symbol ':' and Underscores after the costructors.

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hi all,
I am new to c++ and i would like to know what does the underscores followed by the vaiables name in a constructor of the class, i have a small piece of code like this

C#

server::server(ba::io_service& io_service, int thnum, int port)
    : io_service_(io_service),
      acceptor_(io_service_, ba::ip::tcp::endpoint(ba::ip::tcp::v4(), port)),
      new_connection_(connection::create(io_service_)),
      thp(thnum) {
    // start acceptor in async mode
    acceptor_.async_accept(new_connection_->socket(),
                           boost::bind(&server::handle_accept, this,
                                       ba::placeholders::error));
}

here i have io_service_ after the contructor arguments followed by the ':'
i dont understand any of this why there is a symbol ':' after the arguments and what are those following that symbol ':' actually means .I dont know how to mention them too. pls help me with your simple word to make me undestand.

the entire class which contains the above code is as follows.

C#

class server : private boost::noncopyable {
public:
    server(ba::io_service& io_service, int thnum, int port=10001);

private:
    void handle_accept(const boost::system::error_code& e);

    ba::io_service& io_service_;         /**< reference to io_service */
    ba::ip::tcp::acceptor acceptor_;     /**< object, that accepts new connections */
    connection::pointer new_connection_; /**< pointer to connection, that will proceed next */
    tp::pool thp;                        /**< thread pool object */
};

Posted 25-Oct-11 13:10pm

hariharansa

Updated 26-Oct-11 1:48am

Richard MacCutchan

v2

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-10-25T13:21:00

The underscore? It does not do anything, this is just a valid part of the variable/type/field/function name.

If you mean io_service_, acceptor_ and new_connection_ before the body of the constructor server::server and after ':' — they are field initializers, as well as thp. They are just the names of this class fields to be initialized to the values in round brackets according to C++ syntax before the body of the constructor is started.

See http://www.cprogramming.com/tutorial/initialization-lists-c++.html[^].

This syntax is important because in this way you can initialize a constant; you cannot apply an assignment operator to a constant; see http://stackoverflow.com/questions/1423696/how-to-initialize-a-const-field-in-constructor[^].

For the detail of particular boost classes you use please refer to boost reference of source code.

—SA