is a true Singleton possible with c++?

javacoder (1)
hello forum,

i am very new to c++; but not new to oop. i am a java programmer by profession. teaching myself c++ as my latest "hobby project".

please, can anybody tell me:

what is the c++ idiom for implementing a _real_ Singleton (as in, the GoF design pattern)?

by "real", i mean so that there is no way whatsoever to create more than one instance of the Singleton. if you're thinking, "double-checked locking", etc. please stop there. that's not what i'm getting at.

i'm talking about your basic, simple, canonical Singleton; simply for tutorial purposes. specifically, i want to implement a Singleton that will prevent a client from doing something like this:

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 ...
   Singleton* singleton = Singleton::getInstance();
   singleton->doSummut();
  
   // this shouldn't be allowed
   Singleton* doubleton;
   doubleton->doSummutElse();
   ...


with the implementation i've coded (see below), it is possible to create more than one instance (even with a private constructor). keep in mind: i am a novice with c++; please, be gentle ;¬)

thanks in advance for your help.



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#include <stdlib.h>
#include "Singleton.h"
using namespace std;

void useSingleton(){ 
  Singleton* singleton = Singleton::getInstance();
  singleton->doSummut();
  
  // this shouldn't be allowed
  Singleton* doubleton;
  doubleton->doSummutElse();
}
//
// 
//
int main(int argc, char** argv) {
    useSingleton();
    return (EXIT_SUCCESS);
}


================================

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#ifndef _SINGLETON_H
#define	_SINGLETON_H

#include <stdio.h>
#include <stdlib.h>

class Singleton{ 

public:
    static Singleton* instance;
    static Singleton* getInstance(){ 
        if (instance == 0){ 
           instance = new Singleton;
        } // end if
        return instance;
    }
    
    void doSummut(){ 
    printf("Doing the deed in Singleton.doSummut()\n");
    }
    
    void doSummutElse(){ 
       printf("Doing summut else in Singleton.doSummutElse()\n");
    }
    
private:
    Singleton();
    

};

Singleton* Singleton::instance = 0;

Singleton::Singleton(){ 
}
#endif	/* _SINGLETON_H */
cpjust (9)
Where are you creating a 2nd instance?
All I see is the 1st instance, and an uninitialized pointer.
Duthomhas (13200)
You might want to take a look at this article
http://www.inquiry.com/techtips/cpp_pro/10min/10min0200.asp
ropez (310)
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  // this shouldn't be allowed
  Singleton* doubleton;
  doubleton->doSummutElse();


That code is very dangerous. You don't create another instance of the singleton class, but you pass an uninitialized pointer as this pointer to the doSummutElse() function. Since the function doesn't use any member variables, it probably work, but the compiler should give you a warning about the uninitialized pointer. Make sure you have warnings enabled! If you try to use the this pointer in the doSummutElse function, explicitly or implicitly by using a member variable, your program will probably crash.
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