Linked List with objects

[will93]

Hi, i got problems with linked lists with objects.
I created the linked list with data node of integers and it works without any problems, but now that i try it with a class and its objects it doesnt work.
insert and print seems to work properly, the function ~Lista make the exe crash

//List Header:
#ifndef LISTA_H_
#define LISTA_H_

#include <iostream>
#include "Pringles.h"
using namespace std;

class nodeL{
friend class Lista;
friend ostream& operator<<(ostream&, nodeL*&);

public:
	nodeL(Pringles& p) : nextPtr(0) {P.setT(p.getT()); P.setD(p.getD());}
	Pringles getD(){return P;}
private:
	nodeL* nextPtr;
	Pringles P;
};

ostream& operator<<(ostream& out, nodeL* &n){
	out<< (n->getD()).getT();
	return out;
}


class Lista{
public:
	Lista() : firstPtr(0), lastPtr(0), nElem(0) {}
	~Lista();
	void insertAtFront(Pringles& P);
	bool isEmpty() const {if(firstPtr==0) return true; else return false;}
    void insertAtBack(Pringles& P);
    void removeFront();
    void removeBack();
    void print() const;
private:
	nodeL* firstPtr;
    nodeL* lastPtr;
    int nElem;
};

Lista::~Lista(){

	if(!isEmpty()){
	nodeL* current, *temp;
	current=firstPtr;

	 while(current!=0){

		temp=current;

        cout<<"Deleting element: "; cout<< temp <<endl;
		current=current->nextPtr;
        delete &(temp->getD());
		delete temp;
	 }
	}
}

void Lista::print() const{
	if(!isEmpty()){
	nodeL *current;
	current=firstPtr;
	while(current!=0){
		cout<< current <<" ";
		current=current->nextPtr;
	}
	cout<<endl;
	}
}

void Lista::insertAtFront(Pringles& P){
	nodeL* newPtr=new nodeL(P);

	if(isEmpty()){
		firstPtr=lastPtr=newPtr;
	    nElem++;
	}
	else{newPtr->nextPtr=firstPtr;
	     firstPtr=newPtr;
	     nElem++;
	}
    cout<<"Inserted."<<endl;
}

void Lista::insertAtBack(Pringles& P){
	nodeL* newPtr=new nodeL(P);

	if(isEmpty()){
		firstPtr=lastPtr=newPtr;
	    nElem++;
	}
	else{lastPtr->nextPtr=newPtr;
	     lastPtr=newPtr;
	     nElem++;
	}
	cout<<"Inserted."<<endl;
}

void Lista::removeFront(){
	if(!isEmpty()){
	cout<<"Removing from front: ";
	cout<< firstPtr <<endl;
	nodeL* temp;
	temp=firstPtr;
	firstPtr=temp->nextPtr;

	delete temp;
	nElem--;
	cout<<"Removed."<<endl;
	} else cout<<"List empty!"<<endl;
}

void Lista::removeBack(){
	if(!isEmpty()){
	cout<<"Removing from back: ";
	cout<< (lastPtr) <<endl;
	nodeL* current;
	current=firstPtr;
	while((current->nextPtr)!=lastPtr)
		current=current->nextPtr;
	nodeL* temp=lastPtr;
	lastPtr=current;

	delete temp;
	nElem--;
	cout<<"Removed."<<endl;
	} else cout<<"List empty!"<<endl;
}


#endif /* LISTA_H_ */

//Class Header
#ifndef PRINGLES_H_
#define PRINGLES_H_

#include <string>
using namespace std;

class Pringles{
friend ostream& operator<<(ostream&, Pringles&);
friend istream& operator>>(istream&, Pringles&);
public:
	Pringles(int gr=0, string g="none");
	~Pringles();
	int getD(){return Dimension;}
	string getT(){return *Taste;}
	Pringles operator=(Pringles& P);
	void setD(int gr){Dimension=gr;}
	void setT(string g){*Taste=g;}
private:
	int Dimension;
	string* Taste;
};

Pringles::Pringles(int gr, string g){
	Taste=new string(g);

	Dimension=gr;
}

Pringles::~Pringles(){

	delete Taste;
}

Pringles Pringles::operator=(Pringles& P){
	*Taste=P.getT();
	Dimension=P.getD();
	return *this;
}

ostream& operator<<(ostream& output, Pringles& P){
	output<<"Dimension: "<<P.getD()<<" - Taste: "<<P.getT()<<endl;
	return output;
}

istream& operator>>(istream& input, Pringles& P){
	cout<<"Insert dimension: "; input>>P.Dimension;
	cout<<"Insert taste: "; input>>*P.Taste; cout<<endl;
	return input;
}
#endif /* PRINGLES_H_ */

//MainTest
#include "Lista.h"
#include "Pringles.h"
#include <iostream>
using namespace std;

int main(){
    Pringles P1(10,"Classic"),P2(20,"Paprika"),P3(15,"Sauce");

    Lista L;

    L.insertAtFront(P1);
    L.insertAtBack(P2);
    L.insertAtFront(P3);

    L.print();

    L.removeFront();
    L.removeBack();

    L.print();

    return 0;
}

[keskiverto]

When your line 18 was still int P;, did you have line 55 as delete &(temp->getD());? If not, why the delete is there now?

[coder777]

Line 55 doesn't make sense: the return value of getD() isn't a dynamically allocated object

[will93]

Thanks for the answers but if i remove line 55 the program still doesnt work :(

[coder777]

why did you make Taste a pointer?
At a point that pointer becomes invalid.

Due to this muliple expression packed in a single line the program becomes nearly undebugable

[will93]

I made Taste a pointer because i want to use class string and not char* , and at the same time I have to use dynamic allocations

EDIT: i tried without dynamic allocation and it works (but i dont know why i have to put at line 65 lastPtr->nextPtr and not 0 to let it work), while with char* and dynamic allocation the result is worse than before ...

Now i got a question: how can i use both string and dynamic allocation?

[coder777]

I don't see why Taste is supposed to be a pointer.

I found the bug on line 15: the Pringles object is copied to a local object which deletes Taste after beeing used. Therefore the pointer becomes invalid

[will93]

At the exam i will be forced to use new and delete for string members, this is the reason
Can you be more specific about the bug? i cant find it in line 15

Thank you for the advices!

[coder777]

The result type

Pringles getD(){return P;}

is a copy of P not P itself.

The copy holds the same pointer to Taste as P does.

When the copy goes out of scope its destructor is called and (on line 166) Taste will be destroyed.
Since P and the copy are pointing to the same object Taste within P is now invalid.

Whenever you call getD() of nodeL (like on line 22) Taste will be destroyed

hope you get it?

[will93]

yes now i got it! I have to work with P reference to avoid this, right?
Thank you so much!

[coder777]

I have to work with P reference to avoid this, right?

Yes, consider using const xxx &. That'd stabilize the whole thing.

tip:
to avoid an unwanted copy of an object (usually when a class contains pointer) overload the operator=() and make it private: