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POQt: singleton logging class get different instance accross plugins and thread
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copied!<p>I have spent hours trying to understand why my logger class does not work. The idea is to hook existing <em>qDebug</em>, <em>qWarning</em> and <em>qFatal</em> to one of my slots and also extend the macro list with new ones (<em>qInfo</em> in this example) I tried to follow the following guidelines: <a href="https://stackoverflow.com/questions/6689370/using-a-singleton-class-across-a-qt-application-and-its-plugins">Using a Singleton Class across a Qt Application and its Plugins</a></p> <pre><code>#pragma once #include <QtGui> #include <QMutex> //! Creates a fake call, to have a cleaner design. extern void qInfo(const char* fmt); #define QtTraceMsg QtMsgType(6) #define QtInfoMsg QtMsgType(7) class MsgHandler: public QObject { Q_OBJECT signals: void newMsg(QtMsgType type, const QString &msg); public: static MsgHandler *instance(); static void Handler(QtMsgType type, const char *msg); private: MsgHandler() { qRegisterMetaType<QtMsgType>("QtMsgType"); } // important static MsgHandler* _instance; }; </code></pre> <p>and cpp:</p> <pre><code>#include "MsgHandler.h" MsgHandler* MsgHandler::_instance = NULL; MsgHandler * MsgHandler::instance() { static QMutex mutex; if (_instance == NULL) { mutex.lock(); if (_instance == NULL) _instance = new MsgHandler; mutex.unlock(); } return _instance; } void MsgHandler::Handler(QtMsgType type, const char *msg) { QString s = msg; emit instance()->newMsg(type, s); } /////////////////////////////////////////////////////////////////////////////// void qInfo(const char *msg) { MsgHandler::instance()->Handler(QtInfoMsg, msg); } </code></pre> <p>then in the main I register my handler</p> <pre><code>int main(int argc, char *argv[]) { QApplication a(argc, argv); // Handle error messages qInstallMsgHandler(MsgHandler::Handler); } </code></pre> <p>Everything works perfectly in the main thread. But from a plugin that runs its own thread it fail:</p> <pre><code>void MyPlugin::run() { qWarning("Test debug"); //works fine MsgHandler::instance()->Handler(QtInfoMsg, "info 2"); //creates a new instance! qInfo("test info"); //also creates a new instance! } </code></pre>

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