I have constructed a copy constructor for an ADT queue. The copy constructor works fine. I would want to improve my code, but I don't really know how to shorten it to make it more flexible. Code is given below:
template <typename T>
Queue <T>::Queue(const Queue & other)
{
if (other.first == nullptr)
{
first = nullptr;
nrOfElements = 0;
}
else
{
Node* saveFirst;
Node* walker;
first = other.first;
walker = new Node(first->data);
saveFirst = walker;
while (first->next != nullptr)
{
walker->next = new Node(first->next->data);
walker = walker->next;
first = first->next;
}
walker->next = nullptr;
first = saveFirst;
}
this->nrOfElements = other.nrOfElements;
}
The class Queue also contains an inner private Node class which contains the pointers first, next, etc:
private:
int nrOfElements;
class Node
{
public:
Node* next;
T data;
Node(T data)
{
this->data = data;
}
};
Node* first;
So, I would appreciate any suggestions/examples of how the copy constructor code above could be improved, as I'm a bit lost on the task.
try this one
may be in easier than your code
template <typename T>
Queue <T>::Queue(const Queue & other)
{
Front =NULL;Rear=NULL;Count=0;
Node *p= other.Front;
while(p!=NULL){
EnQueue(p->GetData());
Rear->SetData(p->GetData());
p=p->next;
}
}
First we set NULL value to Front & Rear and 0 for Count second make new Node to get the data from every Node in old Queue and copy it to the new Queue.
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The task was to create a copy constructor for a Forward_list class, and this is the code I have tried. But it keeps showing an error:
terminate called after throwing an instance of 'int'
I have been editing and changing, but I can't seem to get past that error.
class Forward_list
{
public:
class Node
{
public:
// A node will hold data of type T
T data{};
// next will point to the next node in the list
// we initialise next to nullptr
Node* next = nullptr;
// Because we have already intialised the variables
// the default constructor doesn't need to do anything
Node(){}
// if the constructor is called with just one argument.
Node(T input_data, Node* next_node= nullptr)
{
data = input_data;
next = next_node;
}
// Destructor
~Node(){}
};
private:
// private member variables for Forward_list
unsigned size_ = 0;
Node* head_ = nullptr;
public:
Forward_list(const Forward_list<T>& other);
template <typename T>
Forward_list<T>::Forward_list(const Forward_list& other) {
head_ = nullptr;
Node *prev_node = nullptr;
for(Node *other = head_; other != nullptr; other = other->next) {
Node *new_node = new Node;
new_node->data = other->data;
new_node->next = nullptr;
if (!head_)
head_ = new_node;
else
prev_node->next = new_node;
prev_node = new_node;
}
}
Nothing in this code throws an int. So it has to be in code you have not shown. Perhaps in the constructor of T?
Not that it matters, because your copy constructor does not even run its loop at all, since you are looping through the wrong list. You are starting your loop with this->head_ when you need to start it with other.head_ instead.
Also, the Node converting constructor should take its input_data parameter by const reference, and use a member initialization list, to avoid having to default-construct and then assign the data member in separate operations.
Also, the Forward_list copy constructor can be simplified.
Also, make sure you are following the Rule of 3/5/0, if you are not already doing so (you did not show that code).
Try something more like this:
class Forward_list
{
public:
class Node
{
public:
T data;
Node* next;
Node(const T &input_data, Node* next_node = nullptr);
};
Forward_list() = default;
Forward_list(const Forward_list<T>& other);
Forward_list(Forward_list<T>&& other);
~Forward_list();
Forward_list& operator=(Forward_list<T> other);
...
private:
// private member variables for Forward_list
unsigned size_ = 0;
Node* head_ = nullptr;
};
template <typename T>
Forward_list<T>::Node(const T &input_data, Node* next_node)
: data(input_data), next(next_node)
{
}
template <typename T>
Forward_list<T>::Forward_list(const Forward_list& other) {
Node **new_node = &head_;
for(Node *other_node = other.head_; other_node != nullptr; other_node = other_node->next) {
*new_node = new Node(other->data);
++size_;
new_node = &((*new_node)->next);
}
}
template <typename T>
Forward_list<T>::Forward_list(Forward_list&& other) :
head_(std::exchange(other.head_, nullptr)),
size_(std::exchange(other.size_, 0))
{
}
template <typename T>
Forward_list<T>::~Forward_list() {
Node *node = head_;
while (node) {
Node *next = node->next;
delete node;
node = next;
}
}
template <typename T>
Forward_list<T>& Forward_list<T>::operator=(Forward_list other)
{
std::swap(head_, other.head_);
std::swap(size_, other.size_);
return *this;
}
i need to code a linked list for university in c++, mostly to practice coding iterators.
I tested it with some basic cases and it works but after i pass it in valgrind and the test server for the program i get a list of different errors. Maybe somebody can help me not to despair.
(At the end i will append the error list)
template <typename T = float>
class ForwardList
{
struct Node
{
/// Constructs a Node from a data value and a link to the next element.
Node(const T &data, Node *next) : data{data}, next{next} {}
/// A Node owns all nodes after it, so it deletes them on destruction
~Node() { delete next; }
//Performs a deep copy of the Node and all Nodes after it. Bad practice but we got it like that
Node *clone() const
{
if (next == nullptr)
{
return new Node{data, nullptr};
}
else
{
return new Node{data, next->clone()};
}
}
T data;
Node *next;
};
public:
ForwardList() : head(nullptr) {}
/// Copy constructor performs a deep copy of the other list's Nodes
ForwardList(const ForwardList &other)
{
head = other.head->clone();
}
/// Destructor makes sure that all Nodes are correctly destroyed
~ForwardList()
{
while (head->next != nullptr)
{
Node *tmp = head;
head = head->next;
delete tmp;
}
delete head;
}
/// Copy assignment operator uses the copy-and-swap idiom to make a safe
/// assignment
ForwardList &operator=(ForwardList other)
{
swap(*this, other);
return *this;
}
/// Add an element to the front of the list.
void push_front(const T &value)
{
std::cout << "Num: " << numberOfNodes << std::endl;
Node *item = new Node(value, nullptr);
if (head==nullptr)
{
head = item;
}else
{
item->next=head;
head = item;
}
numberOfNodes++;
}
/// Remove the first element of the list. Calling this function on an empty
/// list is undefined behavior. When implementing this function, be careful
/// to delete the one and only the one element that is removed.
void pop_front()
{
Node *item;
item = head->next;
delete head;
head = item;
numberOfNodes--;
}
/// Get a reference to the first element of the list
/// (const and non-const version)
T &front()
{
return head->data;
}
const T &front() const
{
return head->data;
}
/// Return true is the list is empty
bool empty() const
{
return numberOfNodes == 0 ? true : false;
}
std::size_t size() const
{
return numberOfNodes;
}
friend void swap(ForwardList &l, ForwardList &r)
{
Node *tmp = l.head;
l.head = r.head;
r.head = tmp;
}
private:
Node *head;
size_t numberOfNodes = 0;
};
And now the fun part (i will put it on pastebin because its pretty long):
https://pastebin.com/4JAKkJtP
Your issue is that ~Node tries to delete its next, and you also try to walk the list in ~ForwardList. By deleting ~Node(), you let ForwardList handle cleanup and everything works.
The clue here is that valgrind reported use after free, meaning something was deleting a pointer twice. That was a clue to look at everything that deletes a Node* (or really, delete in general).
I read some of the other posts on this topic because there were quite a few, but they didn't really help my situation.
I am getting memory leaks in my implementation of a doubly linked list. I have to make my own so using list is not an option.
here are the two push functions I am using...
template <class T>
void dllist<T>::push_front(T val) {
node* new_node = new node;
new_node->value = val;
new_node->forward = head;
new_node->backward = nullptr;
if (head != nullptr)
head->backward = new_node;
head = new_node;
}
and...
template <class T>
void dllist<T>::push_back(T val) {
node* new_node = new node;
new_node->value = val;
new_node->forward = nullptr;
if (!head)
head = new_node;
else {
node* traveller = head;
while (traveller->forward != nullptr)
traveller = traveller->forward;
traveller->forward = new_node;
new_node->backward = traveller;
}
}
finally, here is my destructor
template <class T>
dllist<T>::~dllist() {
node* current = head;
while (current != nullptr) {
node* forward = current->forward;
delete current;
current = forward;
}
}
In main, I declare an object of type dllist called mylist and I make a few calls to push_front with some integer values and then push_back.
I am using the CRT library to check for leaks and there is a leak at each call to push_back or push_front.
I am confused because I thought I made my destructor correctly. Is there something else Im not seeing?
If anyone could point me in the right direction I'd appreciate it!
Thanks.
MRE
template<class T>
class dllist {
struct node {
T value;
node* forward;
node* backward;
};
node* head;
public:
dllist(); // default constructor
~dllist(); // default destructor
void push_front(T); // push element to the front of the list
void push_back(T); // push element to the back of the list
};
int main() {
{
dllist<int> mylist;
mylist.push_front(10);
mylist.push_front(12);
mylist.push_front(14);
mylist.push_front(16);
mylist.push_front(18);
mylist.push_front(19);
mylist.push_back(11);
mylist.push_back(21);
mylist.push_back(31);
mylist.push_back(41);
mylist.push_back(31);
mylist.push_back(41);
mylist.push_back(222);
}
_CrtDumpMemoryLeaks();
return 0;
}
template <class T>
dllist<T>::dllist() {
head = nullptr;
}
I don't fully understand the concept of templates and am trying to get some help on how to implement one on my linked list below. I'm trying to get my code to be able to support the following types : List< List<std::string> > List<std::string> List<int>. I was wondering if there was any way someone could give me an example of how to convert these items into templates in addition to trying to explain what is happening? I'm new to c++ so any help I can get would be appreciated.
#include <string>
#include <iostream>
#include <cstddef>
using Item = std::string;
// TURN DList into a template!
class DList {
private:
class DListNode {
public:
Item item;
DListNode * next;
DListNode * prev;
DListNode(Item i, DListNode *n=nullptr, DListNode *p=nullptr) {
item = i;
next = n;
prev = p;
}
};
DListNode * head;
DListNode * tail;
public:
class iterator {
DListNode *node;
public:
iterator(DListNode *n = nullptr) {
node = n;
}
Item& getItem() { return node->item; }
void next() { node = node->next; }
void prev() { node = node->prev; }
bool end() { return node==nullptr; }
friend class DList;
};
public:
DList() {
// list is empty
head = nullptr;
tail = nullptr;
}
bool empty() {
return head==nullptr;
}
void append(Item a) {
DListNode *node = new DListNode(a,nullptr,tail);
if ( head == nullptr ) {
// empty list
head = node;
tail = node;
} else {
tail->next = node;
tail = node;
}
}
void insertAfter(iterator it, Item item)
{
if(head == nullptr || it.node == nullptr) { // NULL iterator means insert at head
DListNode *node = new DListNode(item,head); // next=head, prev=NULL
if ( head == nullptr) // same as zyBook
head = tail = node;
else { // if inserting before head, it.node==NULL
head->prev = node;
head = node;
}
} else if (it.node == tail) {
DListNode *node = new DListNode(item,nullptr,tail); // next=NULL, prev=old tail
tail->next = node;
tail = node;
} else {
DListNode *node = new DListNode(item,it.node->next,it.node);
it.node->next = node;
node->next->prev = node;
}
}
void erase (iterator it) {
DListNode *succ = it.node->next; // successor node
DListNode *pred = it.node->prev; // predecessor node
if (succ != NULL)
succ->prev = pred;
if (pred != NULL)
pred->next = succ;
if (it.node == head)
head = succ; // head is following node
if (it.node == tail)
tail = pred; // tail is previous node
delete it.node; // delete the node; not shown in zyBook, but necessary in C/C++
// iterator is now invalid, caller should not use it again
}
iterator begin() {
return iterator(head);
}
iterator reverse_begin() {
return iterator(tail);
}
};
template <typename Item>
std::ostream& operator << (std::ostream& out, DList<Item> &l)
{
out << "{";
auto it = l.begin();
out << it.getItem();
it.next();
for(; !it.end(); it.next())
{
out << ", " << it.getItem();
}
out << "}" << std::endl;
return out;
}
int main()
{
{
DList<std::string> l;
l.append("eggs");
l.append("milk");
l.append("bread");
std::cout << l;
}
{
DList<int> l;
l.append(1);
l.append(2);
l.append(3);
std::cout << l;
}
return 0;
}
Actually, you almost have all you need, but you are still using a regualar class with a concrete type.
using Item = std::string;
class DList { ... };
So first we drop the concrete type:
// using Item = std::string;
class DList { ... }; // sure Item is now undefined...
Then we tell the class to be a template
template <typename Item>
class DList { ... };
Now Item got re-introduced, but instead of being a concrete type, it's now a generic one. That's it, you have a template list (assuming the list is implemented correctly, I didn't check).
Whenever you now instantiate your list:
DList<int>;
DList<std::string>;
// ...
You create a totally new, independent data type (which means especially, that you cannot assign a DList<int> to a pointer to DList<double>, just all alike as you cannot assign a int to a pointer to double either).
When you instantiate a template, every occurence of a template parameter will be replaced with the type you instantiated the template with, e. g. in DList<int>, every occurence of Item will be replaced with int.
Well, all this is just a very short introduction, there's quite a lot to follow yet, but that's rather to be handled in book than in an answer on stackoverflow...
Some notes to your node's constructor, though:
DListNode(Item i /* , ... */) { item = i; }
At very first, you should get used to using constructor's initialiser list (not to be confused with std::initializer_list):
DListNode(Item i /* , ... */) : item(i) { }
You avoid default initiasation + assignment in favour of direct initialisation by value. Additionally, some types (non-default constructible ones, const members and references) only can be initialised that way.
Then you are producing an unnecessary copy:
DListNode(Item i /* , ... */) : item(i) { }
// ^ temporary copy ^ final copy, created from temporary
You avoid that copy, if you accept the item by reference:
DListNode(Item const& i /* , ... */) : item(i) { }
// now copies from reference, one copy less
You can additionally provide move semantics:
DListNode(Item&& i /* , ... */) : item(std::move(i)) { }
so that objects you don't need outside the list any more can be moved into (well, actually their contents). In some cases, this can be much cheaper than a full copy...
All said about the constructor (apart from the initialiser list) applies to the append and insertAfter functions as well.
Initialiser lists and avoiding copies is general advice, unrelated to templates...
I'm new to C++ and have been trying to implement a Singly Linked List, that provides an implementation the destructor, copy constructor and assignment operator. I'm running into compilation issues when trying to implement the copy constructor and the assignment operator.
Here's node.hpp
#ifndef LINKED_LIST_NODE_HPP
#define LINKED_LIST_NODE_HPP
template <typename T>
class Node{
public:
T data;
Node* next;
Node();
Node(T);
Node(const Node&);
~Node();
};
template <typename T>
Node<T>::Node(){}
template <typename T>
Node<T>:: Node(const T data): data(data), next(nullptr){}
template <typename T>
Node<T>::Node(const Node<T>& source) : data(source.data),
next(new Node)
{
(*next) = *(source.next) ;
}
template <typename T>
Node<T>::~Node(){}
#endif //LINKED_LIST_NODE_HPP
This is singly_linked_list.hpp
#ifndef LINKED_LIST_SINGLYLINKEDLIST_HPP
#define LINKED_LIST_SINGLYLINKEDLIST_HPP
#include <iostream>
#include "node.hpp"
template <typename T>
class SinglyLinkedList {
private:
Node<T>* head;
std::size_t count;
public:
SinglyLinkedList();
SinglyLinkedList(const SinglyLinkedList& source);
SinglyLinkedList& operator=(const SinglyLinkedList& source);
~SinglyLinkedList();
void insert(T);
void remove(T);
bool isEmpty();
int length();
void print();
};
template <typename T>
SinglyLinkedList<T>::SinglyLinkedList() : head(nullptr), count(0){}
template <typename T>
template <typename T>
SinglyLinkedList<T>::SinglyLinkedList(const SinglyLinkedList& source){
Node<T>* curr = source.head;
while(curr != nullptr){
Node<T>* p = new Node<T>;
p->data = curr->data;
curr = curr->next;
}
}
//template <typename T>
//SinglyLinkedList<T>::SinglyLinkedList& operator=(const SinglyLinkedList<T>& source){
// //not sure how to implment this.
//}
template <typename T>
SinglyLinkedList<T>::~SinglyLinkedList() {
if(!isEmpty()){
Node<T>* temp = head;
Node<T>* prev = nullptr;
while(temp->next != nullptr){
prev = temp;
temp = temp->next;
delete prev;
}
delete temp;
}
}
template <typename T>
bool SinglyLinkedList<T>::isEmpty() {
return head == nullptr;
}
template <typename T>
void SinglyLinkedList<T>::insert(T item) {
Node<T>* p = new Node<T>(item);
p->next = head;
head = p;
count += 1;
}
template <typename T>
void SinglyLinkedList<T>::remove(T item) {
bool present = false;
if (head->data == item){
Node<T>* temp = head;
head = head->next;
delete(temp);
count -= 1;
return;
}
Node<T>* temp = head;
while (temp->next != nullptr){
if (temp->next->data == item){
Node<T>* removable = temp->next;
temp->next = temp->next->next;
delete(removable);
present = true;
count -= 1;
break;
} else{
temp = temp->next;
}
}
if(!present){
throw std::invalid_argument("item not present in list");
}
}
template <typename T>
int SinglyLinkedList<T>::length() {
return count;
}
template <typename T>
void SinglyLinkedList<T>::print() {
if(isEmpty()){
throw std::invalid_argument("Can't print an empty list!");
}
Node<T>* temp = head;
while(temp != nullptr){
if(temp->next != nullptr){
std::cout<<temp->data;
std::cout<<"->";
}else{
std::cout<<temp->data;
}
temp = temp->next;
}
std::cout<<std::endl;
}
#endif //LINKED_LIST_SINGLYLINKEDLIST_HPP
I've commented out the copy constructor code to make this compile. What is the correct way of doing this? I'm just learning C++.
One issue that introduces complexity is that it is not well defined what the copy constructor of a node should do? Should the next field of the copy point to the next of the original, or it should create a copy of the next and point to that? The former is inadequate and error-prone, the latter would recursively create a copy of the whole list, one node at a time. This will work for lists of small size but will cause stack overflow for lists with many elements due to the depth of the recursive calls.
So to keep things simple, I wouldn't bother with copy constructor of a node.
template <typename T>
class Node {
public:
T data;
Node* next = nullptr;
Node() = default;
Node(const Node&) = delete; // since copying is not well defined, make it impossible to copy a node.
};
Copying a list is a well defined operation, so implementing the copy constructor makes sense. A mistake with your current implementation is that you allocate a new node, only to leak it later (nothing keeps track of the newly allocated node p). What you need looks more like this:
template <typename T>
SinglyLinkedList<T>::SinglyLinkedList(const SinglyLinkedList<T>& source)
: head(nullptr)
, count(0)
{
// deal with the trivial case of empty list
if (source.head == nullptr)
return;
// deal with the case where count >= 1
head = new Node<T>;
head->data = source.head->data;
head->next = nullptr;
count = 1;
Node<T>* lastCopied = source.head; // last node to be copied
Node<T>* lastAdded = head; // last node to be added to the current list
while (lastCopied->next != nullptr)
{
// create new node
Node<T>* p = new Node<T>;
p->data = lastCopied->next->data;
p->next = nullptr;
// link the newly created node to the last of the current list
lastAdded->next = p;
lastAdded = p;
// advance lastCopied
lastCopied = lastCopied->next;
count++;
}
}
Now regarding the assignment operator, luckily you can use the 'copy and swap' idiom that greatly simplifies things.
template <typename T>
SinglyLinkedList<T>& SinglyLinkedList<T>::operator =(SinglyLinkedList<T> source) // note that you pass by value.
{
std::swap(head, source.head);
std::swap(count, source.count);
return *this;
}
My answer would become too long if I tried to explain the copy and swap technique. It is a clever trick to write exception safe code and avoid duplication (implements assignment by using the copy ctor) at the same time. It is worth reading about it here.
Btw, the declaration of your class should look like this
template <typename T>
class SinglyLinkedList
{
private:
Node<T>* head = nullptr;
std::size_t count = 0;
public:
SinglyLinkedList(const SinglyLinkedList& source);
SinglyLinkedList& operator=(SinglyLinkedList source);
// other members here...
};
PS. My code assumes you are using c++11 or a later standard.
I don't like the direction this is headed. I'm going to explain how to do this approach right because it is an excellent lesson on recursion, but because it's recursion it can run the program out of Automatic storage (march off the end of the stack, most likely) with a sufficiently large list. Not cool.
The logic:
Copying a node copies the next node if there is one. This looks something like
template <typename T>
Node<T>::Node(const Node<T>& source) : data(source.data)
{
if (source.next) // if there is a next, clone it
{
next = new Node<T>(*source.next);
}
else
{
next = nullptr;
}
}
This reduces the linked list copy constructor to
template <typename T>
SinglyLinkedList<T>::SinglyLinkedList(const SinglyLinkedList& source){
head = new Node<T>(*source.head); //clone the head. Cloning the head will clone everything after
count = source.count;
}
A helper function may, uh... help here to make the Node copy constructor a bit more idiomatic
template <typename T>
Node<T> * initnext(const Node<T> & source)
{
if (source.next)
{
return new Node<T>(*source.next);
}
else
{
return nullptr;
}
}
template <typename T>
Node<T>::Node(const Node<T>& source) : data(source.data),
next(initnext(source))
{
}
but I don't think you gain much.
So... I don't like the above. What would I do instead? Something a lot like opetroch's solution above, but different enough that I'll write this up.
The node stays brutally stupid. As far as I'm concerned all a Node should ever know is how to store the payload and find other Nodes. This means the linked list should do all of the heavy lifting.
Concept 1: head is nothing but a next pointer. Once you abstract away its differences, unimportant here, you can use it exactly the same way you would next.
Concept 2: If you only know where next points, you have to do a bunch of extra book-keeping to track the previous node to update it's next. But if you take advantage of the previous's next pointing to the current node, you can throw out even more code. By tracking the previous node's next you have all of the information you need.
Concept 3: If you keep a pointer to the previous node's next, you can update that previous node's next any time you want by dereferencing it.
template <typename T>
SinglyLinkedList<T>::SinglyLinkedList(const SinglyLinkedList& obj)
{
Node<T>* tocopy = obj.head;
Node<T>** nextpp = &head; // head is a next. We are now pointing to a pointer to next
while (tocopy) // keep looping until there is no next node to copy
{
*nextpp = new Node<T>(tocopy->data); // copy source and update destination's next
nextpp = &(*nextpp)->next; // advance to point at the next of the node we just added
tocopy= tocopy->next; // get next node to copy
}
count = obj.count;
}
Because this iterates rather than recurses it doesn't eat up Automatic storage (probably the stack) and can keep going until the cows come home.
This logic can also be applied to remove
template <typename T>
void SinglyLinkedList<T>::remove(T item) {
Node<T>** temp = &head; //head is nothing but a next pointer.
// by pointing to where the next is, we don't
// need to track a previous or have special handling
// for the head node
while (*temp){ // because we now have a pointer to a pointer, we need an
// extra dereference
if ((*temp)->data == item){
Node<T>* removable = *temp;
*temp = (*temp)->next;
delete(removable);
count -= 1;
return; // no need for any special magic. Just get out.
} else{
temp = &(*temp)->next; // update the pointer to the next
}
}
// if we got here the node was not found.
throw std::invalid_argument("item not present in list");
}
And following through on head is just a next, we can also gut the destructor:
template <typename T>
SinglyLinkedList<T>::~SinglyLinkedList() {
while(head){ // if head null, list empty
Node<T>* temp = head; // cache so we can delete
head = head->next; // move head
delete temp; //delete removed node
}
}