c++创建订阅者和发布者
此文章默认读者已经对ros有了一定的基础，明白ros之间如何进行数据通信，了解ros的文件结构，了解工作空间，功能包的概念。本篇文章参考创客智造上的教程，他们的讲解比我更详细。百度一下创客智造即可。
订阅者和发布者依托于节点，即订阅者和发布者是在节点中完成的。所以先来说一下节点的定义，步骤可分为：
1.新建代码源文件
2.写代码
3.在cmakelist.txt文件中定义
订阅者和发布者在写代码的步骤中。
下面上一段，roswiki上给出的官方demo，然后对demo做讲解。
talker.cpp
#include "ros/ros.h"
#include "std_msgs/String.h"
#include
//值得一说的是第一行语句，include ros/ros.h ，引入ros的相关头文件，这是所有ros开发的起点
/**
* This tutorial demonstrates simple sending of messages over the ROS system.
*/
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"); //初始化节点，第一二个参数走main里传进来，第三个参数为节点名称
/**
* 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; //实例化节点
/**
* 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
//定义一个发布者，尖括号内的泛型是ros里的消息格式，后面的参数分别为话题名称和消息队列长度
ros::Rate loop_rate(10);
//设置循环的频率
/**
* 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()) //ros：：ok表示ros的状态，当程序出问题或者ctrl+c退出时为false
{
/**
* This is a message object. You stuff it with data, and then publish it.
*/
std_msgs::String msg; //定义需要发布的数据
std::stringstream ss;
ss << "hello world " << count;
msg.data = ss.str();
ROS_INFO("%s", msg.data.c_str()); //ros_info是ros提供的打印信息的方法
/**
* 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(); //类似于线程里的sleep方法
++count;
}
return 0;
}
Listener.cpp
#include "ros/ros.h"
#include "std_msgs/String.h"
/**
* This tutorial demonstrates simple receipt of messages over the ROS system.
*/
//回调函数，用作数据的处理
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);
//实例化订阅者对象，第一个参数为订阅的话题名称，第二个参数为消息队列长度
第三个参数为回调函数，即接收到数据后要执行的方法
/**
* 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;
}
相比较来说，话题的订阅者比发布者更复杂一点，发布者只关注将数据发布出去，订阅者更关注数据接收到之后如何做处理，当然并不是一个节点只能定义一个发布者或者订阅者，一个发布者发布的数据可以是本话题里订阅处理后的数据，最重要的还是让整个系统条理清晰。 节点和话题的命名最好做到结构清晰，让人望文知义。
代码写好之后，接下来就是编译，并且发布到ros系统中。Ros工程使用cmake构建，所以ros的编译要编写cmakelist.txt，然后使用ros的编译命令catkin_make进行编译。所以如果要进行ros更层次的开发，了解一下cmakelist的语法也是必须的。
首先找到功能包根目录下的cmakelist.txt文件，添加如下代码：
include_directories(include ${catkin_INCLUDE_DIRS})
add_executable(talker src/talker.cpp)
#定义增加的可执行文件和源码位置
target_link_libraries(talker ${catkin_LIBRARIES})
#定义连接库
add_dependencies(talker beginner_tutorials_generate_messages_cpp)
#定义依赖
add_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES})
add_dependencies(listener beginner_tutorials_generate_messages_cpp)
cmakelist文件编写好之后，要进入工作空间的根目录，进行编译：
在工作空间根目录下运行命令catkin_make，若不报错则成功
验证一下;
终端1：roscore启动ros
终端2：rosrun 包名（此处为你的包名） talker
终端3：rosrun 包名 listener
观察屏幕打印信息