I am trying to figure out how C++ resource management works, especially in relation to constructors/destructors. This is my test code;
ylist.h
#pragma once
template <class T>
struct ListNode
{
T elem;
struct ListNode* next;
ListNode(T elem):elem(elem),next(nullptr) {}
};
template <class T>
class List
{
ListNode<T> *head;
public:
List() : head(nullptr)
{
}
~List()
{
ListNode<T>* cursor = head;
while (cursor != nullptr)
{
ListNode<T>* next = cursor->next;
delete cursor;
cursor = next;
}
}
void append(T item)
{
ListNode<T>* n = new ListNode<T>(item);
if (head == nullptr)
{
head = n;
}
else
{
ListNode<T>* cursor = head;
while (cursor->next != nullptr)
cursor = cursor->next;
cursor->next = n;
}
}
};
main.cpp
#include <iostream>
#include "ylist.h"
using namespace std;
class myObj
{
int val;
public:
myObj(int val):val(val)
{
cout << "myObj#" << this << "(" << val << ")" << endl;
}
~myObj()
{
cout << "~myObj#" << this << "(" << val << ")" << endl;
}
};
int main()
{
List<myObj> myList;
for (int i = 0; i < 3; i++)
{
myList.append(myObj(i));
}
}
And here is the output;
myObj#00000039614FFAC0(0)
~myObj#00000039614FFAD0(0)
~myObj#00000039614FFAC8(0)
~myObj#00000039614FFAC0(0)
myObj#00000039614FFAC0(1)
~myObj#00000039614FFAD0(1)
~myObj#00000039614FFAC8(1)
~myObj#00000039614FFAC0(1)
myObj#00000039614FFAC0(2)
~myObj#00000039614FFAD0(2)
~myObj#00000039614FFAC8(2)
~myObj#00000039614FFAC0(2)
~myObj#0000019878DF6100(0)
~myObj#0000019878DF5F20(1)
~myObj#0000019878DF6200(2)
According to above output, constructor called for each object once, but destructors called four times.
I was expecting to see multiple temporary copies being constructed/destructed but why is number of constructors/destructos is not matching.
If, for example, I was opening files or db connections on constructors and closing them on destructors, would I get in trouble in above scenario?
Add this copy constructor to your myObj function in order to see how the additional myObj objects are being constructed. Once you have this, you'll see that the number of constructor-calls matches the number of destructor-calls.
myObj(const myObj & rhs):val(rhs.val)
{
cout << "COPY CTOR myObj#" << this << "(" << val << ")" << endl;
}
Related
I have this homework for my C++ class. I have to implement a list class, where I can add from the front or the back. I have implemented it, but there is a big problem.
collectiontest.cpp
#include "list.hpp"
#include <iostream>
int main(void) {
std::cout << "--- Test List ---" << std::endl;
List<int> l;
std::cout << l;
l.add(2);
l.add_front(3);
l.add(1);
std::cout << l;
std::cout << "remove_front: " << l.remove_front() << std::endl;
std::cout << "remove_front: " << l.remove_front() << std::endl;
l.add(5);
std::cout << l;
std::cout << "Contains 5? " << l.contains(5) << std::endl;
std::cout << "remove_back: " << std::endl;
std::cout << l.remove() << std::endl;
std::cout << "remove_back: " << l.remove() << std::endl;
std::cout << l;
return 0;
}
list.hpp
#pragma once
#include "collection.hpp"
#include "node.hpp"
#include <iostream>
#include <string>
template <typename T>
class List : public Collection<T> {
private:
Node<T>* first;
Node<T>* last;
public:
~List() {
if (first != nullptr) {
}
}
void add_front(T value) {
if (first == nullptr) {
std::cout << "First element front" << std::endl;
Node<T> temp(value, nullptr, nullptr);
first = &temp;
last = &temp;
}
else {
if (first == last) {
std::cout << "First and last are same! front-> " << first << "-" << last << std::endl;
Node<T> temp(value, last, nullptr);
last->previous = &temp;
first = &temp;
std::cout << first << "-" << last << std::endl;
}
else {
Node<T> temp(value, first, nullptr);
first->previous = &temp;
first = &temp;
}
}
}
void add(T value) override {
std::cout << "LOL" << std::endl;
if (first == nullptr) {
std::cout << "First element add" << std::endl;
Node<T> temp(value, nullptr, nullptr);
first = &temp;
last = first;
std::cout << "end of add first item" << std::endl;
}
else {
if (first == last) {
std::cout << "First and last are same! add-> " << first << "-" << last << std::endl;
Node<T> temp(value, nullptr, first);
first->next = &temp;
last = &temp;
}
else {
Node<T> temp(value, nullptr, last);
last->next = &temp;
last = &temp;
}
}
}
T remove_front() {
if (first != nullptr) {
T te = first->content;
Node<T>* temp = first;
first = first->next;
return te;
}
return NULL;
}
T remove() override {
if (last != nullptr) {
Node<T>* temp = last;
T te = last->content;
last = last->previous;
last->next = nullptr;
return te;
}
return NULL;
}
bool isEmpty() override {
return (first == nullptr);
}
bool contains(T value) override {
Node<T>* temp = first;
while (temp != nullptr) {
if (temp->content == value) return true;
temp = temp->next;
}
return false;
}
void clear() override {
Node<T>* temp = first->next;
while (temp != nullptr) {
first = temp;
temp = temp->next;
}
}
int getSize() override {
int counter = 0;
Node<T>* temp = first;
while (temp != nullptr) {
counter++;
temp = temp->next;
}
return counter;
}
Node<int>* getFirst() const {
return first;
}
Node<int>* getLast() const {
return last;
}
friend std::ostream& operator<<(std::ostream& os, const List<int>& l);
};
std::ostream& operator<<(std::ostream& stream, const List<int>& l) {
std::cout << "Output called" << std::endl;
Node<int>* temp = l.getLast();
while (temp != nullptr) {
std::cout << "Schleife betreten" << std::endl;
stream << temp->content << " ";
temp = temp->previous;
}
stream << std::endl;
return stream;
}
collection.hpp
#pragma once
template <typename T>
class Collection {
virtual void add(T value) = 0;
virtual int remove() = 0;
virtual bool isEmpty() = 0;
virtual bool contains(T obj) = 0;
virtual void clear() = 0;
virtual int getSize() = 0;
};
node.hpp
#pragma once
template <typename T>
class Node {
public:
Node<T>* next;
Node<T>* previous;
T content;
Node(T value, Node<T>* next, Node<T>* previous) {
content = value;
this->next = next;
this->previous = previous;
}
~Node() {
}
};
I have looked over it several times, but I get the same error.
I debugged my program with Visual Studio, and what happens is collectortest.cpp creates the list and adds the first value with the add() function. In the debugger, I see how the temp node is created, how first is set to the address of temp, and how last is set to the same address. So, when we reach the line where "end of add first item" is supposed to be printed to the console, the variables changes, why??
You can see in this image how the variables all have appropriate values:
Before the output:
But then, when I leave the breakpoint I set at "end of add first item", this happens:
After the output:
There must be something I don't see, because this makes literally no sense. I have looked everywhere, maybe it is just something really dumb, but I am really at my limits here.
Can one of you help and explain what the problem is?
I tried to execute my program and create the nodes appropriately, but I get a strange error in my memory.
From #Wyck:
Node<T> temp(value, nullptr, nullptr);
first = &temp;
You're using a pointer to something that is allocated on the stack and will be destroyed/invalid when it goes out of scope whether you have taken note of the address it used to be at or not.
From #user4581301:
Node<T> temp(value, nullptr, nullptr);
first = &temp;
is one of the increasingly rare times you want to use new.
first = new Node<T>(value, nullptr, nullptr);
From #TedLyngmo:
For the problem #Wyck mentioned, you need to allocate and release memory dynamically using new/delete.
These were the answers posted as comments to the question. My Nodes always got destroyed because they were out of scope. I needed to use new so they got stored in the heap and weren't automatically destroyed if they went out of scope.
Edit: I can't choose my own answer as correct for 2 days, so the question sadly stays open.
I'm trying to implement a templated singly linked list and I'm fairly new to C++
#include <iostream>
#include <string>
#define NEWL "\n"
#define PRINT(s) std::cout << s
#define PRINTL(s) std::cout << s << NEWL
#define PRINTERR(e) std::cerr << e << NEWL
////// Class for a Node
template<class Data> class Node {
Node<Data>* next_ptr;
Data data;
public:
Node(Node<Data>* nxt_ptr) :next_ptr(nxt_ptr) {};
Node(Data d, Node<Data>* nxt_ptr) :data(d), next_ptr(nxt_ptr) {};
Node<Data>* get_next() { return next_ptr; }
Data& get_data() { return data; }
friend std::ostream& operator<<(std::ostream& out, const Node<Data>& node) {
out << node.data;
return out;
};
};
////// Class for a SinglyLinkedList
template<class Data> class SLinkedList {
Node<Data>* head_ptr;
int max_size;
public:
SLinkedList() : head_ptr(nullptr) {};
bool is_empty() {
return head_ptr == nullptr;
};
bool is_full() {
return get_size() == max_size;
};
int get_size() {
if (is_empty()) {
return 0;
}
int count = 0;
for (Node<Data>* it_ptr = head_ptr; it_ptr != nullptr; it_ptr = it_ptr->get_next()) {
count++;
}
return count;
};
void add(Data d) {
if (is_full()) {
throw std::exception("List is full!");
}
Node<Data> new_node(d, head_ptr);
head_ptr = &new_node;
};
void print_content() {
int count = 1;
PRINTL("This list contains:");
for (Node<Data>* it_ptr = head_ptr; it_ptr != nullptr; it_ptr = it_ptr->get_next()) {
PRINTL("\t["<< count << "]" << " at " << it_ptr << " : " << *it_ptr);
count++;
}
}
};
////// Main function
int main()
{
SLinkedList<int> sll;
sll.add(42);
sll.print_content();
}
I can't get this to work. Somehow iterating the list with for-loops does not work. It always results in an Reading Access Violation Exception about a pointer to 0xCCCCCCD0 and I have no idea how to fix this.
Your add function is incorrect
Node<Data> new_node(d, head_ptr);
creates a new function local Node in add. You then set head to the address of that local variable. When the function ends all local variables are destroyed so now head points to an object that no longer exists.
To fix that you need to use the new keyword to create a dynamic object that will live on after the function ends.
Node<Data>* new_node = new Node(d, head_ptr);
head_ptr = new_node;
The down side with this is you need to remember to call delete on all of the nodes you created in the list destructor.
You also have some other bugs in your code. You never set max_size in your constructor so using it at all except to give it a value is undefined behavior as we have no idea what the value of it is going to be. You also never increase the size of the list when you add nodes into the list.
For my current programming assignment I have to construct a binary search tree and use it to insert words from a file in alphabetical order, to make a concordance list. The program is complete and outputs correctly; the main bulk of the program works perfectly. But when the program finishes and destructors are called, it crashes with error in ./concordancebst : free(): invalid pointer. I'm at a loss as to what the problem is, my destructor looks like it would work and looking at code online it seems as though it should work as well. I could probably get around this by using a std::unique_ptr but it seems a bit overkill in the sense of the scope of the program (also overkill in the sense that we haven't covered smart pointers, everything we have done in the class involving dynamic memory has been handled through manual allocating/deallocating).
BST.h
#ifndef BST_H
#define BST_H
#include <string>
class BST {
public:
BST() { root = nullptr; }
~BST();
void insert(const std::string word);
int getCount(const std::string word);
int length();
friend std::ostream& operator << (std::ostream &out_s, BST b);
private:
struct Node {
std::string word;
int count;
Node *left;
Node *right;
Node() { word = ""; count = 0; left = nullptr; right = nullptr;}
~Node();
};
Node *root;
void RInsert(Node* &t, std::string word);
int countNodes(Node *p);
void print(Node *p, std::ostream &out_s);
};
#endif
BST.cpp
#include "BST.h"
#include <string>
#include <iostream>
#include <iomanip>
BST::Node::~Node() {
delete left;
delete right;
}
BST::~BST() {
delete root;
}
int BST::countNodes(Node *p) {
if(p == nullptr)
return 0;
else
return countNodes(p->left) + countNodes(p->right) + 1;
}
int BST::getCount(const std::string word) {
Node *p = root;
while(true) {
if(p == nullptr)
return 0;
else if(word.compare(p->word) < 0)
p = p->left;
else if(word.compare(p->word) == 0)
return p->count;
else
p = p->right;
}
}
int BST::length() {
return countNodes(root);
}
void BST::RInsert(Node* &t, std::string word) {
if(t == nullptr) {
t = new Node;
t->word = word;
t->left = nullptr;
t->right = nullptr;
t->count++;
}
else if(word.compare(t->word) == 0)
t->count++;
else if(word.compare(t->word) < 0)
RInsert(t->left, word);
else //word.compare(t->word > 0)
RInsert(t->right, word);
}
void BST::insert(const std::string word) {
RInsert(root, word);
}
void BST::print(Node *p, std::ostream &out_s) {
if(p != nullptr) {
print(p->left, out_s);
out_s << std::setw(13) << p->word << std::setw(10) << p->count << std::endl;
print(p->right, out_s);
}
}
std::ostream &operator << (std::ostream &out_s, BST b) {
out_s << std::setw(13) << "Word" << std::setw(10) << "Count" << std::endl;
out_s << "---------------------------------------------" << std::endl;
b.print(b.root, out_s);
out_s << "---------------------------------------------" << std::endl;
out_s << "The file contains " << b.length() << " distinct words." << std::endl;
return out_s;
}
std::ostream &operator << (std::ostream &out_s, BST b)
Pass by value^. b is copied.
Since BST has no copy constructor, the default copy constructor is invoked and does not perform a deep copy. b contains copies of the source BST's pointers, and when b is destroyed on return from the function, it takes all of the source's Nodes with it to the grave.
Fixes are twofold:
Most directly, pass by reference.
std::ostream &operator << (std::ostream &out_s, BST & b)
Indirectly, this code violates the Rule of Three. BST and BST::Node need copy constructors and assignment operators to be used safely.
Edit
A minimal test case would be something like:
#include <iostream>
#include "BST.h"
int main()
{
BST t;
t.insert("A");
std::cout << t << std::endl;
return 0;
}
How can I access elements from my Node pointer array? The cout at the bottom returns an address "0xb7738ff4". Even if I make a constructor and set each element to NULL, I cannot modify any of them later on.
#include <iostream>
using namespace std;
class Node
{
public:
char character;
};
class Tree
{
public:
Node* nodes[26];
};
int main() {
Tree t;
//t.nodes[0]->character = 'a';
cout << "\"" << (t.nodes[0]) << "\"";
}
http://ideone.com/XvLme9
t.nodes[0] is returning a Node* pointer. You are passing that pointer as-is to cout, that is why it is printing a memory address (and a random one at that, because you are not initializing the array). If you want to print the character of a node, you have to dereference the Node* pointer, exactly like your commented out code is doing (which is the correct way to do it):
t.nodes[0]->character
You just have to make sure that nodes[0] returns a valid pointer to a real Node object to begin with:
Tree t;
t.nodes[0] = new Node; // <-- here
t.nodes[0]->character = 'a';
std::cout << "\"" << t.nodes[0]->character << "\"" << std::endl;
Don't forget to delete the node when you are done using it. Tree should have a destructor that frees the nodes it owns.
Try something more like this:
#include <iostream>
#include <stdexcept>
class Node
{
public:
char character;
Node(char c = 0);
};
class Tree
{
private:
Node* nodes[26];
int count;
public:
Tree();
~Tree();
Node* add(char c);
Node* get(int idx);
};
Node::Node(char c)
: character(c)
{
}
Tree::Tree()
: count(0)
{
for (int i = 0; i < 26; ++i)
nodes[i] = NULL;
}
Tree::~Tree()
{
for (int i = 0; i < count; ++i)
delete nodes[i];
}
Node* Tree::add(char c)
{
if (count == 26)
throw std::runtime_error("nodes array is at its max capacity");
Node *node = new Node(c);
nodes[count++] = node;
return node;
}
Node* Tree::get(int idx)
{
if ((idx < 0) || (idx >= count))
throw std::out_of_range("invalid index");
return nodes[idx];
}
int main()
{
Tree t;
t.add('a');
std::cout << "\"" << t.get(0)->character << "\"" << std::endl;
}
With that said, you should use std::list or std::forward_list instead of writing your own tree class:
#include <list>
int main()
{
std::list<char> l;
l.push_back('a');
std::cout << "\"" << l.front() << "\"" << std::endl;
}
Or:
#include <list>
class Node
{
public:
char character;
// other things here...
Node(char c = 0);
Node(const Node &src);
Node& operator=(const Node &rhs);
};
Node::Node(char c)
: character(c)
{
}
Node::Node(const Node &src)
: character(src.character)
{
}
Node& Node::operator=(const Node &rhs)
{
character = src.character;
return *this;
}
int main()
{
std::list<Node> l;
l.push_back('a');
std::cout << "\"" << l.front().character << "\"" << std::endl;
}
Hi
I have some issue regarding constructor and destructor. I have list class, which has two inner classes, one private class for the list nodes, and one public iterator class.
Now for the issue, I have written a non-member print function which uses the inner iterator class. When i use this non-member function it will end calling the destructor for the iterator. It doesn't end here though because for some reason it will also call for the list class's destructor. Which causes some problem when I want to print the list content again.
I don't understand why it call the list class destructor as well and wonder if someone kindly can tell me that, and how I should fix it.
I have attached all the code related to the problem
Main
#include <iostream>
#include "sorted_list.h"
#include "iterator.h"
using namespace std;
void list_print(ostream& os, sorted_list list)
{
sorted_list::iteratorn it(&list);
while( ! it.iterator_end())
{
os << "key = " << setw(3) << it.iterator_get_key() << ", "
<< "value = " << setw(5) << it.iterator_get_value() << endl;
it.iterator_next();
}
os << endl;
}
int main()
{
sorted_list a;
a.insert(4,4);
a.insert(5,5);
list_print(cout,a);
list_print(cout,a);
}
sorted_list.cc
#include "sorted_list.h"
sorted_list::sorted_list()
{
cout << "construct sorted_list" << endl;
this->first = 0;
}
sorted_list::~sorted_list()
{
cout << "destruct sorted_list" << endl;
destroy(this->first);
}
void sorted_list::destroy(list_link* item)
{
cout << "destroy list_link" << endl;
if(item)
{
destroy(item->next);
delete item;
}
}
void sorted_list::insert(int key, double value)
{
list_link *curr;
list_link *prev = 0;
curr = first;
while(curr)
{
if(value < curr->value)
break;
prev = curr;
curr = curr->next;
}
if(this->first == 0 || prev == 0) //if empty or add first
{
//first = create(key, value, this->first);
first = new list_link(key, value, this->first);
}
else if(curr == 0)
{
//prev->next = create(key, value, 0);
prev->next = new list_link(key, value, 0);
}
else
{
//prev->next = create(key, value, curr);
prev->next = new list_link(key, value, curr);
}
}
void sorted_list::remove(my_key_type key)
{
list_link *curr = first;;
list_link *prev = 0;
while(curr)
{
if(curr->key == key)
{
list_link *remove;
if(prev == 0)
{
first = curr->next;
delete curr;
curr = first;
}
else
{
remove = curr;
curr = curr->next;
prev->next = curr;
delete remove;
}
continue;
}
prev = curr;
curr = curr->next;
}
}
sorted_list::list_link* sorted_list::clone(list_link* item)
{
list_link* copyItem= new list_link(item->key,item->value,0);
if(item->next!= 0)
copyItem->next=clone(item->next);
return copyItem;
// ADD YOUR CODE HERE ( 4 well formatted lines in reference solution )
}
void sorted_list::copy(sorted_list* my_this_destination)
{
if (my_this_destination->first == 0) // copy if empty
{
cout << "Copy" << endl;
//list_destroy(my_this_destination);
my_this_destination->first = clone(first);
}
}
double sorted_list::find(int key)
{
list_link *travel = this->first;
while(travel)
{
cout << travel->key << "==" << key << endl;
if(travel->key == key)
return travel->key;
travel = travel->next;
}
return -1;
}
int sorted_list::size()
{
list_link *travel = this->first;
int i = 0;
while( travel )
{
travel = travel->next;
i++;
}
return i;
}
sorted_list.h
#ifndef _SORTED_LIST_H_
#define _SORTED_LIST_H_
#include <iostream>
#include <iomanip>
using namespace std;
typedef int my_key_type;
typedef double my_value_type;
class sorted_list
{
public:
sorted_list();
~sorted_list();
void insert(int key, double value);
void remove(my_key_type key);
void copy(sorted_list* my_this_destination);
void destroy();
void init(struct my_list* my_this);
void print();
void print2();
double find(int key);
int size();
private:
class list_link // An inner class inside sorted_list
{
public:
list_link (my_key_type key, my_value_type value, list_link* next = 0);
~list_link();
my_key_type key;
my_value_type value;
list_link *next;
};
list_link* first;
list_link* clone(list_link* item);
void destroy(list_link* item);
// More declarations
public:
class iteratorn
{
public:
iteratorn();
~iteratorn();
iteratorn(sorted_list *item);
list_link* list_begin();
bool iterator_end();
void iterator_next();
int iterator_get_key();
double iterator_get_value();
private:
sorted_list::list_link* current;
};
};
#endif
iteratorn.cc
#include "iterator.h"
#include "sorted_list.h"
sorted_list::iteratorn::iteratorn()
{
}
sorted_list::iteratorn::iteratorn(sorted_list *list)
{
cout << "construct iteratorn" << endl;
this->current = list->first;
}
sorted_list::iteratorn::~iteratorn()
{
cout << "destruct iteratorn" << endl;
}
sorted_list::list_link* sorted_list::iteratorn::list_begin()
{
return current;
}
void sorted_list::iteratorn::iterator_next()
{
current = current->next;
}
int sorted_list::iteratorn::iterator_get_key()
{
return current->key;
}
double sorted_list::iteratorn::iterator_get_value()
{
return current->value;
}
list_link.cc
#include "sorted_list.h"
sorted_list::list_link::list_link(my_key_type key, my_value_type value, list_link* next)
{
this->key = key;
this->value = value;
this->next = next;
}
sorted_list::list_link::~list_link()
{
cout << "list_link destructor" << endl;
}
Your function void list_print(ostream& os, sorted_list list) takes a sorted_list parameter by copy. A quick and dirty fix (that you should do anyways for performance reasons) is the following:
void list_print(ostream& os, const sorted_list& list)
Now, your iteratornclass takes a mutable list, so this won't work as you expect. You will have quite a few methods to change to make this work.
In any case, your real problem is the lack of a proper copy-constructor. Right now, when you "copy" a list, both end up sharing the same elements, but your destructor is written as if each list owns it's own nodes. Define a proper copy operation and it will solve your problem.
More elaborate help on how to solve the problem: (untested)
Change signature:
void list_print(ostream& os, const sorted_list& list);
Declare + define copy constructor:
sorted_list::sorted_list (const sorted_list& other);
Change iteratorn interface to support a const sorted_list:
class sorted_list::iteratorn
{
public:
iteratorn();
~iteratorn();
iteratorn(const sorted_list& list);
const list_link* list_begin() const;
bool iterator_end() const;
void iterator_next();
int iterator_get_key() const;
double iterator_get_value() const;
private:
// You *should* make this `const` but it is not required.
sorted_list::list_link* current;
};
As you can see, the changes are rather minimal, but need to be applied in various places.
const + non-const iterators:
I applied changes here based on the fact that your iteratorn was currently only defining read-only operations on your sorted_list. If you want to support write access to allow changing the value stored in list nodes (never allow changing the key or you won't have a sorted list anymore), you should define two iterator classes. See the STL iterator interface for more details.
You're copying the list by value, so the local copy in list_print() destructs at end of scope. Pass it by const-reference instead.
This in turn means you will have to change your sorted_list to support working with const lists. In particular you need to have a function that returns a const iterator pointing to the beginning of the list:
sorted_list::const_iteratorn begin() const
{
// returns a const_iteratorn pointing at the beginning of this list
}
Notice you need a new kind of iterator: a const_iteratorn, which promises it won't change the list.
Then, inside print_list() initialize a const_iteratorn with the start iterator that sorted_list returns, by copy:
sorted_list::const_iteratorn s(list.begin());
Finally create a second iterator instance that initializes with an end iterator coming from a member function of sorted_list, similar to the begin() function. This will maintain the const-correctness in print_list().
sorted_list::const_iteratorn e(list.end());
while( s != e ) { // two iterators should be able to compare
// ...
s.iterator_next(); // consider ++s
}
Also, as André mentioned, the fact you don't have a proper copy-constructor and assignment operator is a severe issue. Make sure that copying a sorted_list means copying all its elements, so that the new object owns its own list of elements. Do recall the Rule of Three.