How to write a good program in C++: Referencing a value from another function

[Eimyver3]

Hello .
I would like to ask a basic C++ question.
I am currently working on the final project of ROS Basics in 5 Days (C++), which is a maze search.
I am currently creating a service server for collision detection.
I would like to adjust the subscriber to get the laser values and run the assignment, but I am stumped on the assignment.
I would like to read the values from a callback function called laser_callback in a callback function called obstacle_callback for collision as shown in the following code, but I can’t think of a good way to do it.
I’ve tried using pointers, declaring it as a global variable, etc., but I get an error when compiling.

If you could give me some advice on how to solve this problem, I would appreciate it.

#include “ros/ros.h”

    #include "std_srvs/Trigger.h"

    #include "sensor_msgs/LaserScan.h"

    #include "my_turtlebot_topics/laser_turtle_sub.h"

    laser_turtle_sub::laser_turtle_sub()

    {

        sub = nh.subscribe("/kobuki/laser/scan",1000,&laser_turtle_sub::laser_callback,this);

        my_service = nh.advertiseService("/move_bb8_in_circle", &laser_turtle_sub::obstacle_callback,this);

    }

    void laser_turtle_sub::laser_callback(const sensor_msgs::LaserScan::ConstPtr& laser_msg)

    {

        float laser_range_c = laser_msg->ranges[360];

        float laser_range_r = laser_msg->ranges[0];

        float laser_range_l = laser_msg->ranges[719];

        //ROS_INFO("LaserData Center:%f Right:%f Left:%f\n",laser_range_c,laser_range_r,laser_range_l);

    }

    bool laser_turtle_sub::obstacle_callback(std_srvs::Trigger::Request  &req, std_srvs::Trigger::Response &res)

    {

        //サービス要請を受けたとき各方向のレーザーの値を取得する

        res.message = laser_range_c;

        ROS_INFO("%f",res.message);

        return true;

    }

    int main(int argc, char **argv)

    {

    ros::init(argc, argv, "service_move_bb8_in_circle_server");

    ros::NodeHandle nh;

    laser_turtle_sub *lasersub = new laser_turtle_sub();

    //vel_pub = nh.advertise<geometry_msgs::Twist>("cmd_vel", 1000);

    ROS_INFO("Service /move_bb8_in_circle_custom Ready");

    ros::spin();

    return 0;

    }

thank you.

Translated with www.DeepL.com/Translator (free version)

[roalgoal]

Hi, can you share the error message you are receiving while compiling?

[Eimyver3]

Hello．

If you run the above program, you will get the following error because the variable is not declared in obstacle_callback().

As a workaround for this, I implemented declaring the variables as follows. Compilation was successful, but I could not get the proper laser_msg as shown in the following image.
(I’m sorry, sir. The name of the service server has been changed from /move_bb8_in_circle to /turtle_custom.)

I don’t think I understand pointers correctly, but I thought I could use pointers to read the data in laser_range_c if it is from an address. (I’m probably wrong.)
I’ve tried this with * and & everywhere, but none of them worked (I got an error that it was not declared).

So I’m looking for a simple way to read the value of laser_range_c declared in another function like this, but I’ve hit a wall.

I would be grateful for any advice.

[roalgoal]

Have you tried simplifying your code? For example, advertising the service inside main() instead of inside a callback function? I see that calling the service fails, do you have it available/created correctly?

[Eimyver3]

Thank you for the advice.
I was reviewing my code again after your advice, and I realized my mistake.

In my previous code, I had given a float laser message to the response string type “message” in trigger.srv!
So when I ran “rosservice call ~” as shown in the image above ,
This is why when you run “rosservice call ~” as shown in the image above, you get an unintended display.
(After displaying the value of the laser, a mysterious ? is displayed after displaying the laser value…)

As a solution, I created the following code and ran it to get the following result.

laser_turtle_sub.h

    #ifndef LASER_TURTLE_SUB_H
    #define LASER_TURTLE_SUB_H
    #include <ros/ros.h>
    #include "sensor_msgs/LaserScan.h"
    #include "std_srvs/Trigger.h"



    class laser_turtle_sub{
        protected:
            ros::NodeHandle nh;
            ros::Subscriber sub;
            ros::ServiceServer my_service;
            sensor_msgs::LaserScan laser;

        public:
            laser_turtle_sub();
            float laser_range_c;
            void laser_callback(const sensor_msgs::LaserScan::ConstPtr& laser_msg);
            bool obstacle_callback(std_srvs::Trigger::Request  &req, std_srvs::Trigger::Response &res);
    };

    #endif

laser_turtle_service_server.cpp

    #include "ros/ros.h"
    #include "std_srvs/Trigger.h"
    #include "sensor_msgs/LaserScan.h"
    #include "my_turtlebot_topics/laser_turtle_sub.h"

    static float laser_range_c;
    static float laser_range_r;
    static float laser_range_l;

    laser_turtle_sub::laser_turtle_sub(){
        sub = nh.subscribe("/kobuki/laser/scan",1000,&laser_turtle_sub::laser_callback,this);
        my_service = nh.advertiseService("/turtle_custom", &laser_turtle_sub::obstacle_callback,this);
    }

    void laser_turtle_sub::laser_callback(const sensor_msgs::LaserScan::ConstPtr& laser_msg)
    {
        laser_range_c = laser_msg->ranges[360];
        laser_range_r = laser_msg->ranges[0];
        laser_range_l = laser_msg->ranges[719];

        //ROS_INFO("LaserData Center:%f Right:%f Left:%f\n",laser_range_c,laser_range_r,laser_range_l);
    }

    bool laser_turtle_sub::obstacle_callback(std_srvs::Trigger::Request  &req, std_srvs::Trigger::Response &res)
    {
        ROS_INFO("recieve a call");
        ROS_INFO("%f",laser_range_c);
        //サービス要請を受けたとき各方向のレーザーの値を取得する
        if(laser_range_c > 2){
            res.message = "safe";
            ROS_INFO("%s",res.message);
        }
        else{
            res.message = "crush";
            ROS_INFO("%s",res.message);
        }

        return res.success = true;
    }

    int main(int argc, char **argv)
    {
        ros::init(argc, argv, "service_turtle_server");
        ros::NodeHandle nh;

        laser_turtle_sub *lasersub = new laser_turtle_sub();
        ros::Rate rate(2);

        ROS_INFO("Service /turtle_custom Ready");
        ros::spin();

        return 0;
    }

Anyway, thanks for the advice. Thank you very much for your advice, I was able to review my code calmly.