

shared-ptr

boost-asio

c++17

现场演示