Virtual Functions Polymorphism and Inheritance - c++

Question

I know the title sucks... but I have no clue what actual issue I have got at my hands...
I have defined a base class for linked lists and a specialized class that inherits the base class.
Base:
list.h
#ifndef LIST_H
#define LIST_H
#include <string>
#include "../../declarations.h"
#include "element.h"
class List: public Element{
/*
* #brief is a generic list class, designed to be inherited by more
* specialised list emulating classes.
*/
public: // methods
/// generates empty list.
List(){
init();
cout << "defining List" << endl;
}
/// Removes first list element
bool remove_first(){
if (!is_empty()){
Element* newFirst = first;
delete first;
first = newFirst->next;
return true;
}
else{
std::cerr << "list already empty" << endl;
return false;
}
}
/// removes last list element
bool remove_last(){
// store length fo list in min_len for switch
unsigned int min_len = 0;
if (!is_empty()){
if ((first->next) != 0){
min_len = 2;
}
else {
min_len = 1;
}
}
// as switch doesn't allow for comparisons, map any value greater
// 1 to 2 if (min_len>1) min_len = 2;
switch (min_len){
case (2):{
// Iterate over list, always look two elements ahead.
// If the next element points to 0 you have found your new last
// element (the current next to last one). Delete current-> next
// to remove the last element and let the new last one point to
// zero.
Element* current = first;
while((current->next)->next != 0){
current = (current->next);
}
delete (current->next);
current->next = 0;
return true;
}
// if the list contaisn only one element use remove_first()
// as the logic for that case is already present there
case (1): return remove_first();
case (0):{
std::cerr << "list already empty" << endl;
return false;
}
}
}
// Declare several virtual functions for appending elements
// that get defined in the iheriting classes.
virtual void append_front(int i){};
virtual void append_front(char c){};
virtual void append_front(std::string s){};
virtual void append_back(int i){};
virtual void append_back(char c){};
virtual void append_back(std::string s){};
/// todo
std::string head(){}
/// returns length of list
unsigned length(){
unsigned counter = 0;
Element* current = first;
while(current != 0) {
++counter;
current = current->next;
}
return counter;
}
/// determines whether list is empty
bool is_empty(){
return first == 0;
}
// printing depends on type of list, so this method gets defined in
// inheriting class
virtual void print(){};
private: // methods
void init(){
// initialise empty list with a NULL pointer
first = 0;
}
protected: // data members
/// points to first list element or zero, if list is empty
Element* first;
};
#endif
Derived:
transitionsList.h
#ifndef TRANSITION_LIST_H
#define TRANSITION_LIST_H
#include "../../../generic/list.h"
#include "../../elements/transition/transition.h"
class TransitionList: public List {
public:
void append_front(char c){
// generate list element and store in pointer e
Element* e;
e = new Transition();
// store parameter c in the object *e points to
e->set_content(c);
// Either list is empty. In thet case object at &e becomes first
// list element and also the last.
// Or list wasn't empty and e->next points to the current first, then
// object *e points to becomes first.
if (is_empty()) {
first = e;
e->next = 0;
}
else {
e->next = first;
first = e;
}
}
void append_back(char c){
if (is_empty()) {
append_front(c);
}
else {
Element* e;
e = new Transition();
e->set_content(c);
e->next = 0;
// Go through the list until you find the last element, let it
// then ppint to the new last element.
Element* current = first;
while(current->next != 0) {
current = current->next;
}
current->next = e;
}
}
void print(){
cout << "[";
// go through the list, starting with first element
Element* current = first;
// as long as the last list element is not the current one, keep
// printing elements.
while (current != 0) {
cout << " " << current->get_content();
// let current be the next of current current, to move
// through the list
current = current->next;
} // while
cout << " ]";
}
};
#endif
These two depend on a few other classes:
element.h:
#ifndef ELEMENT_H
#define ELEMENT_H
#include "../../declarations.h"
class State;
class Element{
/*
* #brief is a generic list element class, designed to be inherited by
* more specialised list element classes.
*/
public:
Element(){}
virtual ~Element(){}
virtual void set_content(int i){};
virtual void set_content(char c){};
virtual void set_content(std::string s){};
virtual char get_content() = 0;
Element* next; ///< is the pointer to the next transition in the list
};
#endif
state.h:
#ifndef STATE_H
#define STATE_H
#include "../../declarations.h"
class List;
class State{
/**
* #brief is a class emulating a trie state
*/
public: // methods
State(){
cout << "defining State" << endl;
init();
}
private: // methods
void init(){
}
private: // variables
// Pointer required due to forward declarations
List* transitions; ///< is the list of states dominated by THIS.
// count stores how often the word the current path codes for has been
// found in the training data.
int count; ///< is the number of occurances of some word in this path.
};
#endif
and transition.h
#ifndef TRANSITION_H
#define TRANSITION_H
#include "../../../generic/element.h"
#include "../../state.h"
typedef char cont;
class Transition: public Element {
public:
void set_content(cont c){
content = c;
};
cont get_content(){
return content;
};
State* successor;
Transition* next; ///< is the pointer to the next transition in the list
protected:
cont content; ///< is the character this transition codes for
};
#endif
When I now run this cpp-file:
list_test.cpp:
#include "transitionList.h"
int main(){
TransitionList* derivedList;
List *baseList = &derivedList;
}
I get the error:
list_test2.cpp:5:8: error: cannot initialize a variable of type 'List *' with an rvalue of type 'TransitionList **'
Which I don't understand, because I am under the (apparently phony) impression, that this would be analogous to what I saw in this tutorial:
https://youtu.be/DudHooleNVg?t=4m59s
Where is
int main(){
TransitionList* derivedList;
List *baseList = &derivedList;
}
not analogous to
int main(){
Ninja* n;
Enemy *enemy1 = &n;
}
:/ Very confused.