CODEWITHSUNDEEP
c++
Homie
Homie

Reputation: 79

Undefined reference to error in c++ (using a singleton class)

I've made a class named Client and the definition goes below in client.h:

#ifndef __CLIENT_H__
#define __CLIENT_H__

#include <string>

using namespace std;

class Client {
public:
    Client(string n, string p) : name(n), password(p) {logged_in = true;}
    void set_name(string n, string p) {name=n; password=p; }
private:
    string name;
    string password;
    bool logged_in;
};

#endif

then I've inherited a singleton class from Client named Admin in admin.h:

#ifndef __ADMIN_H__
#define __ADMIN_H__

#include "client.h"

using namespace std;

class Admin : public Client {
public:
    static Admin* get_admin(string n, string p)
    {
        if (Admin::n == 0)
            Admin::admin = new Admin(n, p);
        else
            Admin::admin->set_name(n, p);
        return Admin::admin;
    }
private:
    Admin(string n, string p) : Client(n, p) {Admin::n++;}
    static int n;
    static Admin* admin;
};

int Admin::n = 0;

#endif

and here is my main.cpp:

#include <iostream>
#include <string>
#include "client.h"
#include "admin.h"

using namespace std;

int main()
{
    Admin* administrator = Admin::get_admin("ghamar", "utcom90");
}

and the problem is that when I want to compile main.cpp using g++ (g++ main.cpp), I get the following error:

/tmp/cc15VnHc.o: In function `Admin::get_admin(std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >)':
main.cpp:(.text._ZN5Admin9get_adminESsSs[Admin::get_admin(std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >)]+0x5f): undefined reference to `Admin::admin'
main.cpp:(.text._ZN5Admin9get_adminESsSs[Admin::get_admin(std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >)]+0xa0): undefined reference to `Admin::admin'
main.cpp:(.text._ZN5Admin9get_adminESsSs[Admin::get_admin(std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >)]+0xd1): undefined reference to `Admin::admin'

the important part of the error says:

undefined reference to 'Admin::admin'

Can anybody help me out with this? thanks a lot for your time.

Upvotes: 1

Views: 1266

Answers (4)

pippin1289
pippin1289

Reputation: 4939

You need to define Admin outside the class since it is static.

Admin* Admin::admin = NULL;

Upvotes: 1

ildjarn
ildjarn

Reputation: 62995

You have two problems here, and are only seeing the errors for one so far.

You need to define Admin::n and Admin::admin in a single source file. Currently the former is defined in a header (which will cause multiple-definition errors once more than one source file includes that header), and the latter is not defined at all.

Remove the definition of Admin::n from admin.h, and add the following to a single source file:

#include "admin.h"

int Admin::n = 0;
Admin* Admin::admin = 0;

Upvotes: 2

Mike Seymour
Mike Seymour

Reputation: 254751

As well as declaring the static members in the class definition, you also need to define them in exactly one source file:

#include "admin.h"

int Admin::n = 0;
Admin* Admin::admin = 0;

You should remove the definition of n from the header; that will give "multiple definition" errors if you include the header from more than one source file. The = 0 bits are optional, since static objects are zero-initialised by default.

You should also remove the underscores from the start of the include guards; those are reserved, and using them could cause conflicts with names used by the standard library implementation.

Upvotes: 2

hmjd
hmjd

Reputation: 122011

You have declared admin, but not defined it (yet you have defined n):

int Admin::n = 0; 
Admin* Admin::admin;    // <-- add this

Upvotes: 1

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