Make C++ class partially constexpr and save RAM

I have written code for a controller that has buttons and lamps. It is based on Arduino/ATmega2560. RAM is very limited. Changing to another device is no option.

Each button and lamp is modelled as a C++ class instance. They are derived from an abstract class Thing, which contains a pointer mpTarget that indicates connections between Things. The targets are initially assigned at runtime and do not change during runtime.

I need to save memory. As I have ~600 Things, it would help me alot if all those mpTargets were stored in ROM (and not in RAM), because the targets are known at compile time.

So I played with constexpr. However, it seems to be impossible to make a class only partially constexpr. I cannot make the whole class constexpr, because there are also member variables that change during runtime (mState in the example below).

What would be a good way to achieve this behavior? constexpr? Templates? Anything?

Sample code:

#include <iostream>

class Thing
{
  public:
  // only called in setup()
  void setTarget(Thing & pTarget)
  {
    mpTarget = & pTarget;
  }
  virtual void doSomething(int value) {};
  protected:
  // known at compile time - can we make this constexpr?
  Thing * mpTarget;
};

class Button : public Thing
{
  public:
  void setState(int newState)
  {
    mState = mState + newState;
    mpTarget->doSomething(mState);
  }
  
  private:
  // changes during runtime
  int mState;
};
class Lamp: public Thing
{
  public:
  virtual void doSomething(int value)
  {
    std::cout << value << std::endl;
  };
};


Button myButton;
Lamp myLamp;

int main()
{
  myButton.setTarget(myLamp);
  while (1)
  {
    myButton.setState(123);
  }
}