[c++]The importance of dynamic allocation [closed] - c++

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I wrote a doubly linked list, and trying to add an append()(insert at the end)and len()(caculate the number of member in the list). I just don't understand why it doesn't work now. Here is the simplest code:
#include<iostream>
using namespace std;
class linkedList{
private:
struct node{
node* last;
node* next;
char* str;
};
node sentinel;
public:
linkedList();
~linkedList();
int len();
void append(char*);
};
linkedList::linkedList(){
sentinel.last=&sentinel;
sentinel.next=&sentinel;
sentinel.str="I am sentinel!!";
};
linkedList::~linkedList(){};
int linkedList::len(){
node* currentNode=&sentinel;
int count=0;
while ((*currentNode).next!=&sentinel){
count++;
currentNode=(*currentNode).next;
cout<<(*currentNode).str<<endl;
}
return count;
}
void linkedList::append(char* str){
node newNode;
newNode.str=str;
newNode.last=sentinel.last;
(*sentinel.last).next=&newNode;
sentinel.last=&newNode;
newNode.next=&sentinel;
}
int main(){
linkedList myList;
myList.append("Hello");
myList.append("World");
int length=myList.len();
cout<<length<<endl;
return 0;
}
What I am doing is just add two new nodes into the linked list, and caculate the total number of my nodes. it should return 2. but why it doesn't work?

Answer 1

newNode in your code below will go out of scope as soon as append is finished executing. Assigning it's memory address as a pointer to more global member is likely going to end in a segfault.
void linkedList::append(char* str){
node newNode;
newNode.str=str;
newNode.last=sentinel.last;
(*sentinel.last).next=&newNode;
sentinel.last=&newNode;
newNode.next=&sentinel;
}
Try allocating your node on the heap using new node, possibly using a shared_ptr to make memory management a bit simpler.
void linkedList::append(char* str){
node *newNode = new node;
newNode->str=str;
newNode->last=sentinel.last;
(*sentinel.last).next=newNode;
sentinel.last=newNode;
newNode->next=&sentinel;
}
With this approach, be sure to cleanup the nodes when destructing your linkedlist, via the delete operator on each node.
Alternatively, look into using shared_ptr's to a Node instead of raw pointers, which will always call delete when the linkedlist (and nobody else) is pointing to the node.

Answer 2

Use the new keyword to allocate a new node:
void linkedList::append(char* str){
node *newNode = new node();
newNode->str=str;
newNode->last=sentinel.last;
(*sentinel.last).next=newNode;
sentinel.last=newNode;
newNode->next=&sentinel;
}