Using static class in c++

Question

I have some simple hypothetical static class in C++:

#ifndef __STAT_H_
#define __STAT_H_

class Stat {

 private:
  static vector<int> v;

 public:

  static void add_num(int num);
  static void clear_nums();
  static void get_count();
};

#endif

And the ccp file is so:

#include "Stat.h"

vector<int> v;


void Stat::add_num(int num) {
  v.push_back(num);
}

void Stat::clear_nums() {
  v.clear();
}

int Stat::get_num_count() {
  return v.size();
}

Now when I include in main.cpp file "Stat.h" and try to use some static method:

Stat::add_num(8);

the error during compilation is

undefined reference to 'Stat::add_num(int)'

What can be the problem in this case? Thank you.

EDIT: sorry about addresses vector, it should be v there

Most likely one of these (I'm looking at you, didn't link implementation): stackoverflow.com/questions/12573816/…. You also might find this interesting: stackoverflow.com/questions/228783/… — Qaz
– Qaz, Commented Dec 9, 2012 at 8:04
You should explain what compilation command you are using, and what compiler you have. — Basile Starynkevitch
– Basile Starynkevitch, Commented Dec 9, 2012 at 8:05
Note: Your include guard is not valid according to the standard. All names beginning with a leading underscore followed by a capital letter are reserved by the implementation in the global namespace. (And as it is a macro, that is included) Names with double underscores are disallowed in all contexts. But that is unrelated to your question. — Billy ONeal
– Billy ONeal, Commented Dec 9, 2012 at 8:12
This is unrelated to the error, but there is no concept of static classes in C++. — juanchopanza
– juanchopanza, Commented Dec 9, 2012 at 8:25

unknownuser · Accepted Answer · 2012-12-09 08:07:44Z

3

It sounds like you have not included stat.cpp in compilation. So your linker cannot find an implementation for the static methods.

answered Dec 9, 2012 at 8:07

unknownuser

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6 Comments

Billy ONeal Over a year ago

+1. I would additionally note that the declaration of v is wrong. It declares a vector v, not Stat::v.

Sergey Over a year ago

I included stat.h into main.cpp

Billy ONeal Over a year ago

@Sergey: But you didn't tell the compiler about stat.cpp -- which is where Stat::add_num is defined. So the compiler (or rather, in this case the linker) is perfectly okay in saying "I don't know where to find this definition".

unknownuser Over a year ago

See stackoverflow.com/questions/3202136/…

Sergey Over a year ago

if I change stat.h to stat.cpp in include directive, it's the same error

|

billz · Accepted Answer · 2012-12-09 08:18:18Z

1

You need to link Stat.o in g++ command, say:

g++ -c -o Stat.o Stat.cpp
g++ -o Stat main.cpp Stat.o

I guess in your Stat.cpp:

vector<int> v;

should be:

vector<int> Stat::v;

There is no compile error if you define local v in Stat.cpp but I guess you intent to use Stat::v

edited Dec 9, 2012 at 8:18

answered Dec 9, 2012 at 8:11

billz

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1 Comment

Billy ONeal Over a year ago

This is also an error; but it is not the one the user is having a problem with.

Dennis · Accepted Answer · 2012-12-09 09:20:06Z

Here's my take on your program, just for reference sake.

Stat.h

#ifndef STAT_H
#define STAT_H

#include <vector>
using std::vector;

class Stat
{
 public:
  static void add_num(int num);
  static void clear_nums();
  static int get_count();

 private:
  static vector<int> v;
};

#endif

Stat.cpp

#include "Stat.h"

vector<int> Stat::v;

void Stat::add_num(int num) { v.push_back(num); }

void Stat::clear_nums() { v.clear(); }

int Stat::get_count() { return v.size(); }

main.cpp

#include "Stat.h"

int main()
{
  Stat s;
  s.add_num(8);
}

Makefile

CC = g++
OBJS = Stat.o
DEBUG = -g
CFLAGS = -Wall -c $(DEBUG)
LFLAGS = -Wall $(DEBUG)

all: build clean

build: $(OBJS)
        $(CC) main.cpp $(LFLAGS) $(OBJS) -o stat

Stat.o: Stat.h
        $(CC) $(CFLAGS) Stat.cpp

clean:
        -rm -f *.o

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