我可以将指针作为指向类的指针传递，以便我可以使用分配给类中指针的任何内容吗？

Question

I have two objects of the same class in my program. I set the focused object to a third variable, which is a pointer. So I can switch the focused object with this third pointer variable and have access at every part of my code without having to know which object is focused.

This looks like the following:

class MainClass
{
    OtherClass otherClass;

    Field fieldA;
    Field fieldB;
    Field *focusedField = &fieldA;

    void someMethod(){
         otherClass.othermethod();         

         focusedField = &fieldB;
         
         otherClass.othermethod();
    }

    void MainClass()
      : otherClass(focusedField);
    {
        
    }
}

But now I want to use this pointer in another class like this:

class OtherClass{
    OtherClass(Field *f){
        focusedField = f;
    }

    Field *focusedField;
    
    void otherMethod(){
        std::cout << focusedField->getState() << std::endl;
    }
}

And this works partly...
The first call of the otherMethod method uses the fieldA, which is correctly, but the second call, after I changed it to fieldB, is still using fieldA.

I want to achieve that I pass the focusedField to the OtherClass and it changes if I change it in the MainClass.

Answer 1

To expand on JS's comment , if you want to “track” a value, you store a pointer or reference to it. That's still true if the value is itself a pointer. For const-correctness, you should declare the OtherClass member (and constructor parameter) as Field *const & since it doesn't need to change which Field is focused. If it doesn't need to write to whatever focused Field either, use const Field *const & .

It's worth noting that using a pointer to pointer might be better both as a member (because it allows assignment to have the usual semantics) and as a constructor parameter (to advertise that a temporary is inappropriate as an argument). It still serves the same sharing purpose and allows the same choices of const placement.