How to display the function of latch? [closed]

edit

I want to show the function of latch in publisher. According to the description of the latch: "When a connection is latched, the last message published is saved and automatically sent to any future subscribers that connect." So I write a small program to show the function of latch. Below is my code, I just did a modification slightly according to the ROS tutorials.

#include "ros/ros.h"
#include "std_msgs/String.h"
#include <sstream>
int main(int argc, char **argv)
{
    ros::init(argc, argv, "talker");

    ros::NodeHandle n;

    ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000, true);

    ros::Rate loop_rate(100);

    int count = 0;
    while(chatter_pub.getNumSubscribers() == 0)
        loop_rate.sleep();


    std_msgs::String msg;
    std::stringstream ss;
    ss << "hello world " << count;
    msg.data = ss.str();

    ROS_INFO("%s", msg.data.c_str());

    chatter_pub.publish(msg);
}

But when I run the subscribers, it doesn't show me the last message published.

Do you know why?