Question

I’m trying to use a Singleton Class (its a program-wide debug logger called ‘PrisLog’) across my Qt Application. The program also has plugins. I want to make my singleton class available to those plugins, but this doesn’t work. From what I can tell, trying to use the class in the plugin results in another instance being created.

-The singleton class is just a *.cpp and *.h file, nothing else. I’ve linked both my main application and the plugin to these files individually… is this the right way to do it?

-I’ve attached my singleton class’s code below, though I think I’ve created the class correctly. If I use it from within separate classes in my main application, it works as expected (one instance).

EDIT: Linking both the application and plugin to the same static lib (the singleton class) works. Here’s how my qmake *.pro files looked:

MySingletonLib.pro

TEMPLATE = lib

CONFIG += staticlib

HEADERS += \
mysingletonlib.h

SOURCES += \
mysingletonlib.cpp

MyPlugin.pro (also incl #include mysingletonlib.h in myplugin.h)

INCLUDEPATH += path/to/MySingletonLib

LIBS += -Lpath/to/MySingletonLib -lMySingletonLib

MyPlugin.pro (also incl #include mysingletonlib.h in myapp.h)

INCLUDEPATH += path/to/MySingletonLib

LIBS += -Lpath/to/MySingletonLib -lMySingletonLib

And the original code:

#ifndef PRISLOG_H
#define PRISLOG_H

#include <QFile>
#include <QDir>
#include <QString>
#include <QMutex>
#include <QDebug>
#include <QMutexLocker>
#include <QTextStream>
#include <QDateTime>

// PrisLog (singleton) class definition
class PrisLog
{

public:
    static PrisLog* Instance();

    void SetLogsPath(QString);
    QString GetLogsPath();

    void SetDebugDestination(QString);
    void SetElmRxDestination(QString);
    void SetElmTxDestination(QString);
    void SetDlgDestination(QString);

    QTextStream* GetDebugStream();
    QTextStream* GetElmRxStream();
    QTextStream* GetElmTxStream();
    QTextStream* GetDlgStream();

    QMutex* GetDebugMutex();

private:
    PrisLog();                          // private constructor
    PrisLog(const PrisLog&);            // prevent copy constructor
    PrisLog& operator=(const PrisLog&); // prevent assignment

    static PrisLog* m_Instance;
    static bool m_InitFlag;

    QString m_appPath;

    QFile m_DebugFile;
    QTextStream m_DebugStream;
    QMutex m_DebugMutex;

    QFile m_ElmRxFile;
    QTextStream m_ElmRxStream;

    QFile m_ElmTxFile;
    QTextStream m_ElmTxStream;

    QFile m_DlgFile;
    QTextStream m_DlgStream;

};

// thread-UNSAFE writer, but less expensive
// use: single stream <--> single thread!
class PrisLogWriter
{

public:
    PrisLogWriter(QTextStream*);
    ~PrisLogWriter();

    QTextStream* m_stream;
};

// thread-UNSAFE writer, but less expensive
// this version does not include any formatting
// use: single stream <--> single thread!
class PrisLogRawWriter
{

public:
    PrisLogRawWriter(QTextStream*);
    ~PrisLogRawWriter();

    QTextStream* m_stream;
};

// thread-safe writer
// use: single stream <--> many threads
class PrisLogSafeWriter
{

public:
    PrisLogSafeWriter(QTextStream*, QMutex*);
    ~PrisLogSafeWriter();

    QTextStream* m_stream;

private:
    QMutex* m_mutex;
};


#define PRISLOGDEBUG (*(PrisLogSafeWriter(PrisLog::Instance()->GetDebugStream(), PrisLog::Instance()->GetDebugMutex()).m_stream))
#define PRISLOGELMRX (*(PrisLogWriter(PrisLog::Instance()->GetElmRxStream()).m_stream))
#define PRISLOGELMTX (*(PrisLogWriter(PrisLog::Instance()->GetElmTxStream()).m_stream))
#define PRISLOGDLG (*(PrisLogRawWriter(PrisLog::Instance()->GetDlgStream()).m_stream))


#endif // PRISLOG_H

Answer 1

I think you should take this class to a statically linked, but separeted shared dll/so.

If your host application does not use this class, the linker simply won’t link it into the binary and your plugin could not use it. Additionally: your binary has no library class interface.