Search and protecting singly linked list in C++ - c++

Question

I have one questions regarding searching elements on a Singly Linked List of ints, in this case, using C++. I'm creating my own version of list for exercising. This is the code
Let's suppose I have two search functions. I know we need to traverse the entire list until find the element because we don't have direct access like arrays.
The two functions are:
bool search(int n); // Traverse the list till find n.
bool search(Node* node, int n); Traverse the list till find n only after *node (included)
1 case: My list has the following elements: [0, 1, 2, 3]
If I search for 3 I easily find at the end of the list. Nice.
QUESTIONS:
2 case: My list has the following elements: [0, 1, 2, 3, 3, 3, 4, 5, 6]
If I search for 3 with:
bool search(int n);
I'm going to get the first 3 element always, except if I have a reference to the second or third 3 element to pass to that function:
bool search(Node* node, int n);
My questions is if that is the correct search algorithm in a singly linked list. The two types of functions or if I should have other types.
Bellow is the code for my actual code (I didn't put the code for searching):
SingleLinkedList.h
struct Node {
int data;
Node* next;
Node(int d = 0)
: data {d}, next {nullptr}
{}
};
class SinglyLinkedList {
public:
SinglyLinkedList();
~SinglyLinkedList();
void display();
bool addFirst(const int); // Add a node to the beginning of the list.
bool addFirst(Node*); // Add a node to the beginning of the list.
bool addLast(const int); // Add a node to the end of the list.
bool addLast(Node*); // Add a node to the end of the list.
private:
Node* head;
Node* tail;
};
SinglyLinkedList.h
#include "SinglyLinkedList.h"
#include <iostream>
SinglyLinkedList::SinglyLinkedList()
: head {nullptr}, tail {nullptr}
{}
SinglyLinkedList::~SinglyLinkedList() {
Node* iterationNode = head;
Node* actualNode {nullptr};
while (iterationNode != nullptr) {
actualNode = iterationNode;
iterationNode = iterationNode->next;
delete actualNode;
}
}
void SinglyLinkedList::display() {
std::cout << "################### Displaying Linked List ###################" << std::endl;
if (head == nullptr) {
std::cout << "Linked List is empty!" << std::endl;
}
else {
Node* iterationNode = head;
std::cout << "[ ";
while (iterationNode != nullptr) {
std::cout << iterationNode->data << " ";
iterationNode = iterationNode->next;
}
iterationNode = nullptr;
std::cout << "]" << std::endl;
}
std::cout << "##############################################################" << std::endl;
}
bool SinglyLinkedList::addFirst(const int n) {
Node* element = new Node {n};
if (head == nullptr) {
head = element;
tail = element;
}
else {
element->next = head;
head = element;
}
return true;
}
bool SinglyLinkedList::addFirst(Node* element) {
if (head == nullptr) {
head = element;
tail = element;
}
else {
element->next = head;
head = element;
}
return true;
}
bool SinglyLinkedList::addLast(const int n) {
Node* element = new Node {n};
if (head == nullptr) {
head = element;
tail = element;
}
else {
tail->next = element;
tail = element;
}
return true;
}
bool SinglyLinkedList::addLast(Node* element) {
if (head == nullptr) {
head = element;
tail = element;
}
else {
tail->next = element;
tail = element;
}
return true;
}
Program.cpp
#include <iostream>
#include "SinglyLinkedList.h"
int main() {
{
SinglyLinkedList list;
list.display();
list.addFirst(5);
list.addFirst(4);
list.addFirst(3);
Node* secondNode = new Node {2};
list.addFirst(secondNode);
Node* firstNode = new Node {1};
list.addFirst(firstNode);
Node* zeroNode = new Node;
list.addFirst(zeroNode);
list.addLast(6);
list.display();
}
system("pause");
}
Another question is, how can I protect my struct in a way the user of the program can not mess up changing the links/references directly. For example, in the Program.cpp, any programmer could simply do this:
secondNode->next = zeroNode

Answer 1

The answer to your first question depends on what you need. If you are doing this as a learning project, implement whatever you see fit. What you have described is appropriate for search by value.
The best way to prevent users from directly accessing your Node members in cases like this is to completely abstract the Node type away. You can do this simply by declaring and defining Node in your source file and use forward declarations of Node* in your header. Users who include your header will then not have any notion of your Node type whatsoever.
// SinglyLinkedList.h
class SinglyLinkedList {
//...//
struct Node* head; // head node is forward declared
//...//
}
// SinglyLinkedList.cc
struct Node {
//...
};
// define ll methods
If you do want the user to know about the Node type, one solution is to make its members private, create a public value accessor method, and make the Node a friend of the SinglyLinkedList class.