Access violation reading location 0xC0000005 C++ -

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my add function has problem dereferencing first , first pointing nothing because of that. don't know how fix isn't null pointer. 

    struct node     {         int data;         node *next;     };      class linkedlist     {         node *first;         node *last;         int count;         public:          linkedlist()         {             first = null;             last = null;             count = 0;         }           void add(int item)         {             if (first == null)             {                 first->data = item;                 last->data = item;                 last->next = null;                 first->next = last;                 count = 1;             }             else             {                 node *newnode = new node;                 newnode->data = last->data;                 newnode->next = last;                 last->data = item;                 last->next = null;                 count ++;             }         }

you have lot of code in common between if , else.

        if (first == null)         {             first->data = item;             last->data = item;             last->next = null;             first->next = last;             count = 1;         }         else         {             node *newnode = new node;             newnode->data = last->data;             newnode->next = last;             last->data = item;             last->next = null;             count ++;         }

in if, increment count 0 1. in else, also increment it.

count always getting incremented. don't need type twice.

        if (first == null)         {             first->data = item;             last->data = item;             last->next = null;             first->next = last;         }         else         {             node *newnode = new node;             newnode->data = last->data;             newnode->next = last;             last->data = item;             last->next = null;         }         count ++;

you're setting last->data item in both of them.

and you're setting last->next null in both of them.

        if (first == null)         {             first->data = item;             first->next = last;         }         else         {             node *newnode = new node;             newnode->data = last->data;             newnode->next = last;         }         last->data = item;         last->next = null;         count ++;

you forgot create new node when it's first new node.

        if (first == null)         {             node *newnode = new node;   // added             first = newnode;            // added             last = newnode;             // added             first->data = item;             first->next = last;         }         else         {             node *newnode = new node;             newnode->data = last->data;             newnode->next = last;         }         last->data = item;         last->next = null;         count ++;

the first->data = item in if redundant. first same last there, , last->data = item happening.

        if (first == null)         {             node *newnode = new node;             first = newnode;              last = newnode;             // removed             first->next = last;         }         else         {             node *newnode = new node;             newnode->data = last->data;             newnode->next = last;         }         last->data = item;         last->next = null;         count ++;

and since first , newnode have the same value in if, can use variable names interchangeably.

        if (first == null)         {             node *newnode = new node;              first = newnode;            // these 2 pointers equal!             last = newnode;             newnode->next = last;       // (same pointer)         }         else         {             node *newnode = new node;             newnode->data = last->data;             newnode->next = last;         }         last->data = item;         last->next = null;         count ++;

now, almost everything in else in if. can all moved out.

        node *newnode = new node;          if (first == null)         {             first = newnode;             last = newnode;         }         else         {             newnode->data = last->data;         }         newnode->next = last;         last->data = item;         last->next = null;         count ++;

that code should more understandable now, too. lesson: don't repeat yourself. :)


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