C++ const getter method with lazy initialization

Question

What is the proper way to implement a getter method for a lazily-initialized member variable and maintain const-correctness? That is, I would like to have my getter method be const, because after the first time it is used, it's a normal getter method. It is only the first time (when the object is first initialized) that const does not apply. What I would like to do:

class MyClass {
  MyClass() : expensive_object_(NULL) {}
  QObject* GetExpensiveObject() const {
    if (!expensive_object_) {
      expensive_object = CreateExpensiveObject();
    }
    return expensive_object_;
  }
private:
  QObject *expensive_object_;
};

Can I eat my cake and have it too?

Answer 1

That's fine and is the typical way of doing it.

You will have to declare expensive_object_ as mutable

mutable QObject *expensive_object_; 

mutable basically means "I know I'm in a const object, but modifying this won't break const-ness."