How to design a singleton class in C++ on Linux using boost::call_once?

Question

I am designing a thread-safe singleton class in C++ on Linux using boost::call_once.

I got the following compiler error:

/tmp/ccLanvZI.o: In function `SingleTon::getInstance()':
singleTon.cpp:(.text._ZN9SingleTon11getInstanceEv[SingleTon::getInstance()]+0x1b): undefined reference to `SingleTon::p'
/tmp/ccLanvZI.o: In function `SingleTon::makeInstance()':
singleTon.cpp:(.text._ZN9SingleTon12makeInstanceEv[SingleTon::makeInstance()]+0x21): undefined reference to `SingleTon::SingleTon()'
singleTon.cpp:(.text._ZN9SingleTon12makeInstanceEv[SingleTon::makeInstance()]+0x2c): undefined reference to `SingleTon::p'
collect2: ld returned 1 exit status
make: *** [singlton] Error 1

After seeing some posts, I still do not know how to handle this error. How do I change the void (SingleTon::)() to void (*)()?

#include <iostream>
#include <pthread.h>
#include <boost/thread/mutex.hpp>
#include <boost/thread/once.hpp>
#include <boost/bind.hpp>
#include <boost/threadpool.hpp>
#include <boost/thread/thread.hpp>

boost::once_flag flag = BOOST_ONCE_INIT;

class SingleTon
{
    private:
        static SingleTon *p;
        SingleTon();
        SingleTon(const SingleTon&);
        SingleTon& operator=(const SingleTon&);
        void makeInstance();

    public:
        SingleTon* getInstance();

        ~SingleTon()
        {
            if (p)
                delete p;
        }  
};

void SingleTon::makeInstance() 
{
    p = new SingleTon;
    return;
}

SingleTon* SingleTon::getInstance()
{
    boost::call_once( makeInstance  , flag); // error !!!
    return p;
}


int main()
{
    SingleTon *s;
    s = SingleTon::getInstance();
    return 0;
}

Can't resist: the title has the answer. You made the "Error of designing a Singleton class". — sehe, May 27 '12 at 21:51
"I am designing a thread-safe singleTon class in C++" -- err, good luck! — Xeo, May 27 '12 at 23:01

score 4 · Accepted Answer · answered May 27 '12 at 21:54

4

The error indicates boost::call_once doesn't work on member function pointers. This is because you forgot to make both makeInstance and getInstance static.

answered May 27 '12 at 21:54

AardvarkSoup

1,071
8
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I have updated the code and still have errors for adding some code to do test in main(), any help is apprciated. thanks ! – user1002288 May 27 '12 at 22:47

Jonathan Wakely · Answer 2 · 2012-05-28T21:41:12.703

2

Why have you written a destructor? Which instance do you plan to destroy? Oh wait, there's only one, cos it's a singleton, so in that case why does the destructor try to delete the instance? Infinitely recursive failure.

Singletons smell anyway, avoid them.

edited May 28 '12 at 21:41

answered May 28 '12 at 21:32

Jonathan Wakely

166,810
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Damn - wish I'd spotted that :) +1 anyway. – Fraser May 28 '12 at 21:37

score 1 · Answer 3 · edited May 23 '17 at 12:11

As AardvarkSoup (tasty and nutritious) pointed out, you need makeInstance() and getInstance() to be static.

Also, you need to define SingleTon(), not just declare it, since it's used in makeInstance().

Finally, you need to initialise the static member p, like:

SingleTon* SingleTon::p(nullptr);

This makes the code compile. However, designing a thread-safe singleton class is non-trivial. See this question for piles of links to very full explanations as to why you should try and avoid using singletons.

How to design a singleton class in C++ on Linux using boost::call_once?

3 Answers3