Qt Singleton Class

[alexismedina]

Hello Devs,

I'm trying to use a singleton class, here's my code (it's really simple):

dao.h

class DAO
{
private:
    DAO();
    ~DAO();
     static DAO* m_dao;
public:
    static DAO* getInstance();
    ...
}

class DAO
{
private:
    DAO();
    ~DAO();
     static DAO* m_dao;
public:
    static DAO* getInstance();
    ...
}

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dao.cpp

#include "dao.h"
 
DAO::DAO() { }
 
DAO::~DAO() { }
 
DAO* DAO::getInstance() {
 
    if ( ! m_dao ) {    // << This causes a crash 
        m_dao = new DAO();
    }
 
    return m_dao;
}

#include "dao.h"

DAO::DAO() { }

DAO::~DAO() { }

DAO* DAO::getInstance() {

    if ( ! m_dao ) {    // << This causes a crash 
        m_dao = new DAO();
    }

    return m_dao;
}

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When trying to build the project, I have this message:

dap.cpp:9: undefined reference to `DAO::m_dao'
dao.cpp:10: undefined reference to `DAO::m_dao'
:-1: error: collect2: ld returned 1 exit status

Am I missing something?. I'm using Qt 4.6.32 with vista 64, Qt Creator 1.3.1

[zuck]

Try this:

dao.cpp

DAO* DAO::m_dao = 0;

DAO* DAO::m_dao = 0;

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[SneakyPeterson]

I believe your problem comes from the fact that you are trying to test for null using
( ! m_dao )
I was able to compile a very similar case, but I test using
if(m_dao == NULL)

[alexismedina]

Try this:

dao.cpp

DAO* DAO::m_dao = 0;

DAO* DAO::m_dao = 0;

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This one did the trick! Thanks.