Can I use, in a friend function, an object created in class constructor?

Question

#include "MPU6050.h"

class DATA{
    public:
    DATA(int A);
    void Sensor();
    }


DATA::DATA(int A){
MPU6050 mpu(A);
}

void Sensor(){
mpu.setFullScaleAccelRange(3);
// the problem is here, "mpu" object is not declared for this friend function
}

int main(){
DATA mpu1(0x68)       
/* I'm using two MPU6050 on I2C bus so I need to address them.
 this example contains only one MPU6050 object*/
    return 0;
}

Compiler Error : "mpu was not declared in this scope"

While writing this code I had a feeling that the mpu object is not declared for Sensor friend function. Is there a way to deal with this case?

I'm a C++ Beginner.

Thanks

Answer 1

I am a bit rusty on my C++ (it's been 15+ years) but I think the issue is that you didn't declare the mpu variable as a private (or protected) member of the class. Instead it only gets a declaration (and I'm not sure that's done in the right way, my (Java-influenced) brain says you should likely do MPU6050 mpu = mpu(A); to make the correct declaration) in the DATA constructor, which means its visibility is limited to the constructor, hence the Sensor() member function cannot access it.

Net, what you want to do is change the class declaration to:

class DATA{
    MPU6050 mpu;
    public:
    DATA(int A) : mpu(A) {};
    void Sensor();
    }