3.8 创建话题的发布和订阅节点

     前面我们有介绍话题以及节点的概念，也尝试使用了话题的命令行工具对话题发布了消息，本节我们将通过创建节点的方式来持续发布和订阅话题消息。

3.8.1 编辑发布节点

  我们来通过源码实现一个节点，创建一个名为 chatter 的话题，并向其发布话题消息“ hello world”。

3.8.1.1 创建并编辑源码文件

  首先还是进入我们之前创建的功能包 beginner_tutorials 的目录下，创建一个 src 文件夹，今后我们在功能包 beginner_tutorials 中创建的源码文件一般默认就存放在这个 src 文件夹中。

roscd beginner_tutorials
mkdir src

  创建源码文件，并输入以下代码内容：

cd src
gedit talker.cpp

  27 #include "ros/ros.h"
  28 #include "std_msgs/String.h"
  29 
  30 #include <sstream>
  31 
  32 /**
  33  * This tutorial demonstrates simple sending of messages over the ROS system.
  34  */
  35 int main(int argc, char **argv)
  36 {
  37   /**
  38    * The ros::init() function needs to see argc and argv so that it can perform
  39    * any ROS arguments and name remapping that were provided at the command line.
  40    * For programmatic remappings you can use a different version of init() which takes
  41    * remappings directly, but for most command-line programs, passing argc and argv is
  42    * the easiest way to do it.  The third argument to init() is the name of the node.
  43    *
  44    * You must call one of the versions of ros::init() before using any other
  45    * part of the ROS system.
  46    */
  47   ros::init(argc, argv, "talker");
  48 
  49   /**
  50    * NodeHandle is the main access point to communications with the ROS system.
  51    * The first NodeHandle constructed will fully initialize this node, and the last
  52    * NodeHandle destructed will close down the node.
  53    */
  54   ros::NodeHandle n;
  55 
  56   /**
  57    * The advertise() function is how you tell ROS that you want to
  58    * publish on a given topic name. This invokes a call to the ROS
  59    * master node, which keeps a registry of who is publishing and who
  60    * is subscribing. After this advertise() call is made, the master
  61    * node will notify anyone who is trying to subscribe to this topic name,
  62    * and they will in turn negotiate a peer-to-peer connection with this
  63    * node.  advertise() returns a Publisher object which allows you to
  64    * publish messages on that topic through a call to publish().  Once
  65    * all copies of the returned Publisher object are destroyed, the topic
  66    * will be automatically unadvertised.
  67    *
  68    * The second parameter to advertise() is the size of the message queue
  69    * used for publishing messages.  If messages are published more quickly
  70    * than we can send them, the number here specifies how many messages to
  71    * buffer up bef