C++ Static Variable giving error

Question

I am learning C++ and have a weird issue with a lab I am working on. Where I don't seem to be referencing things right would be my guess.

It basically throws an error like it can't find the static int nCounters declared in the Class Header file.

//Start of counter.h

class Counter
{

private:
  int counter;
  int limit;
  static int nCounters;

public:
  Counter(int x, int y);
  void increment();
  void decrement();
  int getValue();
  int getNCounters();

};

//end of counter.h

//Start of counter.cpp

#include "counter.h"

Counter::Counter(int x, int y)
{
    counter = x;
    limit = y;
    nCounters++;
}

void Counter::increment()
{
    if (counter < limit)
    {
        counter++;
    }
}

void Counter::decrement()
{
    if (counter > 0)
    {
        counter--;
    }
}

int Counter::getValue()
{
    return counter;
}

int Counter::getNCounters()
{
    return nCounters;
}

//end of counter.cpp

//Start of counter_test.cpp

#include <iostream>
#include "counter.cpp"
using namespace std;
int main(){
    Counter derp(5,5);

    cout << derp.getValue();

    return 0;
}

//end of counter_test.cpp

If I remove static it compiles and runs just fine. It must be something I don't understand here. — Tim Herbert, Feb 21 '18 at 01:05
There are dozens if not hundreds of answered Stack Overflow questions found by a search like "C++ undefined reference static member". — aschepler, Feb 21 '18 at 01:12
@aschepler Google search for *undefined reference static variable c++ site:stackoverflow.com* says "About 48,200 results" — user4581301, Feb 21 '18 at 01:13

Achal · Accepted Answer · 2018-02-21T01:15:24.183

1

You need to define the static variable. static int nCounters; is just a declaration not definition.

Define like this

int Counter :: nCounters;

for e.g

class Counter {
        private:
                static int nCounters; /*declaration */
        public:
                /* Member function of class */
};
int Counter :: nCounters;/*definition, this is must  */
int main() {
        /* some code **/
        return 0;
}

edited Feb 21 '18 at 01:15

answered Feb 21 '18 at 01:08

Achal

11,821
2
15
37

Awesome works perfectly after adding this in my counter.cpp file. I knew I must have been missing something somewhere. – Tim Herbert Feb 21 '18 at 01:25

score 1 · Answer 2 · answered Feb 21 '18 at 01:09

1

As this variable is the same across all classes and is not defined when you instantiate a Counter unlike normal member variables, you have to add the following line somewhere in your code (Best placed in counter.cpp):

Counter::nCounters = 0;

This sets the defines and sets the initial value of nCounters.

On a side note, #include "counter.cpp" is bad practice. Instead #include "counter.h" and link the .cpp file to your executable.

answered Feb 21 '18 at 01:09

Arnav Borborah

11,357
8
43
88

I also did what you suggested and moved my counter.cpp file to the footer of the counter.h file. Thanks that is a bit cleaner. – Tim Herbert Feb 21 '18 at 01:26

C++ Static Variable giving error

2 Answers2