Just out of curiosity, I modified the simple c++ publisher and simple python publisher. I got rid of the while loop and did the following.

c++

loop_rate.sleep();

msg.data = "hello world 1";
chatter_pub.publish(msg);
ros::spinOnce();
loop_rate.sleep();

msg.data = "hello world 2";
chatter_pub.publish(msg);
ros::spinOnce();
loop_rate.sleep();

msg.data = "hello world 3";
chatter_pub.publish(msg);
ros::spinOnce();
loop_rate.sleep();

python

rate.sleep()

hello_str = "1-hello world py"
pub.publish(hello_str)
rate.sleep()

hello_str = "2-hello world py"
pub.publish(hello_str)
rate.sleep()

hello_str = "3-hello world py"
pub.publish(hello_str)
rate.sleep()

Now when I run both publishers one after the other, rostopic echo gives me the following, which is expected

data: "1-hello world py"
---
data: "2-hello world py"
---
data: "3-hello world py"
---
data: "hello world 1"
---
data: "hello world 2"
---
data: "hello world 3"
---

The problem is, c++ subscriber callback is not triggered for the first messages. To be exact, it gets triggered after only the 3rd message for c++ publisher and after the 2nd message for python publisher. After that it works fine. This is the c++ subscriber output I get

[ INFO] [1569905819.772859593]: I heard: [2-hello world py]
[ INFO] [1569905819.874401168]: I heard: [3-hello world py]
[ INFO] [1569905825.212111970]: I heard: [hello world 3]

Python subscriber works fine

[INFO] [1569905819.678143]: /listener_17329_1569905813773I heard 1-hello world py
[INFO] [1569905819.772875]: /listener_17329_1569905813773I heard 2-hello world py
[INFO] [1569905819.875100]: /listener_17329_1569905813773I heard 3-hello world py
[INFO] [1569905825.012191]: /listener_17329_1569905813773I heard hello world 1
[INFO] [1569905825.112169]: /listener_17329_1569905813773I heard hello world 2
[INFO] [1569905825.212244]: /listener_17329_1569905813773I heard hello world 3

Can someone please explain what's happening here? I don't get it.

---

c++ publisher - talker.cpp

#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);

  ros::Rate loop_rate(10);
  std_msgs::String msg;
  std::stringstream ss;
  loop_rate.sleep();

  msg.data = "hello world 1";
  ROS_INFO("%s", msg.data.c_str());
  chatter_pub.publish(msg);
  ros::spinOnce();
  loop_rate.sleep();

  msg.data = "hello world 2";
  ROS_INFO("%s", msg.data.c_str());
  chatter_pub.publish(msg);
  ros::spinOnce();
  loop_rate.sleep();

  msg.data = "hello world 3";
  ROS_INFO("%s", msg.data.c_str());
  chatter_pub.publish(msg);
  ros::spinOnce();
  loop_rate.sleep();

  return 0;
}

python publisher - talker.py

#!/usr/bin/env python
import rospy
from std_msgs.msg import String

def talker():
    pub = rospy.Publisher('chatter', String, queue_size=10)
    rospy.init_node('talker', anonymous=True)
    rate = rospy.Rate(10) # 10hz
    rate.sleep()

    hello_str = "1-hello world py"
    rospy.loginfo(hello_str)
    pub.publish(hello_str)
    rate.sleep()

    hello_str = "2-hello world py"
    rospy.loginfo(hello_str)
    pub.publish(hello_str)
    rate.sleep()

    hello_str = "3-hello world py"
    rospy.loginfo(hello_str)
    pub.publish(hello_str)
    rate.sleep()

if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

Subscribers are identical to the ones provided in the example pages (c++, python)