How can I move these global variables to be

Question

I'm taking a sample I found on the web about how to get ROS and arduino to communicated together over serial. I have the sample working and now I'm moving the idea of the sample into my OOP project and am running into some trouble.

My sketch works but when I move a certain global variable to a protected member it breaks my sketch.

How do I declare this global variable

ros::Publisher chatter("chatter", &str_msg);

as a protected variable

protected:
 ros::Publisher chatter("chatter", &str_msg);

without getting this error

expected identifier before string constant

sample .ino

/*
 * rosserial Publisher Example
 * Prints "hello world!"
 */
#include <ros.h>
#include <std_msgs/String.h>
ros::NodeHandle nh;
std_msgs::String str_msg;
ros::Publisher chatter("chatter", &str_msg);
char hello[13] = "hello world!";
void setup()
{
 nh.initNode();
 nh.advertise(chatter);
}
void loop()
{
 str_msg.data = hello;
 chatter.publish( &str_msg );
 nh.spinOnce();
 delay(1000);
}

ROSController class

#include <ros.h>
#include <std_msgs/String.h>
class ROSController {
 protected:
 ros::NodeHandle _nh;
 std_msgs::String str_msg;
// ros::Publisher chatter("chatter", &str_msg);
 int _throttle;
 bool _is_on = false;
 public:
 void loop();
 ROSController();
 double* getAttitude();
 bool isOn() {
 return this->_is_on;
 }
 int getThrottlePerc() {
 return this->_throttle / ROSController::THR_MAX;
 }
 static const int THR_MAX;
};
const int ROSController::THR_MAX = 100;
ROSController::ROSController() {
 this->_nh.initNode();
// this->_nh.advertise(chatter);
}
double* ROSController::getAttitude() {
 return new double[3]{0, 0, 0};
}
void ROSController::loop() {
// str_msg.data = new char[] {"hello"};
// chatter.publish( str_msg );
 this->_nh.spinOnce();
 delay(1000);
}

• Majenko: I don't understand what you are trying to do there. Some context would be nice - ideally some actual code that demonstrates just what you are trying to do (before and after maybe?)
• Jacksonkr: I added my sample .ino and a little more of the backstory. It's really such a small issue so I was hoping I could keep the question just as simple but I suppose to be thorough you'll have to see the bigger picture.
• Majenko: And where are you trying to move that global variable to?
• Jacksonkr: I've added my ROSController class to the bottom of the question
• Nick Gammon: There's no such thing as a "protected" global variable. Protected variables belong inside classes.

Answer 1

The problem you are having is you are trying to both define and declare a class member variable at the same time. You can't. Instead you need to split it:

protected:
 ros::Publisher chatter;
ROSController::ROSController() : chatter("chatter", &str_msg)
{
 this->_nh.initNode();
// this->_nh.advertise(chatter);
}

That is, set up the variable as a protected member, then have the constructor call the constructor for you with the right parameters.

(I think that is the right syntax off the top of my head).

• Majenko: Then your Publisher class has no default constructor. Add one.
• Jacksonkr: Hey, that worked! I don't understand what you did on line 4. What is that called? I want to research it and find out more about it.
• Majenko: It's called an initialization list - you can read a little more about it here: tutorialspoint.com/cplusplus/cpp_constructor_destructor.htm

Answer 2

You can't call the constructor in the class definition. Example:

class foo
 {
 protected:
 String bar ("fubar");
 };
void setup ()
 {
 } // end of setup
void loop ()
 {
 } // end of loop

That gives this error:

sketch_sep08b:4: error: expected identifier before string constant
 String bar ("fubar");
 ^
sketch_sep08b:4: error: expected ',' or '...' before string constant
exit status 1
expected identifier before string constant

---

Instead declare the class (without the constructor) and call the constructor in the constructor for the parent class, like this:

class foo
 {
 protected:
 String bar;
 public:
 foo () : bar ("fubar") {}; // constructor
 };
void setup ()
 {
 } // end of setup
void loop ()
 {
 } // end of loop