ROS advertise函数参数详解

突然想知道advertise函数里第三个参数到底是什么含义，网上博客找了一圈每一个解释清楚的，这里记录一下

首先看一下官方的注释：

* \brief Advertise a topic, simple version
*
* This call connects to the master to publicize that the node will be
* publishing messages on the given topic.  This method returns a Publisher that allows you to
* publish a message on this topic.
*
* This version of advertise is a templated convenience function, and can be used like so
*
*   ros::Publisher pub = handle.advertise<std_msgs::Empty>("my_topic", 1);
*
* \param topic Topic to advertise on
*
* \param queue_size Maximum number of outgoing messages to be
* queued for delivery to subscribers
*
* \param latch (optional) If true, the last message published on
* this topic will be saved and sent to new subscribers when they
* connect
*
Publisher advertise(const std::string& topic, uint32_t queue_size, bool latch = false)

这里面第三个参数 bool latch，如果设置为true，会保存发布方Publisher的最后一条消息，如果有新的订阅对象订阅该发布方时，发布方会自动把这条消息发送给订阅者；这样做的目的是降低Publisher发布的频率，提高话题使用效率，举个例子：一个发布地图的Publisher只在最开始发布了一次，当latch设置为TRUE时，如果后面有新的subscriber订阅到该发布时，就算此时发布方已经停止发布了，也会把之前发布过的最后一条消息保留发给新订阅该话题的对象。