ASIO 直接从异步解析获取 tcp 端点

我正在寻找使用 ASIO 独立库（不是 Boost ASIO），我正在尝试设置客户端以连接到特定端口上的服务器。

我看到在波特漏斗例如，无需处理迭代器即可获取端点。

asio::io_service io_service;

// Determine the location of the server.
tcp::resolver resolver(io_service);
tcp::resolver::query query(host_name, port);
tcp::endpoint remote_endpoint = *resolver.resolve(query);

我正在尝试使用解析器 async_resolve() 成员函数来解析查询。

这是我目前拥有的代码：

asio::io_service IOService;
asio::ip::tcp::resolver resolver(IOService);
asio::ip::tcp::resolver::query query(ADDRESS, PORT);
resolver.async_resolve(query,
            [this](const tcp::endpoint srvEndpoint, std::error_code error)
            {
                IOService->post(
                    [this, error, srvEndpoint]
                    {
                         handle_resolve_handler(error, srvEndpoint);
                    });
            });

有没有一种方法可以完成 porthopper 示例中所示的操作，但异步执行？

Oh, but

tcp::endpoint remote_endpoint = *resolver.resolve(query);

与迭代器打交道非常多！它使用它来取消引用。注意到那个可爱的星星了吗？这是指针间接操作员。

至于你的电话：

resolver.async_resolve(query,
            [this](const tcp::endpoint srvEndpoint, std::error_code error)
            {
                IOService->post(
                    [this, error, srvEndpoint]
                    {
                         handle_resolve_handler(error, srvEndpoint);
                    });
            });

That does not satisfy the completion handler requirements. Indeed, trying to compile it with Boost Asio² gives a slew of errors: Live On Coliru:

main.cpp:12:14:   required from here
/usr/local/include/boost/asio/ip/basic_resolver.hpp:163:5: error: static assertion failed: ResolveHandler type requirements not met
     BOOST_ASIO_RESOLVE_HANDLER_CHECK(
     ^
/usr/local/include/boost/asio/ip/basic_resolver.hpp:163:5: error: no match for call to '(Demo::doResolve()::<lambda(boost::asio::ip::tcp::endpoint, boost::system::error_code)>) (const boost::system::error_code&, const boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp>&)'
     BOOST_ASIO_RESOLVE_HANDLER_CHECK(

文档说:

你瞧，你的迭代器又出现了！这绝非偶然。该库的设计是这样的，无论选择什么接口，异步调用都将始终返回相同的数据。

Cobbling it together: Live On Coliru

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

namespace asio = boost::asio;
using boost::system::error_code;
using asio::ip::tcp;

struct Demo {
    Demo(asio::io_service& svc) : _svc(svc) {}
    void doResolve() {
        resolver.async_resolve(query, [this](error_code error, tcp::resolver::iterator it) {
                tcp::endpoint ep = error? tcp::endpoint{} : *it;
                _svc.post([this, error, ep] { handle_resolve_handler(error, ep); });
            });
    }

  private:
    asio::io_service& _svc;
    tcp::resolver resolver     {_svc};
    tcp::resolver::query query {"www.google.com", "https"};

    void handle_resolve_handler(error_code ec, tcp::endpoint srvEndpoint) {
        std::cout << "handle_resolve_handler: " << ec.message() << " " << srvEndpoint << "\n";
    }
};

int main() {
    asio::io_service svc;
    Demo x(svc);
    x.doResolve();

    svc.run();
}

Prints³:

handle_resolve_handler: Success 216.58.213.196:443

1 参见使用协程时的区别（yield or yield[ec]), asio::use_future etc.: 如何将 error_code 设置为 asio::yield_context

² 基本上s/boost::system::error_code/std::error_code/

³ 在具有网络访问权限的系统上