Undefined reference to static variable [duplicate]

Question

Possible Duplicate:
C++: undefined reference to static class member

I'm using MinGW. Why static variable is not working

[Linker error] undefined reference to `A::i' 

#include <windows.h>

    class A { 
        public:     
        static int i;
        static int init(){

            i = 1;  

        }

    };

int WINAPI WinMain (HINSTANCE hThisInstance,
                    HINSTANCE hPrevInstance,
                    LPSTR lpszArgument,
                    int nFunsterStil){
    A::i = 0;
    A::init();

    return 0;
}

you should change return of init to void, or return something — Karthik T
– Karthik T, Commented Jan 15, 2013 at 5:09
This question has been asked countless times before: stackoverflow.com/questions/272900/… stackoverflow.com/questions/12117859/… stackoverflow.com/questions/3672088/… stackoverflow.com/questions/7787059/… and many more — Adam Rosenfield
– Adam Rosenfield, Commented Jan 15, 2013 at 5:11

billz · Accepted Answer · 2013-01-15 05:10:37Z

48

You only declared A::i, need to define A::i before using it.

class A  
{ 
public:     
  static int i;
  static void init(){
     i = 1;  
  }
 };

int A::i = 0;

int WINAPI WinMain (HINSTANCE hThisInstance,
                HINSTANCE hPrevInstance,
                LPSTR lpszArgument,
                int nFunsterStil)
{
  A::i = 0;
  A::init();

  return 0;
}

Also your init() function should return a value or set to void.

edited Jan 15, 2013 at 5:10

answered Jan 15, 2013 at 5:04

billz

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2 Comments

Goodies Over a year ago

What if it is private? Can you still access it to define it?

Chris Tophski Over a year ago

@Goodies I had no problem with a private static member in my test. Though I needed a hint from the first link of this comment: stackoverflow.com/questions/14331469/… of this question. I think this seemed to be the problem with private members in your case. If it's a structural problem with member visibility in the class definition and you need a certain member declaration order, you may also have multiple private/public/protected blocks (each with one or more members) in sequence.

Olaf Dietsche · Accepted Answer · 2013-01-15 05:07:47Z

19

You have declared A::i inside your class, but you haven't defined it. You must add a definition after class A

class A {
public:
    static int i;
    ...
};

int A::i;

answered Jan 15, 2013 at 5:07

Olaf Dietsche

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