Example 3) A little state machine (with performance benefits compared to if-else, if there is a higher number of states)
 
struct SimpleStateMachine
{
    SimpleStateMachine() {current_state=state_A;}
    void step() {current_state->step();}
  private:
    struct State {
        virtual void step()=0;
    } * current_state;
    State state_A {  // derived from struct State
        virtual void step() {current_state=state_B;}
    };
    State state_B {  // derived from struct State
        virtual void step_v() {current_state=state_A;}
    }
};

// nested object
 
class Nested {
    virtual void foo_nested() {}
}
 
class Outside {
    void foo_outside() {..}
    Nested nested {
        void foo_nested() {
            // finds the method in class Outside
            foo_outside();
        }
    };  // nested object
}
 
Outside().nested.foo_nested();
 
 
// CRTP solution
 
template<class T>
class NestedT {
    void foo_nested() {
        static_cast<T*>(this)->foo_outside();
    }
}
 
class Outside extends NestedT<Outside> {
    void foo_outside();
}
 
Outside().foo_nested();