简短的故事：socket.cancel()之后的调用socket.async_connect(...)可以正常工作，即调用完成处理程序；但是对于socket.cancel()之后的asio::async_connect(socket, ...)，永远不会调用完成处理程序。
从长远来看，这里是完整的源代码：
#include <iostream>
#include <vector>
#include <chrono>
#include <ctime>
#include <boost/asio.hpp>
namespace sys = boost::system;
namespace asio = boost::asio;
using tcp = asio::ip::tcp;
static const int count = 10;
struct client_t {
using sock_ptr_t = std::unique_ptr<tcp::socket>;
using resolve_results_t = tcp::resolver::results_type;
client_t(asio::io_context& io_context_): io_context{io_context_}, resolver{io_context}{
}
void start() {
tcp::endpoint endpoint(asio::ip::address::from_string("93.184.216.34"), 80);
resolver.async_resolve("www.example.com", "80", [&](auto ec, auto results){
this->on_resolve(ec, results);
});
}
void on_resolve(const sys::error_code& ec, resolve_results_t results) {
auto now = std::chrono::system_clock::now();
time_t tt = std::chrono::system_clock::to_time_t(now);
std::cerr << "on_resolve " << this << " " << ec.message() << ", at " << ctime(&tt);
if (!ec) {
sock = std::make_unique<tcp::socket>(io_context);
/*
sock->async_connect(*results.begin(), [this](auto& ec){
this->on_connect(ec);
});
*/
asio::async_connect(*sock, results.begin(), results.end(), [this](auto& ec, auto it){
this->on_connect(ec);
});
}
}
void on_connect(const sys::error_code& ec) {
auto now = std::chrono::system_clock::now();
time_t tt = std::chrono::system_clock::to_time_t(now);
std::cerr << "on_connect " << this << " " << ec.message() << ", at " << ctime(&tt);
}
void cancel() {
std::cout << "cancellation attempt for " << this << "\n";
if (sock) {
sys::error_code ec;
sock->cancel(ec);
std::cout << "cancellation result: " << ec.message() << "\n";
}
}
asio::io_context& io_context;
sock_ptr_t sock;
tcp::resolver resolver;
};
using client_ptr_t = std::unique_ptr<client_t>;
using container_t = std::vector<client_ptr_t>;
int main() {
asio::io_context io_context;
std::cerr << "starting\n";
container_t container;
for(int i = 0; i < count; ++i) {
auto client = std::make_unique<client_t>(io_context);
client->start();
container.emplace_back(std::move(client));
}
boost::asio::deadline_timer timer(io_context);
timer.expires_from_now(boost::posix_time::milliseconds(5));
timer.async_wait([&](auto& ec){
for(auto& it : container) {
it->cancel();
}
auto now = std::chrono::system_clock::now();
time_t tt = std::chrono::system_clock::to_time_t(now);
std::cout << "cancelled at " << ctime(&tt);
});
io_context.run();
std::cerr << "finished\n";
return 0;
}
输出样本为：
starting
on_resolve 0x985000 Success, at Sat Oct 26 22:59:22 2019
on_resolve 0x9854c0 Success, at Sat Oct 26 22:59:22 2019
on_resolve 0x985600 Success, at Sat Oct 26 22:59:22 2019
on_resolve 0x985730 Success, at Sat Oct 26 22:59:22 2019
on_resolve 0x985830 Success, at Sat Oct 26 22:59:22 2019
on_resolve 0x985700 Success, at Sat Oct 26 22:59:22 2019
on_resolve 0x985a90 Success, at Sat Oct 26 22:59:22 2019
on_resolve 0x985bb0 Success, at Sat Oct 26 22:59:22 2019
on_resolve 0x985cd0 Success, at Sat Oct 26 22:59:22 2019
on_resolve 0x985e80 Success, at Sat Oct 26 22:59:22 2019
cancellation attempt for 0x985000
cancellation result: Success
cancellation attempt for 0x9854c0
cancellation result: Success
cancellation attempt for 0x985600
cancellation result: Success
cancellation attempt for 0x985730
cancellation result: Success
cancellation attempt for 0x985830
cancellation result: Success
cancellation attempt for 0x985700
cancellation result: Success
cancellation attempt for 0x985a90
cancellation result: Success
cancellation attempt for 0x985bb0
cancellation result: Success
cancellation attempt for 0x985cd0
cancellation result: Success
cancellation attempt for 0x985e80
cancellation result: Success
cancelled at Sat Oct 26 22:59:22 2019
（我必须按ctrl + c才能停止程序）。
但是，如果我使用sock->async_connect(...)（上面代码中的注释部分），则一切正常，即
starting
on_resolve 0x6d6000 Success, at Sat Oct 26 22:59:03 2019
on_resolve 0x6d64c0 Success, at Sat Oct 26 22:59:03 2019
on_resolve 0x6d6600 Success, at Sat Oct 26 22:59:03 2019
on_resolve 0x6d6730 Success, at Sat Oct 26 22:59:03 2019
on_resolve 0x6d6830 Success, at Sat Oct 26 22:59:03 2019
on_resolve 0x6d6700 Success, at Sat Oct 26 22:59:03 2019
on_resolve 0x6d6a90 Success, at Sat Oct 26 22:59:03 2019
on_resolve 0x6d6bb0 Success, at Sat Oct 26 22:59:03 2019
on_resolve 0x6d6cd0 Success, at Sat Oct 26 22:59:03 2019
on_resolve 0x6d6e80 Success, at Sat Oct 26 22:59:03 2019
cancellation attempt for 0x6d6000
cancellation result: Success
cancellation attempt for 0x6d64c0
cancellation result: Success
cancellation attempt for 0x6d6600
cancellation result: Success
cancellation attempt for 0x6d6730
cancellation result: Success
cancellation attempt for 0x6d6830
cancellation result: Success
cancellation attempt for 0x6d6700
cancellation result: Success
cancellation attempt for 0x6d6a90
cancellation result: Success
cancellation attempt for 0x6d6bb0
cancellation result: Success
cancellation attempt for 0x6d6cd0
cancellation result: Success
cancellation attempt for 0x6d6e80
cancellation result: Success
cancelled at Sat Oct 26 22:59:03 2019
on_connect 0x6d6000 Operation canceled, at Sat Oct 26 22:59:03 2019
on_connect 0x6d64c0 Operation canceled, at Sat Oct 26 22:59:03 2019
on_connect 0x6d6600 Operation canceled, at Sat Oct 26 22:59:03 2019
on_connect 0x6d6730 Operation canceled, at Sat Oct 26 22:59:03 2019
on_connect 0x6d6830 Operation canceled, at Sat Oct 26 22:59:03 2019
on_connect 0x6d6700 Operation canceled, at Sat Oct 26 22:59:03 2019
on_connect 0x6d6a90 Operation canceled, at Sat Oct 26 22:59:03 2019
on_connect 0x6d6bb0 Operation canceled, at Sat Oct 26 22:59:03 2019
on_connect 0x6d6cd0 Operation canceled, at Sat Oct 26 22:59:03 2019
on_connect 0x6d6e80 Operation canceled, at Sat Oct 26 22:59:03 2019
finished
为什么呢？我已经阅读了文档，但没有发现任何提示我在做什么错。
答案 0 :(得分：2)
如文档中所述，asio::async_connect
...尝试将套接字连接到一系列端点中的一个...对序列中的每个端点一次，直到成功建立连接为止。
如果提供了多个端点，则取消一个未完成的连接尝试将导致for(auto& p: fs::recursive_directory_iterator(dirname))
{
if (fs::is_regular_file(p))
{
//do rename
}
}
从下一个端点重试（循环here's）。
在取消套接字后尝试asio::async_connect。这将阻止close重试。
asio::async_connect