error related to static member functions and varaibles

kai85 · #1 19-May-2005, 23:57

CPP / C++ / C Code:

#include<iostream.h>
class static_type{
	static int i;
public:
	static void init(int x){i=x;}
	void show(){cout<<i;}
};
main()
{

	static_type::init(5);
	static_type a;
	a.show();
	return 0;
}

im compiling with VC++ ,here's the error i get:

error LNK2001: unresolved external symbol "private: static int static_type::i" (?i@static_type@@0HA)
Debug/a.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.

what should i do, usually when i get this type of error i just close Vc++ and then re-enter and compile the program again and ... thats about it ! i don't get the error again, but with this one ... doesn't work.
need ur help fast.
thanks kai85.

LuciWiz · #2 20-May-2005, 02:47

You need to "declare" your static variables in exactly one place before using your class:

CPP / C++ / C Code:

class static_type{
	static int i;
public:
	static void init(int x){i=x;}
	void show(){cout<<i;}
};

int static_type::i;

You can initialise it to a value in the same place if you need to.

Quote:

Originally Posted by kai85

what should i do, usually when i get this type of error i just close Vc++ and then re-enter and compile the program again and ... thats about it ! i don't get the error again, but with this one ... doesn't work.

You should send this to Microsoft! I wonder what they would say to that complaint!

Best regards,
Lucian

kai85 · #3 20-May-2005, 04:43

Quote:

Originally Posted by LuciWiz

You need to "declare" your static variables in exactly one place before using your class:

CPP / C++ / C Code:

class static_type{
	static int i;
public:
	static void init(int x){i=x;}
	void show(){cout<<i;}
};

int static_type::i;

You can initialise it to a value in the same place if you need to.

You should send this to Microsoft! I wonder what they would say to that complaint!

Best regards,
Lucian

ok i did that , no more errors .
but why?
why do i need to declare istatic varables in exactly one place before i use them ? and by this exactly one place do u mean before the main function?
thanks kai85.

cable_guy_67 · #4 20-May-2005, 06:11

Quote:

Originally Posted by kai85

why do i need to declare istatic varables in exactly one place before i use them ? and by this exactly one place do u mean before the main function?
thanks kai85.

Because if you declare it more than one place you will anger the little imps that live in your compiler and they will refuse to work. They will claim confusion and direct you to their union rep.

Well, maybe not exactly but it paints a pretty picture, don't it?

Using the code example you can see how Luci seperated your declaration of your static integer in the definition of your class:

CPP / C++ / C Code:

class static_type{
  static int i;
public:
  static void init(int x){i=x;}
  void show(){cout<<i;}
};

from your definition in the implementation of the class member variable.

CPP / C++ / C Code:

int static_type::i;

You could do a number of things.

Include the whole thing in your main code file
Have a seperate file for your class containing the above code
Have a class header file with the definition of the class and an implementation file for your class that would contain the definition of your static member (as well as any other code you may have). That would be a .h and .cpp file respectively.

For the first one it would look like:

CPP / C++ / C Code:

#include <iostream>
using std::cout;
using std::endl;

class static_type{
  static int i;
public:
  static void init(int x){i=x;}
  void show(){cout<<i;}
};

int static_type::i;

int main(){
  // a useless test program
  static_type my_static_int;
  my_static_int.init(100);
  cout << "My static int is ";
  my_static_int.show();
  cout << endl;
  return 0;
}

The second one also uses inclusion guards on the header file as good practice.

CPP / C++ / C Code:

#include "static_int.h"
#include <iostream>
using std::cout;
using std::endl;

int main(){
  // a useless test program
  static_type my_static_int;
  my_static_int.init(100);
  cout << "My static int is ";
  my_static_int.show();
  cout << endl;
  return 0;
}

and your header

CPP / C++ / C Code:

#ifndef STATIC_INT_H
#define STATIC_INT_H

#include <iostream>
using std::cout;

class static_type{
  static int i;
public:
  static void init(int x){i=x;}
  void show(){cout<<i;}
};

int static_type::i;
#endif

Finally the third example from my list which is good when things start getting larger and more complex.

CPP / C++ / C Code:

// main.cpp
#include "static_int.h"
#include <iostream>
using std::cout;
using std::endl;

int main(){
  // a useless test program
  static_type my_static_int;
  static_type your_static_int;

  my_static_int.init(100);
  cout << "My static int is ";
  my_static_int.show();
  cout << endl;

  your_static_int.init(200);
  cout << "My static int is ";
  my_static_int.show();
  cout << endl;
  cout << "Your static int is ";
  my_static_int.show();
  cout << endl;

  return 0;
}

CPP / C++ / C Code:

// static_int.h
#ifndef STATIC_INT_H
#define STATIC_INT_H

#include <iostream>
using std::cout;

class static_type{
  static int i;
public:
  static void init(int x){i=x;}
  void show(){cout<<i;}
};
#endif

CPP / C++ / C Code:

// static_int.cpp
#include "static_int.h"

int static_type::i;

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