Program throws error when I increment a private static variable in C++

Question

I created a private static variable that keeps track of the number of elements in the linked list.

struct node
{
        int data;
        node *next;
};    
class linkedList
    {
            private:
                    node *head,*tail;
                    static int listSize;
            public:
                linkedList()
                {
                    head=NULL;
                    tail=NULL;
                }
                void insert(int n)
                {
                        node *temp=new node;
                        temp->data=n;
                        temp->next=NULL;
                        if(head == NULL)
                        {
                                head=temp;
                                tail=temp;
                        }
                        else
                        {
                                tail->next=temp;
                                tail=temp;
                        }
                        linkedList::listSize+=1;
                }
    };
    void main()
    {
         linkedList l;
         l.insert(10);
         l.insert(20);
    }

The compiler throws an error when it reaches the line linkedList::listSize+=1;

error: ‘linkedList’ has not been declared.

[MCVE](https://stackoverflow.com/help/mcve) would help a lot here. — Eljay, May 15 '18 at 15:38
That is not a valid C++ code. C++ can't be learned by guessing. A good [C++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) is in order. — Ron, May 15 '18 at 15:39
`static int listSize;` means you can only ever have one linked list. Reconsider making this `static`. — user4581301, May 15 '18 at 15:41
It should be `int linkedList::listSize+=1;`, but anyway you can't access `listSize` from n on class members as it is private...or is your actual code rather `void linkedList::insert()`... — Jabberwocky, May 15 '18 at 15:41
I took out the `int`, added the missing code best I could, and it works on my machine. I think the bug is in the code not shown. — Eljay, May 15 '18 at 15:41
The problem is likely header related. My money is on a circular include. — user4581301, May 15 '18 at 15:42
@user4581301 Save your money. This is pure guesswork from the OP's side. — Ron, May 15 '18 at 15:44
@Ron Agreed. Guesswork from my side, too. Not much can be done with this question but close it for the time being. — user4581301, May 15 '18 at 15:47
node is a structure that holds a variable named data of type int and a pointer to next node. @MichaelWalz — Prasanth Ganesan, May 15 '18 at 15:47
@PrasanthGanesan you need to make all clarifications in your question. As it stands here there is too much information missing. — Jabberwocky, May 15 '18 at 15:49
[This compiles fine](https://www.ideone.com/3SIOh9), except for the missing `main`. Please provide a [MCVE]. — Jabberwocky, May 15 '18 at 15:51
There are still typos in your program, `inser(20);` instead of `insert(20);` and `:` instead of `;` in `linkedList()`. — Jabberwocky, May 15 '18 at 15:56
sorry man, Those are typos which I made while editing. They are fine in the program. — Prasanth Ganesan, May 15 '18 at 15:59
@PrasanthGanesan never edit code in your question. Just copy/paste it. — Jabberwocky, May 15 '18 at 16:12

score 2 · Accepted Answer · answered May 15 '18 at 15:59

Once your typos corrected (inser(20) instead of insert(20) and : instead of ; in linkedList(), your program almost compiles.

There is just one thing missing: you need to implement the listSize variable somewhere for example by putting int linkedList::listSize; before main:

...
int linkedList::listSize;   /(/ <<< add this

void main()
{
  linkedList l;
  l.insert(10);
  l.insert(20);
}

But why are you using a static variable for counting the elements of the list? You probably want listSize to be an ordinary (non static) class member, just as head and tail:

class linkedList
{
private:
  node * head, *tail;
  int listSize;      // no static
public:
  ...

and drop the int linkedList::listSize; suggested before.

Program throws error when I increment a private static variable in C++

1 Answers1