Streamline singleton behavior in a class QgsSingleton

If a singleton is unused in an application, it's not going to be created just
to be deleted instantanously on app exit.
QgsCoordinateTransformCache will also be cleaned on exit.

src/core/CMakeLists.txt

@@ -557,6 +557,7 @@ SET(QGIS_CORE_HDRS
   qgssimplifymethod.h
   qgssnapper.h
   qgsspatialindex.h
+  qgssingleton.h
   qgstolerance.h
   qgsvectordataprovider.h
   qgsvectorlayercache.h

src/core/qgsapplication.cpp

@@ -14,12 +14,13 @@
  ***************************************************************************/
 
 #include "qgsapplication.h"
+#include "qgscrscache.h"
+#include "qgsexception.h"
+#include "qgsgeometry.h"
 #include "qgslogger.h"
 #include "qgsmaplayerregistry.h"
-#include "qgsproviderregistry.h"
 #include "qgsnetworkaccessmanager.h"
-#include "qgsexception.h"
-#include "qgsgeometry.h"
+#include "qgsproviderregistry.h"
 
 #include <QDir>
 #include <QFile>
@@ -607,11 +608,13 @@ void QgsApplication::initQgis()
 
 void QgsApplication::exitQgis()
 {
-  delete QgsMapLayerRegistry::instance();
-
+  // Cleanup providers
   delete QgsProviderRegistry::instance();
 
-  delete QgsNetworkAccessManager::instance();
+  // Cleanup known singletons
+  QgsMapLayerRegistry::cleanup();
+  QgsNetworkAccessManager::cleanup();
+  QgsCoordinateTransformCache::cleanup();
 }
 
 QString QgsApplication::showSettings()

src/core/qgscrscache.cpp

@@ -18,20 +18,15 @@
 #include "qgscrscache.h"
 #include "qgscoordinatetransform.h"
 
-
-QgsCoordinateTransformCache* QgsCoordinateTransformCache::instance()
-{
-  static QgsCoordinateTransformCache mInstance;
-  return &mInstance;
-}
-
 QgsCoordinateTransformCache::~QgsCoordinateTransformCache()
 {
   QHash< QPair< QString, QString >, QgsCoordinateTransform* >::const_iterator tIt = mTransforms.constBegin();
   for ( ; tIt != mTransforms.constEnd(); ++tIt )
   {
     delete tIt.value();
   }
+
+  mTransforms.clear();
 }
 
 const QgsCoordinateTransform* QgsCoordinateTransformCache::transform( const QString& srcAuthId, const QString& destAuthId, int srcDatumTransform, int destDatumTransform )

src/core/qgscrscache.h

@@ -19,16 +19,16 @@
 #define QGSCRSCACHE_H
 
 #include "qgscoordinatereferencesystem.h"
+#include "qgssingleton.h"
 #include <QHash>
 
 class QgsCoordinateTransform;
 
 /**Cache coordinate transform by authid of source/dest transformation to avoid the
 overhead of initialisation for each redraw*/
-class CORE_EXPORT QgsCoordinateTransformCache
+class CORE_EXPORT QgsCoordinateTransformCache : public QgsSingleton<QgsCoordinateTransformCache>
 {
   public:
-    static QgsCoordinateTransformCache* instance();
     ~QgsCoordinateTransformCache();
     /**Returns coordinate transformation. Cache keeps ownership
         @param srcAuthId auth id string of source crs
@@ -41,7 +41,6 @@ class CORE_EXPORT QgsCoordinateTransformCache
     void invalidateCrs( const QString& crsAuthId );
 
   private:
-    static QgsCoordinateTransformCache* mInstance;
     QMultiHash< QPair< QString, QString >, QgsCoordinateTransform* > mTransforms; //same auth_id pairs might have different datum transformations
 };
 

src/core/qgsmaplayerregistry.cpp

@@ -19,19 +19,6 @@
 #include "qgsmaplayer.h"
 #include "qgslogger.h"
 
-//
-// Static calls to enforce singleton behaviour
-//
-QgsMapLayerRegistry *QgsMapLayerRegistry::mInstance = 0;
-QgsMapLayerRegistry *QgsMapLayerRegistry::instance()
-{
-  if ( mInstance == 0 )
-  {
-    mInstance = new QgsMapLayerRegistry();
-  }
-  return mInstance;
-}
-
 //
 // Main class begins now...
 //

src/core/qgsmaplayerregistry.h

@@ -23,21 +23,20 @@
 #include <QSet>
 #include <QObject>
 #include <QStringList>
+
+#include "qgssingleton.h"
 class QString;
 class QgsMapLayer;
 
 /** \ingroup core
 * This class tracks map layers that are currently loaded and provides
 * a means to fetch a pointer to a map layer and delete it.
 */
-class CORE_EXPORT QgsMapLayerRegistry : public QObject
+class CORE_EXPORT QgsMapLayerRegistry : public QObject, public QgsSingleton<QgsMapLayerRegistry>
 {
     Q_OBJECT
 
   public:
-    //! Returns the instance pointer, creating the object on the first call
-    static QgsMapLayerRegistry * instance();
-
     //! Return the number of registered layers.
     int count();
 
@@ -239,9 +238,10 @@ class CORE_EXPORT QgsMapLayerRegistry : public QObject
     //! private singleton constructor
     QgsMapLayerRegistry( QObject * parent = 0 );
 
-    static QgsMapLayerRegistry *mInstance;
     QMap<QString, QgsMapLayer*> mMapLayers;
     QSet<QgsMapLayer*> mOwnedLayers;
+
+    friend class QgsSingleton; // Let QgsSingleton access private constructor
 }; // class QgsMapLayerRegistry
 
 #endif //QgsMapLayerRegistry_H

src/core/qgsnetworkaccessmanager.cpp

@@ -86,19 +86,6 @@ class QgsNetworkProxyFactory : public QNetworkProxyFactory
     }
 };
 
-//
-// Static calls to enforce singleton behaviour
-//
-QgsNetworkAccessManager* QgsNetworkAccessManager::sInstance = 0;
-QgsNetworkAccessManager* QgsNetworkAccessManager::instance()
-{
-  if ( sInstance == 0 )
-  {
-    sInstance = new QgsNetworkAccessManager();
-  }
-  return sInstance;
-}
-
 QgsNetworkAccessManager::QgsNetworkAccessManager( QObject *parent )
     : QNetworkAccessManager( parent )
     , mUseSystemProxy( false )

src/core/qgsnetworkaccessmanager.h

@@ -24,6 +24,8 @@
 #include <QNetworkProxy>
 #include <QNetworkRequest>
 
+#include "qgssingleton.h"
+
 /*
  * \class QgsNetworkAccessManager
  * \brief network access manager for QGIS
@@ -41,15 +43,11 @@
  * that the fallback proxy should not be used for, then no proxy will be used.
  *
  */
-class CORE_EXPORT QgsNetworkAccessManager : public QNetworkAccessManager
+class CORE_EXPORT QgsNetworkAccessManager : public QNetworkAccessManager, public QgsSingleton<QgsNetworkAccessManager>
 {
     Q_OBJECT
 
   public:
-    //! returns a pointer to the single instance
-    // and creates that instance on the first call.
-    static QgsNetworkAccessManager* instance();
-
     QgsNetworkAccessManager( QObject *parent = 0 );
 
     //! destructor
@@ -100,7 +98,6 @@ class CORE_EXPORT QgsNetworkAccessManager : public QNetworkAccessManager
     QNetworkProxy mFallbackProxy;
     QStringList mExcludedURLs;
     bool mUseSystemProxy;
-    static QgsNetworkAccessManager* sInstance;
 };
 
 #endif // QGSNETWORKACCESSMANAGER_H

src/core/qgsproviderregistry.cpp

@@ -208,7 +208,7 @@ QgsProviderRegistry::~QgsProviderRegistry()
 
   while ( it != mProviders.end() )
   {
-    QgsDebugMsg( QString( "cleanup:%1" ).arg( it->first ) );
+    QgsDebugMsg( QString( "cleanup: %1" ).arg( it->first ) );
     QString lib = it->second->library();
     QLibrary myLib( lib );
     if ( myLib.isLoaded() )
@@ -405,7 +405,7 @@ QWidget* QgsProviderRegistry::selectWidget( const QString & providerKey,
 
 #if QT_VERSION >= 0x050000
 QFunctionPointer QgsProviderRegistry::function( QString const & providerKey,
-                                                QString const & functionName )
+    QString const & functionName )
 {
   QLibrary myLib( library( providerKey ) );
 

src/core/qgssingleton.h

@@ -0,0 +1,57 @@
+/***************************************************************************
+    qgssingleton.h
+     --------------------------------------
+    Date                 : 24.11.2014
+    Copyright            : (C) 2014 Matthias Kuhn
+    Email                : matthias dot kuhn at gmx dot ch
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#ifndef QGSSINGLETON_H
+#define QGSSINGLETON_H
+
+template <typename T>
+class QgsSingleton
+{
+  public:
+    static T* instance()
+    {
+      if ( sInstance == 0 )
+      {
+        sInstance = createInstance();
+      }
+      return sInstance;
+    }
+
+    static void cleanup()
+    {
+      delete sInstance;
+      sInstance = 0;
+    }
+
+  protected:
+    virtual ~QgsSingleton() {}
+
+    explicit QgsSingleton()
+    {
+      Q_ASSERT( sInstance == 0 );
+      sInstance = static_cast<T*>( this );
+    }
+
+  private:
+    static T* sInstance;
+    static T* createInstance()
+    {
+      return new T;
+    }
+};
+
+template <typename T> T* QgsSingleton<T>::sInstance = 0;
+
+#endif // QGSSINGLETON_H