How to have a variable be of a class type be set from a reference (Arduino C++)

Question

Seems like a simple question, but my knowledge of C++ is sadly not very good.

So I have Joint declared as a class in another file. I want another class named Dancer to accept Joint instance from outside and save it in it's own instance.

I have something like this so far.

// Dancer.h
class Dancer {
  public:
    Joint joint;
    void setJoint(Joint newJoint);
    void animate();
}

// Dancer.cpp
Dancer::setJoint(Joint newJoint) {
  joint = newJoint;
}
Dancer::animate() {
  joint.moveTo(random(90));
}

// MainArduinoSketch.ino
Joint frontLeftJoint;
void setup() {
  dancer.setJoint(frontLeftJoint);
}

The problem is that my manipulation of the joint no longer works, though I get no errors or crashes. I clearly have an instance, which keeps values and responds to methods, but it's not the same instance. It's acting like I cloned the joint, rather passed the joint around.

I have a hunch I probably need some pointers somewhere to keep the reference, but I'm not sure exactly what needs to be a pointer, and how to call methods on instances referenced though a pointer.

So how can my code above be changed so that an instance of Dancer can get passed an instance of Joint and have it be the exact same instance of joint?

Answer 1

If you wish to talk about the exact same instance, you need to indeed use a reference or a pointer (they are fairly similar in what happens under the hood).

The choice falls to a pointer in this case, because you can only initialize a reference, but not set its value later on anymore. Since you have a setJoint, this is clearly a reason to use a pointer.

To keep the rest of the code mostly the same, we will let setJoint take a reference argument however, and then take its address to gain access to a pointer to the memory location that was passed by reference to the function.

When accessing members (be they data or function) of objects to which we have a pointer, there are two options: You can either dereference the pointer and then use the resulting object as you normally would, or you can use the -> operator to do this in one step.

Putting everything together, this is the result:

// Dancer.h
class Dancer {
  public:
    Joint* joint;
    void setJoint(Joint& newJoint);
    void animate();
}

// Dancer.cpp
Dancer::setJoint(Joint& newJoint) {
  joint = &newJoint;
}
Dancer::animate() {
  joint->moveTo(random(90));
}

// MainArduinoSketch.ino
Joint frontLeftJoint;
void setup() {
  dancer.setJoint(frontLeftJoint);
}