用C++写一个简单的订阅者

打开一个终端，进入到beginner_tutorials包下面：

cd ~/catkin_ws/src/beginner_tutorials

建立文件src/listener.cpp：

vim src/listener.cpp

将下面的代码复制到文件中：

#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;
}

保存退出。

下面看一下代码的解释：

void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
  ROS_INFO("I heard: [%s]", msg->data.c_str());
}

当一个消息到达chatter话题时，这个回调函数将会被调用。

ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);

订阅chatter话题，当一个新的消息到达时，ROS将会调用chatterCallback()函数。第二个参数是对列的长度，会将收到的消息缓冲下来，一共可以缓冲1000条消息，满1000之后，后面的到达的消息将会覆盖前面的消息。

NodeHandle::subscribe()将会返回一个ros::Subscriber类型的对象，当订阅对象被销毁以后，它将会自动从chatter话题上撤销。

ros::spin();

ros::spin()进入了一个循环，非常快的调用消息的回调函数。不要担心，如果它没有什么事情可做时，它也不会浪费太多的CPU。

当ros::ok()返回false时，ros::spin()将会退出。

这就意味着，当ros::shutdown()被调用，或按下CTRL+C等情况，都可以退出。

下面总结一下写一个订阅者的步骤：（1）初始化ROS系统（2）订阅chatter话题（3）Spin，等待消息的到来（4）当一个消息到达时，chatterCallback()函数被调用。

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下面看一下如何构建节点。我们之前用过catkin_create_pkg创建过package.xml和CMakeLists.txt(目录：catkin_ws/src/beginner_tutorials/CMakeLists.txt)，这时候你的CMakeLists.txt看起来应该是下面这个样子，包括前面所做的修改，注释部分可以除去：

cmake_minimum_required(VERSION 2.8.3)
project(beginner_tutorials)

## Find catkin and any catkin packages
find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)

## Declare ROS messages and services
add_message_files(DIRECTORY msg FILES Num.msg)
add_service_files(DIRECTORY srv FILES AddTwoInts.srv)

## Generate added messages and services
generate_messages(DEPENDENCIES std_msgs)

## Declare a catkin package
catkin_package()

将下面几行代码添加到CMakeLists.txt的最后。最终你的CMakeLists.txt文件看起来样该是下面这个样子：

cmake_minimum_required(VERSION 2.8.3)
project(beginner_tutorials)

## Find catkin and any catkin packages
find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)

## Declare ROS messages and services
add_message_files(FILES Num.msg)
add_service_files(FILES AddTwoInts.srv)

## Generate added messages and services
generate_messages(DEPENDENCIES std_msgs)

## Declare a catkin package
catkin_package()

## Build talker and listener
include_directories(include ${catkin_INCLUDE_DIRS})

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)

构建完之后，这将会创建两个可执行文件，talker和listener。它们将会产生在~/catkin_ws/devel/lib/share/<package name>目录下，这也就是说构建好的东西，会放在devel的目录中，原本的东西会留在src文件中。

下面开始构建，在你的工作空间根目录下输入：

catkin_make

这样我们就完成了，在某个话题上的消息的发布者和订阅者的可执行模块的创建。