ROS学习：用C++写一个发布者和订阅者文件。

作为学习ros的初学者，写下这篇文章，帮助自己理清ros工作的一个流程，在此感谢前辈的文章添加链接描述
ubuntu16.04
创建工作空间（workspace)
工作空间可以作为一个独立的项目进行编译，存放ROS程序的源文件、编译文件和执行文件。建立工作空间的方法如下：

$ mkdir -p ~/catkin_ws/src
$ cd ~/catkin_ws/src

虽然这时候的工作空间是空的，但是我们依旧可以进行编译：

$ cd ..
$ catkin_make

这时候，会在当前文件夹下生成devel，build这两个子文件夹，在devel文件夹下能看到几个setup.*sh文件.
接下来把工作空间在bash中注册:

$ source devel/setup.bash

或在加入: echo "source ~/catkin_ws/devel/setup.bash" >> ~/.bashrc
这样就不用每次都source一下
要验证是否已经在bash中注册可以使用如下命令：

$ echo $ROS_PACKAGE_PATH

若是输出如下信息：

/home/youruser/catkin_ws/src:/opt/ros/indigo/share:/opt/ros/indigo/stacks

则代表创建成功。
创建一个ROS功能包（Package）
在一个工作空间内，可能会包含多个ROS功能包。而最基本ROS功能包中会包括CmakeLists.txt和Package.xml这两个文件，其中Package.xml中主要包含本项目信息和各种依赖（depends），而CmakeLists.txt中包含了如何编译和安装代码的信息。
首先切换到工作空间：

$ cd ~/catkin_ws/src

现在可以使用catkin_create_pkg命令去创建一个叫beginner_tutorials的包，这个包的依赖是std_msgs、roscpp、rospy。

$ catkin_create_pkg beginner_tutorials std_msgs rospy roscpp

接下来在工作空间编译这个工程包：

$ cd ~/catkin_ws
$ catkin_make

一个简单的发布（Publisher）、订阅(Subscriber)程序
写一个发布（Publisher）节点
节点（node）是连接到ROS网络中可执行的基本单元。我们在这创建一个发布者—“talker”节点，这个节点持续对外发布消息。
直接进入我们的beginner_tutorials目录，在src目录中右键创建一个talker.cpp文件，里面的内容如下：

#include "ros/ros.h"
#include "std_msgs/String.h"
 
#include 
int main(int argc, char **argv)
{
  /**
   * The ros::init() function needs to see argc and argv so that it can perform
   * any ROS arguments and name remapping that were provided at the command line. For programmatic
   * remappings you can use a different version of init() which takes remappings
   * directly, but for most command-line programs, passing argc and argv is the easiest
   * way to do it.  The third argument to init() is the name of the node.
   *
   * You must call one of the versions of ros::init() before using any other
   * part of the ROS system.
   */
  ros::init(argc, argv, "talker");
 
  /**
   * NodeHandle is the main access point to communications with the ROS system.
   * The first NodeHandle constructed will fully initialize this node, and the last
   * NodeHandle destructed will close down the node.
   */
  ros::NodeHandle n;
 
  /**
   * The advertise() function is how you tell ROS that you want to
   * publish on a given topic name. This invokes a call to the ROS
   * master node, which keeps a registry of who is publishing and who
   * is subscribing. After this advertise() call is made, the master
   * node will notify anyone who is trying to subscribe to this topic name,
   * and they will in turn negotiate a peer-to-peer connection with this
   * node.  advertise() returns a Publisher object which allows you to
   * publish messages on that topic through a call to publish().  Once
   * all copies of the returned Publisher object are destroyed, the topic
   * will be automatically unadvertised.
   *
   * The second parameter to advertise() is the size of the message queue
   * used for publishing messages.  If messages are published more quickly
   * than we can send them, the number here specifies how many messages to
   * buffer up before throwing some away.
   */
  ros::Publisher chatter_pub = n.advertise("chatter", 1000);
 
  ros::Rate loop_rate(10);
 
  /**
   * A count of how many messages we have sent. This is used to create
   * a unique string for each message.
   */
  int count = 0;
  while (ros::ok())
  {
    /**
     * This is a message object. You stuff it with data, and then publish it.
     */
    std_msgs::String msg;
 
    std::stringstream ss;
    ss << "hello world " << count;
    msg.data = ss.str();
 
    ROS_INFO("%s", msg.data.c_str());
 
    /**
     * The publish() function is how you send messages. The parameter
     * is the message object. The type of this object must agree with the type
     * given as a template parameter to the advertise<>() call, as was done
     * in the constructor above.
     */
    chatter_pub.publish(msg);
 
    ros::spinOnce();
 
    loop_rate.sleep();
    ++count;
  }
 
 
  return 0;
}

写一个订阅（Subscriber）节点
还是在src目录下，创建一个listener.cpp文件。内容如下：

#include "ros/ros.h"
#include "std_msgs/String.h"
 
/**
 * This tutorial demonstrates simple receipt of messages over the ROS system.
 */
void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
  ROS_INFO("I heard: [%s]", msg->data.c_str());
}
 
int main(int argc, char **argv)
{
  /**
   * The ros::init() function needs to see argc and argv so that it can perform
   * any ROS arguments and name remapping that were provided at the command line. For programmatic
   * remappings you can use a different version of init() which takes remappings
   * directly, but for most command-line programs, passing argc and argv is the easiest
   * way to do it.  The third argument to init() is the name of the node.
   *
   * You must call one of the versions of ros::init() before using any other
   * part of the ROS system.
   */
  ros::init(argc, argv, "listener");
 
  /**
   * NodeHandle is the main access point to communications with the ROS system.
   * The first NodeHandle constructed will fully initialize this node, and the last
   * NodeHandle destructed will close down the node.
   */
  ros::NodeHandle n;
 
  /**
   * The subscribe() call is how you tell ROS that you want to receive messages
   * on a given topic.  This invokes a call to the ROS
   * master node, which keeps a registry of who is publishing and who
   * is subscribing.  Messages are passed to a callback function, here
   * called chatterCallback.  subscribe() returns a Subscriber object that you
   * must hold on to until you want to unsubscribe.  When all copies of the Subscriber
   * object go out of scope, this callback will automatically be unsubscribed from
   * this topic.
   *
   * The second parameter to the subscribe() function is the size of the message
   * queue.  If messages are arriving faster than they are being processed, this
   * is the number of messages that will be buffered up before beginning to throw
   * away the oldest ones.
   */
  ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);
 
  /**
   * ros::spin() will enter a loop, pumping callbacks.  With this version, all
   * callbacks will be called from within this thread (the main one).  ros::spin()
   * will exit when Ctrl-C is pressed, or the node is shutdown by the master.
   */
  ros::spin();
 
  return 0;
}

编译创建的节点
在编译我们创建的节点之前，我们还需要编辑Cmakelist.txt文件（注意：是beginner_tutorials项目包下的CMakelist文件），告诉编辑器我们需要编辑什么文件，需要什么依赖。打开相应的CMakelist文件
在文件中添加如下语句：

include_directories(include ${catkin_INCLUDE_DIRS})（找到这个位置，添加以下语句）

add_executable(talker src/talker.cpp)
target_link_libraries(talker ${catkin_LIBRARIES})
add_dependencies(talker beginner_tutorials_generate_messages_cpp)

add_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES})
add_dependencies(listener beginner_tutorials_generate_messages_cpp)

将目录切换到工作空间目录，并执行catkin_make运行命令：

$ cd ~/catkin_ws
$ catkin_make

若是没有报错则文件编译完成。
测试程序的正确性
首先，我们得要启动ROS核心程序roscore。

$ roscore

若是前面没有添加echo "source ~/catkin_ws/devel/setup.bash" >> ~/.bashrc
则每开一个终端就需要注册一下：

$ cd ~/catkin_ws
$ source ./devel/setup.bash

运行talker.cpp节点

rosrun beginner_tutorials talker 

当运行成功时，则会出现如下信息：

[ INFO] [1547964361.358836271]: hello world 12
[ INFO] [1547964361.458837732]: hello world 13
[ INFO] [1547964361.558842476]: hello world 14
[ INFO] [1547964361.658843518]: hello world 15
[ INFO] [1547964361.758828707]: hello world 16
[ INFO] [1547964361.858848536]: hello world 17

这就表示发布（Publisher）节点已经正确的运行了。
接下来运行listener节点：$ rosrun beginner_tutorials listener
这时候会看到如下信息：

`[ INFO] [1547964474.752571964]: I heard: [hello world 23]
[ INFO] [1547964474.852573978]: I heard: [hello world 24]
[ INFO] [1547964474.952543237]: I heard: [hello world 25]
[ INFO] [1547964475.052528214]: I heard: [hello world 26]
[ INFO] [1547964475.152539526]: I heard: [hello world 27]
[ INFO] [1547964475.252531352]: I heard: [hello world 28]
[ INFO] [1547964475.352789340]: I heard: [hello world 29]
`

这说明订阅节点（listener）已经成功的接收到了发布节点（talker）发布的信息。至此，整个程序结束！