Cannot access private member in singleton class destructor

Question

I'm trying to implement this singleton class. But I encountered this error:

'Singleton::~Singleton': cannot access private member declared in class 'Singleton' This is flagged in the header file, the last line which contains the closing brace.

Can somebody help me explain what is causing this problem? Below is my source code.

Singleton.h:

 class Singleton { public: static Singleton* Instance() { if( !pInstance ) { if( destroyed ) { // throw exception } else { Create(); } } return pInstance; } private: static void Create() { static Singleton myInstance; pInstance = &myInstance; } Singleton() {} Singleton( const Singleton& ); Singleton& operator=( const Singleton& ); ~Singleton() { pInstance = 0; detroyed = false; } static Singleton* pInstance; static bool destroyed; };

Singleton.cpp:

 Singleton* Singleton::pInstance = 0; bool Singleton::destroyed = false;

Inside my main function:

 Singleton* s = Singleton::Instance();

If I make the destructor as public, then the problem disappears. But a book (Modern C++ Design) says it should be private to prevent users from deleting the instance. I actually need to put some code for cleanup for pInstance and destroyed inside the destructor.

By the way, I'm using Visual C++ 6.0 to compile.

I think it should work and can u give code flashing an error ? — Ashish
– Ashish, Commented Jan 25, 2010 at 7:52
@Mac: Code flashing? I'm not sure I understand what this means. — jasonline
– jasonline, Commented Jan 25, 2010 at 7:54
Neeraj: It does? So I guess this only occurs in Visual C++?! — jasonline
– jasonline, Commented Jan 25, 2010 at 7:56
If that really is some weird VC6 bug and you don't need any behaviour in the destructor you could use static Singleton* instance = new Singleton; - or just don't use the singleton pattern and use a free function that returns the instance in question. — Georg Fritzsche
– Georg Fritzsche, Commented Jan 25, 2010 at 8:25

Michael Burr · Accepted Answer · 2010-01-25 08:10:12Z

2

You should probably have let us know that the version of Visual C++ you're working with is VC6. I can repro the error with that.

At this point, I have no suggestion other than to move up to a newer version of MSVC if possible (VC 2008 is available at no cost in the Express edition).

Just a couple other data points - VC2003 and later have no problem with the Singleton destructor being private as in your sample.

answered Jan 25, 2010 at 8:10

Michael Burr

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4 Comments

jasonline Over a year ago

@Michael: Sorry for missing that info. Thank you for your information.

Michael Burr Over a year ago

VC6 is pretty dang old nowadays - it makes sense to use it if you're maintaining code written for it, but for new projects please use something more modern. You'll save yourself a lot of unnecessary headaches (there's enough headache inducing stuff in C++ that you shouldn't pile it on with VC6 if you don't have to).

jasonline Over a year ago

Yes, I'm maintaining old code so I thought of trying to compile just in this environment. Unfortunately, it's not working. I'll just have to try it on other compilers I guess.

Michael Burr Over a year ago

Or maybe just live with the dtor being public if you have to use VC6.

R Samuel Klatchko · Accepted Answer · 2010-01-25 07:56:28Z

The code you posted looks doesn't have any problem, so the problem must be in some other part of your source code.

The error message will be preceded by the filename and line number where the problem is occurring. Please look at the line and you'll see a bit of code that is either trying to call delete on a singleton pointer or is trying to construct an instance of singleton.

The error message will look something like this (the file and line number are just an example):

c:\path\to\file.cpp(41) : error C2248: 'Singleton::~Singleton': cannot access private member declared in class 'Singleton'

So in that case, you would want to see what is happening at line 41 in file.cpp.

It's actually pointing at the header file, at the end of the file (the closing brace). And those are the only source codes in my workspace right now.
Does this happen when compiling singleton.cpp or some other source file? What is the code right before #include "singleton.h"?
@R Samuel Klatchko: Compiling singleton.cpp. Just another include file for stdafx.h. Someone was already able to reproduce it in one of the answers posted.

Stephen C · Accepted Answer · 2010-01-25 08:30:11Z

2

I'm no C++ or VC expert, but your example looks similar to the one described on this page ... which the author calls a compiler bug.

answered Jan 25, 2010 at 8:30

Stephen C

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1 Comment

jasonline Over a year ago

@Stephen: Thanks for the info. Your link provides a list of search results. I browsed over and I think it's this one - codeguru.com/forum/archive/index.php/t-236067.html.

Georg Fritzsche · Accepted Answer · 2010-01-25 07:58:25Z

Personally i haven't put destructors in my singletons unless i am using a template singleton class, but then i make them protected.

template<class T> class Singleton { public: static T &GetInstance( void ) { static T obj; return obj; } static T *GetInstancePtr( void ) { return &(GetInstance()); } protected: virtual ~Singleton(){}; Singleton(){}; };

then write my class as

class LogWriter : public Singleton<LogWriter> { friend class Singleton<LogWriter>; }

@Craig: Thanks for sharing. I actually need the destructor to do some cleanup though. That's why. If I make it as protected, I get the same error saying cannot access protected member declared in...

Ashish · Accepted Answer · 2010-01-25 08:00:39Z

-1

class Singleton { static Singleton *pInstance = NULL; Singleton(){}; public: static Singleton * GetInstance() { if(!pInstance) { pInstance = new Singleton(); } return pInstance; } static void RemoveInstance() { if(pInstance) { delete pInstance; pInstance = NULL; } } };

answered Jan 25, 2010 at 8:00

Ashish

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2 Comments

jasonline Over a year ago

@Mac: Thanks for sharing. But with this one, you actually need the caller to call RemoveInstance() right for the cleanup. I was thinking of an implementation where you don't need to ask the caller to call a cleanup function.

Loki Astari Over a year ago

This is the standard anti-pattern for writing a singleton. Please learn the correct (or should I say better ways to do it). Scott Myers has some really good examples.

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