C++ how to get a static variable in a class?

Question

I can't get the syntax correct for with static variables and c++ classes.

This is a simplified example to show my problem.

I have one function that updates a variable that should be the same for all objects, then I have another functions that would like to use that variable. In this example I just return it.

#include <QDebug>

class Nisse
{
    private: 
        //I would like to put it here:
        //static int cnt;
    public: 
        void print()
        {
            //And not here...!
            static int cnt = 0;
            cnt ++;
            qDebug() << "cnt:" << cnt;
        };

        int howMany()
        {
            //So I can return it here.
            //return cnt;
        }
};

int main(int argc, char *argv[])
{
    qDebug() << "Hello";
    Nisse n1;
    n1.print();

    Nisse n2;
    n2.print();
}

The current local static in the print-function is local to this function, but I would like it to be "private and global in the class".

It feels like I am missing some basic:s c++ syntax here.

/Thanks

Solution:

I was missing the

int Nisse::cnt = 0;

So the working example looks like

#include <QDebug>

class Nisse
{
    private: 
        static int cnt;
    public: 
        void print()
        {
            cnt ++;
            qDebug() << "cnt:" << cnt;
        };

        int howMany()
        {
            return cnt;
        }
};

int Nisse::cnt = 0;

int main(int argc, char *argv[])
{
    qDebug() << "Hello";
    Nisse n1;
    n1.print();

    Nisse n2;
    n2.print();

    qDebug() << n1.howMany();
}

What's your problem when you put it 'global' in the class? – Benoit Thiery Mar 17 '11 at 16:22 — Benoit Thiery, Mar 17 '11 at 16:22
Then I don't think it will be shared between the objects? – Johan Mar 17 '11 at 16:26 — Johan, Mar 17 '11 at 16:26
Yes it will be shared if you declare it static. – Benoit Thiery Mar 17 '11 at 16:26 — Benoit Thiery, Mar 17 '11 at 16:26

Matt K · Accepted Answer · 2011-03-17T16:30:08.693

6

Your commented out code is halfway there. You also need to define it outside the class with a int Nisse::cnt = 0; statement.

EDIT: like this!

class Nisse
{
    private: 
        static int cnt;
    public: 
...     
};
int Nisse::cnt = 0;

edited Mar 17 '11 at 16:30

answered Mar 17 '11 at 16:20

Matt K

13,370
2
32
51

score 3 · Answer 2 · edited May 23 '17 at 12:14

3

Initializing private static members

edited May 23 '17 at 12:14

Community

1
1

answered Mar 17 '11 at 16:23

Ptival

9,167
36
53

score 2 · Answer 3 · answered Mar 17 '11 at 16:31

You can't initialize a static member variable in the class definition.

From your comments, it seems you are a bit confused with C++ syntax, especially since your methods are also defined there like you would in Java.

file name Nisse.h

#include <QDebug>

class Nisse
{
    private: 
        static int cnt;
    public: 
        void print();
        int howMany();

};

file named Nisse.cc

Nisse::cnt = 0;

void Nisse::print()
{
    cnt++;
    qDebug() << "cnt:" << cnt;
}

int Nisse::howMany()
{
    //So I can return it here.
    return cnt;
}

score 0 · Answer 4 · answered Mar 17 '11 at 16:28

0

In the header file:

class Nisse
{
    private: 
        static int cnt;

    public: 
...
};

In the cpp file:

int Nisse::cnt = 0;

answered Mar 17 '11 at 16:28

Benoit Thiery

6,325
4
22
28

C++ how to get a static variable in a class?

4 Answers4

file name Nisse.h

file named Nisse.cc