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)