c++: Problems with a singleton (as a so)

eantoranz · 02-16-2006, 02:38 PM

I'm trying to create a singleton class. I think I have been able to code it correctly after spending sometime learning a thing or two I don't know very well about C++.

Anyway.... I have put this class in a so. But when I start the application that uses this library, it complais because it can't find one symbol (one the class' static members).

header file (I'm using QT):

Code:

#include <QSqlDatabase>

#ifndef SMILE_H_
#define SMILE_H_

#endif /*SMILE_H_*/

class SmileDB {
	
private:
	static SmileDB *instance;

public:
	static SmileDB *getInstance();
	
	void setDatabase(QSqlDatabase *database);

private:
	QSqlDatabase *database;
};

Here's the implementation:

Code:

#include <QSqlDatabase>
#include "smiledb.h"

SmileDB * SmileDB::getInstance() {
	if (! SmileDB::instance) {
		SmileDB::instance = new SmileDB();
	}
	
	return SmileDB::instance;
}

void SmileDB::setDatabase(QSqlDatabase *database) {
	if (! this->database) {
		this->database = database;
	}
}

When I start the application, I get this:

Code:

symbol lookup error: lib/libsmiledb.so.1: undefined symbol: _ZN7SmileDB8instanceE

Any ideas what I'm missing?

graemef · 02-16-2006, 05:08 PM

In the implementation you need to initialise the static as follows:

SmileDB * SmileDB::instance = 0;

You also want the constructor to be private.

dmail · 02-16-2006, 05:34 PM

Quote:

Originally Posted by graemef

In the implementation you need to initialise the static as follows:

SmileDB * SmileDB::instance = 0;
...

or you could use a lazy singleton

Code:

class SmileDB 
{
public:
    static SmileDB *getInstance()
    {
        static SmileDb* instance = 0;
        if(!instance)instance = new SmileDB();
        return instance;
    }
.......

although i much prefer returning a refernce instead of a pointer, but this depends of the lifetime of the singleton. I use a template class for singletons which have the life time of the app and register an atexit func to delete the instance; where getinstance returns a ref.
also you may want to include a few funcs in private so that they are not auto created.

Code:

SmileDB(const SmileDB&){;}
SmileDB& operator=(const SmileDB&){;}

eantoranz · 02-17-2006, 09:54 AM

Can't get it to work as a so. I'll try the static way to see how it goes.

eantoranz · 02-17-2006, 12:44 PM

I get exactly the same error... only that in compiletime, instead of runtime.

Code:

smiledb.o: In function `SmileDB::getInstance()':
smiledb.cpp:(.text+0x29): undefined reference to `SmileDB::instance'
smiledb.cpp:(.text+0x5a): undefined reference to `SmileDB::instance'

I think it's a problem in my code. Can you show me one very simple example of a singleton (c++) so that I can extend from it? both the .h and the .cpp

With just a getInstance() will be enough.. and thanks for your help so far.

dmail · 02-17-2006, 01:05 PM

this is a very basic version.

Code:

class Singleton
{
	public:
	static Singleton& manager( void )
	{
		static Singleton* instance =0;
		if(!instance)instance = new Singleton();
		return *instance;
	}
	void foo( void){;}
	private:
	Singleton(){;}
	~Singleton(){;}
};

foo would be called like so:

Code:

Singleton::manager().foo();

eantoranz · 02-17-2006, 01:50 PM

Quote:

Originally Posted by graemef

In the implementation you need to initialise the static as follows:

SmileDB * SmileDB::instance = 0;

You also want the constructor to be private.

As graemef had said, I had to write that beautiful line... but I hadn't realized it was a static part.... so it's out of any method scope:

Code:

#include "smiledb.h"

SmileDB * SmileDB::instance = 0;

SmileDB::SmileDB() {}

Now... about the dmail's way... I read somewhere that this way is better, cause it avoids memory leaks.. could you elaborate on this, please? (I'm going to give it a try right away.... the marvels of using a CVS

)

eantoranz · 02-17-2006, 02:39 PM

How do you like it? ;-)

.h

Code:

#include <QSqlDatabase>

#ifndef SMILE_H_
#define SMILE_H_

#endif /*SMILE_H_*/

/**
 * Clase SmileDB que es la conexion a la DB de smile
 * 
 */
class SmileDB {
	
private:
	SmileDB();
#ifdef LAZY
private:
#else
public:
#endif
	~SmileDB();
	
#ifndef LAZY
private:
	static SmileDB * instance;
#endif

public:
	/**
	 * Retornar la instancia de SmileDB
	 */
#ifdef LAZY
	 static SmileDB & getInstance();
#else
	 static SmileDB * getInstance();
#endif
	
	/**
	 * Guardar la conexion a DB (si no ha sido definida previamente)
	 */
	void setDatabase(QSqlDatabase *database);
	
private:
	QSqlDatabase *database;
};

.cpp

Code:

#include <QSqlDatabase>
#include "smiledb.h"

#ifndef LAZY
SmileDB * SmileDB::instance = 0;
#endif

SmileDB::SmileDB() {}
SmileDB::~SmileDB() {
#ifndef LAZY
	delete SmileDB::instance;
#endif
}

#ifdef LAZY
SmileDB & SmileDB::getInstance() {
	static SmileDB* instance;
	if (! instance) {
		instance = new SmileDB();
	}
	return *instance;
}
#else
SmileDB * SmileDB::getInstance() {
	if (! SmileDB::instance) {
		SmileDB::instance = new SmileDB();
	}
	return SmileDB::instance;
}
#endif

void SmileDB::setDatabase(QSqlDatabase *database) {
	if (! this->database) {
		this->database = database;
	}
}

eantoranz · 02-19-2006, 02:43 PM

Both implementations work (thank you very much).

Now I have a little trickier problem. How can you make a singleton that can be extended? Say, you have a number of classes that you want to make singletons, so you have a single instance of each one of them. I don't want to have to write the same code over and over again in each one. Is there an easy way to accomplish it?

dmail · 02-19-2006, 03:09 PM

For a single thread, you can use this template

Code:

#ifndef _SINGLETON_H_
#	define _SINGLETON_H_

namespace pattern
{
template< class T >
class Singleton
{
public:

	inline static T& manager(void)//getter for the singleton
	{
		if( ! m_instance )//check
		{	//create the instance of the derived class
			m_instance = new T();
			//register the function to be called at exit
			atexit( Pattern::Singleton<T>::destroy );	
		}
		return (*m_instance);//return the reference
	}
protected:
	//called when the app exits
	static void destroy(void)
	{ 
		if(m_instance){ delete m_instance; m_instance = 0; } 
	}
	Singleton(void){;}//constructor
	~Singleton(void){;}//destructor
private:
	static T* volatile m_instance;//the instance of the 

	//declare two other functions so the compiler doesn't auto create
	//them, these are in private so that there is no way of calling them
	Singleton(const Singleton&){;}
	Singleton& operator=(const Singleton&){;}
};

};//namespace pattern

//intailise the static
template<class T>T* volatile Pattern::Singleton<T>::m_instance = 0;
#endif //_SINGLETON_H_

and to inherit from this use something like

Code:

class Singleton_timer : public Pattern::Singleton<Singleton_timer>
{ 
public:
//public func here

private:
	friend class Pattern::Singleton<Singleton_timer>;
	Singleton_timer( void ){;}
	~Singleton_timer( void ){;}
};

//typedef a smaller name for this class
typedef Pattern::Singleton<Singleton_timer> Timer;

Therefore a singleton timer would be called like so:

Code:

Timer::manager().func_to_call_here();

eantoranz · 02-20-2006, 09:11 AM

It works... but I have a problem. I have to set the contructor of the extending class as public, because if I set it to private, it wont compile:

Code:

../comun/formas/login/../../smiledb/../singleton.h: In static member function 'static T& Smile::Singleton<T>::getInstance() [with T = Smile::SmileDB]':
../comun/formas/login/login.cpp:27:   instantiated from here
../comun/formas/login/../../smiledb/smiledb.h:16: error: 'Smile::SmileDB::SmileDB()' is private
../comun/formas/login/../../smiledb/../singleton.h:40: error: within this context

dmail · 02-20-2006, 09:38 AM

Really?
Did you include this line in the inherited class?

Code:

	friend class Pattern::Singleton<CLASS_NAME_HERE>;

If you did can I see your code please, because this does and should work. ;( It gives the singleton access to private in the inherited class.