CODEWITHSUNDEEP
c++pointersconstructorlinked-list
Tangleman
Tangleman

Reputation: 133

Linked List overload operator = Run time error

So I am really frustrated as to why this is happening. I am implementing a class that is similar to std::string but this is using Link List instead of arrays. my overloaded operator = is not working for some odd reason. below you can see, when I print the pointer inside the method the string is copied to the link list, however when I create a string object with this pointer to return, then the console prints out infinite junk. any ideas to what I am missing here ? (I am only pasting related code)

static int NumAllocations = 0;

struct ListNode {
    char info;
    ListNode *next;
    ListNode () : info('0'), next(0) {}
    ListNode ( char c ) : info (c), next(0) {}


};


class MyString {
 private:
    ListNode *head;
    static void delstring (ListNode *l);
    static int strlen (ListNode * head);
    static ListNode* strcpy (ListNode *dest, ListNode *src);
 public:

MyString::MyString () : head(0) {}

MyString::MyString (ListNode *l) : head(l) {}

MyString::MyString( const MyString & s ) {
    if (s.head == 0)
        head = 0;
    else
        head = strcpy(head, s.head);
}


MyString MyString::operator = (const MyString & s ){               
    ListNode *renew = NULL;
    if (head != s.head) {
        if (head != 0)
            delstring(this -> head);

        head = strcpy(head, s.head);
        // printList(head); (this prints out the string just fine, so it must be the                                   constructor ? but what about it ?!

        MyString res (head);
        return res;
    }
}


MyString::~MyString(){
    if (head == 0)
        return;
    ListNode *temp = NULL;
    do {
        temp = head -> next;
        delete head;
        -- NumAllocations;
        head = temp;
    } while (temp != 0);
}

STATIC PUBLIC FUNCTIONS 

ListNode* MyString::strcpy (ListNode *dest, ListNode *src){
    dest = new ListNode (src -> info);
    ++ NumAllocations;
    ListNode *iter = dest;
    for (ListNode *ptr = src -> next; ptr != 0; ptr = ptr ->next){
        iter -> next = new ListNode (ptr -> info);
        iter = iter -> next;
        ++ NumAllocations;
    }
    return dest;
}


void MyString::delstring (ListNode *l){
    if (l == 0)
        return;
    ListNode *temp = NULL;
    do {
        temp = l -> next;
        delete []l;

        -- NumAllocations;
        l = temp;
    } while (temp != 0);
    l = 0;
}

Upvotes: 1

Views: 205

Answers (1)

WhozCraig
WhozCraig

Reputation: 66194

Two things are fundamentally wrong with your assignment operator.

  • Not all control paths return values.
  • You shouldn't need the temp-final copy in the first place. The function should be returning a reference, specifically *this.

So...

MyString& MyString::operator = (const MyString & s )
{               
    if (head != s.head) 
    {
        if (head != 0)
            delstring(this -> head);
        head = strcpy(head, s.head);
    }
    return *this;
}

Also, everything I see in this code says the ListNode objects are allocated individually and linked together, yet in the delstring member you do this:

void MyString::delstring (ListNode *l)
{
    if (l == 0)
        return;
    ListNode *temp = NULL;
    do {
        temp = l -> next;
        delete []l;  // <<==== vector delete of single allocated item

        -- NumAllocations;
        l = temp;
    } while (temp != 0);
    l = 0;
}

Maybe try this instead:

void MyString::delstring (ListNode *& l)
{
    while (l)
    {
        ListNode *temp = l;
        l = l->next;
        delete temp;
        --NumAllocations;
    }
}

note this takes a pointer-reference rather than a pointer. it will set the caller's pointer to nullptr once the list is empty (assuming you properly terminated your list on construction, and it looks like you do).

Upvotes: 2

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