c++ interrupt handling data

Sorry for the cryptic title, I wasn't quite sure how to make it without writing this post. I'm working on a raspberry Pi project that is dealing with motor drives and interrupts. The issue I'm having is getting the data from the interrupts. I'm trying to keep this all in C++, so am struggling with static data.

motor.h

class Motors
{
  public:
  Motors()
  {...}
  static void tickA2D( Motors *myObj );
}

motor.cpp

// telemetry 
static volatile float Vbatt;

void Motors::tickA2D( Motors *myObj ) {
  Vbatt = telem; // this works fine
}

lawnmower.cpp

std::string elecData = "{\"vbatt\":" + std::to_string(mtr->Vbatt)...  //fails
std::string elecData = "{\"vbatt\":" + std::to_string(Motors::Vbatt)...  //fails

console:

lawnmower.cpp:152: undefined reference to `Motors::Vbatt'

How can I access the variable Vbatt from lawnmower.cpp? I have 3 other variables as well, but I'm trying to keep this simple.

Answer

First, you should declare Vbatt in the class definition in motor.h .

class Motors
{
public:
    Motors() { ... }

    static void tickA2D(Motors *myObj);

    // Add this line to declare Vbatt
    static volatile float Vbatt;
};

Next, you should define Vbatt in motor.cpp.

#include "motor.h"

// actually define the static member here
volatile float Motors::Vbatt = 0.0;

Last, you can access it in lawnmower.cpp.

std::string elecData = "{\"vbatt\":" + std::to_string(Motors::Vbatt) + "}";

Note:

You marked Vbatt as volatile, which is great for interrupt/shared memory stuff. But:

• volatile tells the compiler not to optimize out reads and writes (because the value can change outside normal program flow like in ISRs).

• It does not make access thread-safe (so don’t forget atomic or mutexes if needed later, depending on complexity)