Merging two linked lists throws _CrtlsValidHeapPointer(block) in MSVC - c++

Question

I am teaching myself data structures in C++ and my current challenge is to create, and merge, two linked lists. However, Microsoft Visual Studio throws the following error:
Debug Assertion Failed! Program: LinkedList\Debug\LinkedList.exe File: minkernel\crts\ucrt\src\appcrt\heap\debug_heap.cpp Line: 904 Expression: _CrtlsValidHeapPointer(block)
However, it seems to work fine in Wandbox: Source code in Wandbox Online Compiler Where does it go wrong?
This is the source code:
#include <iostream>
#include <cstdlib>
class Node {
public:
int data;
Node* next;
};
class LinkedList {
public:
LinkedList() {
head = nullptr;
}
~LinkedList() {
Node* temp = head;
while (head) {
head = head->next;
delete temp;
temp = head;
}
};
void addNode(int value);
void display();
void merge(LinkedList& list2);
private:
Node* head;
};
void LinkedList::addNode(int value) {
Node* newnode = new Node();
newnode->data = value;
newnode->next = nullptr;
if (head == nullptr) {
head = newnode;
} else {
Node* temp = head; // head is not NULL
while (temp->next != nullptr) {
temp = temp->next; // go to end of list
}
temp->next = newnode; // linking to newnode
}
}
void LinkedList::display() {
if (head == nullptr) {
std::cout << "List is empty!" << std::endl;
} else {
Node* temp = head;
while (temp != nullptr) {
std::cout << temp->data << " ";
temp = temp->next;
}
std::cout << std::endl;
}
}
void LinkedList::merge(LinkedList& list2) {
Node* node = new Node();
node->next = nullptr;
Node* temp = node;
Node* head1 = head;
Node* head2 = list2.head;
while (head1 != nullptr && head2 != nullptr) {
if (head1->data <= head2->data) {
temp->next = head1;
temp = temp->next;
head1 = head1->next;
} else {
temp->next = head2;
temp = temp->next;
head2 = head2->next;
}
}
while (head1 != nullptr && head2 == nullptr) {
temp->next = head1;
temp = temp->next;
head1 = head1->next;
}
while (head2 != nullptr && head1 == nullptr) {
temp->next = head2;
temp = temp->next;
head2 = head2->next;
}
temp = temp->next;
delete temp;
head = node->next;
}
int main() {
LinkedList list;
list.addNode(1);
list.addNode(2);
std::cout << "Linked List Data: " << list.display() << std::endl;
LinkedList list2;
list2.addNode(3);
list.merge(list2);
list.display();
return 0;
}

Answer 1

That MSVC error indicates that you're doing something bad with memory. In this case it seems you're deleting memory twice. Freeing a pointer twice is Undefined Behavior and compilers/runtimes are not required to diagnose the problem.
Your merge seems to be expecting sorted lists, but addNode does not enforce this. Also, once that merge is done, the nodes of list2 will be contained in both the returned list and the original list2. When the LinkedList destructor runs you will (try to) delete those nodes twice. The Microsoft Compiler in Debug builds can tell you about these problems.
There are also issues with memory leaks (e.g., the node allocated in merge is not freed).

Answer 2

The Expression: _CrtIsValidHeapPointer(block) indicates that memory has corrupted, possibly due to writing past the end of a buffer or writing past the end of a class because the writer assumes the class is larger than was allocated (like when it is expecting a derived class and gets a base class, or object of a different class).
Please open Debug->Windows->Exception Settings, In the Exception Settings window, expand the node for a category of exceptions -> Common Language Runtime Exceptions( meaning .NET exceptions), and select the check box for a specific exception within that category System.AccessViolationException. You could also select an entire category of exceptions if you don't know which exception to choose. And then you could see the initial problem.
For more information about how to manage exceptions, please refer to this document below:
https://learn.microsoft.com/zh-cn/visualstudio/debugger/managing-exceptions-with-the-debugger?view=vs-2019&redirectedfrom=MSDN