Question

The following code using boost::asio will not compile:

    #ifndef _SERVER_H_
    #define _SERVER_H_

    #include "Connection.h"

    class Server
    {
    public:
      Server(boost::asio::io_service& io_service);

    private:
      void start_accept();

      void handle_accept(Connection::pointer new_connection,const boost::system::error_code& error);


      boost::asio::ip::tcp::acceptor acceptor_;
    };

    #endif
    -------------------------------------------------------------------------------------------------
    #include "Server.h"

    Server::Server(boost::asio::io_service& io_service)
        : acceptor_(io_service, boost::asio::ip::tcp::endpoint(boost::asio::ip::tcp::v4(), 9985)){
            start_accept();
    }

    void Server::start_accept(){

        Connection::pointer new_connection =
          Connection::create(acceptor_.io_service());

        acceptor_.async_accept(new_connection->socket(),
            boost::bind(&Server::handle_accept, this, new_connection,
              boost::asio::placeholders::error));
    }


    void Server::handle_accept(Connection::pointer new_connection,const boost::system::error_code& error){
        if (!error)
        {
          new_connection->start();
          start_accept();
        }
      }
--------------------------------------------------------------------------------------------------
#include <Server.h>
#include <iostream>


int main()
{
  try
  {
    boost::asio::io_service io_service;
    Server server1(io_service);
    io_service.run();
  }
  catch (std::exception& e)
  {
    std::cerr << e.what() << std::endl;
  }

  return 0;
}

This is the error it produces in Visual C++:

error LNK2019: unresolved external symbol "public: __thiscall Server::Server(class boost::asio::io_service &)" (??0Server@@QAE@AAVio_service@asio@boost@@@Z) referenced in function _main

What does this error mean?

Answer 1

This means that whatever source file you put Server‘s constructor definition into isn’t being compiled and/or linked.