Const correctness for numerous data provider methods

Author: nyalldawson

doc/api_break.dox

@@ -18,6 +18,21 @@ This page tries to maintain a list with incompatible changes that happened in pr
 
 \section qgis_api_break_3_0 QGIS 3.0
 
+\subsection qgis_api_break_3_0_DataProviders Data Providers
+
+<ul>
+<li>Many methods in QgsDataProvider, QgsVectorDataProvider and QgsRasterDataProvider have been made const-correct.
+This has no effect on PyQGIS code, but c++ code implementing third-party providers will need to update the
+signatures of these methods to match. Affected methods are:
+<ul>
+<li>QgsDataProvider: crs(), extent(), isValid(), supportsSubsetString(), subsetString()</li>
+<li>QgsVectorDataProvider: getFeatures(), minimumValue(), maximumValue(), uniqueValues(), enumValues(), defaultValue(),
+attributeIndexes(), pkAttributeIndexes(), isSaveAndLoadStyleToDBSupported()</li>
+<li>QgsRasterInterface: extent()</li>
+</ul
+</li>
+</ul>
+
 \subsection qgis_api_break_3_0_QgsVectorFileWriter QgsVectorFileWriter
 
 <ul>

python/core/qgsdataprovider.sip

@@ -54,12 +54,11 @@ class QgsDataProvider : QObject
     virtual ~QgsDataProvider();
 
 
-    /** Get the QgsCoordinateReferenceSystem for this layer
-     * @note Must be reimplemented by each provider.
-     * If the provider isn't capable of returning
-     * its projection an empty srs will be return, ti will return 0
+    /* Returns the coordinate system for the data source.
+     * If the provider isn't capable of returning its projection then an invalid
+     * QgsCoordinateReferenceSystem will be returned.
      */
-    virtual QgsCoordinateReferenceSystem crs() = 0;
+    virtual QgsCoordinateReferenceSystem crs() const = 0;
 
 
     /**
@@ -82,21 +81,21 @@ class QgsDataProvider : QObject
 
 
     /**
-     * Get the extent of the layer
+     * Returns the extent of the layer.
      * @return QgsRectangle containing the extent of the layer
      */
-    virtual QgsRectangle extent() = 0;
+    virtual QgsRectangle extent() const = 0;
 
 
     /**
      * Returns true if this is a valid layer. It is up to individual providers
-     * to determine what constitutes a valid layer
+     * to determine what constitutes a valid layer.
      */
-    virtual bool isValid() = 0;
+    virtual bool isValid() const = 0;
 
 
     /**
-     * Update the extents of the layer. Not implemented by default
+     * Update the extents of the layer. Not implemented by default.
      */
     virtual void updateExtents();
 
@@ -109,16 +108,16 @@ class QgsDataProvider : QObject
      */
     virtual bool setSubsetString( const QString& subset, bool updateFeatureCount = true );
 
-    /** Provider supports setting of subset strings */
-    virtual bool supportsSubsetString();
+    /** Returns true if the provider supports setting of subset strings. */
+    virtual bool supportsSubsetString() const;
 
     /**
      * Returns the subset definition string (typically sql) currently in
      * use by the layer and used by the provider to limit the feature set.
      * Must be overridden in the dataprovider, otherwise returns a null
      * QString.
      */
-    virtual QString subsetString();
+    virtual QString subsetString() const;
 
 
     /**

python/core/qgsvectordataprovider.sip

@@ -98,8 +98,10 @@ class QgsVectorDataProvider : QgsDataProvider
 
     /**
      * Query the provider for features specified in request.
+     * @param request feature request describing parameters of features to return
+     * @returns iterator for matching features from provider
      */
-    virtual QgsFeatureIterator getFeatures( const QgsFeatureRequest& request = QgsFeatureRequest() ) = 0;
+    virtual QgsFeatureIterator getFeatures( const QgsFeatureRequest& request = QgsFeatureRequest() ) const = 0;
 
     /**
      * Get feature type.
@@ -134,7 +136,7 @@ class QgsVectorDataProvider : QgsDataProvider
      * and maximal values. If provider has facilities to retrieve minimal
      * value directly, override this function.
      */
-    virtual QVariant minimumValue( int index );
+    virtual QVariant minimumValue( int index ) const;
 
     /**
      * Returns the maximum value of an attribute
@@ -144,7 +146,7 @@ class QgsVectorDataProvider : QgsDataProvider
      * and maximal values. If provider has facilities to retrieve maximal
      * value directly, override this function.
      */
-    virtual QVariant maximumValue( int index );
+    virtual QVariant maximumValue( int index ) const;
 
     /**
      * Return unique values of an attribute
@@ -154,7 +156,7 @@ class QgsVectorDataProvider : QgsDataProvider
      *
      * Default implementation simply iterates the features
      */
-    virtual void uniqueValues( int index, QList<QVariant> &uniqueValues /Out/, int limit = -1 );
+    virtual void uniqueValues( int index, QList<QVariant> &uniqueValues /Out/, int limit = -1 ) const;
 
     /** Calculates an aggregated value from the layer's features. The base implementation does nothing,
      * but subclasses can override this method to handoff calculation of aggregates to the provider.
@@ -170,15 +172,15 @@ class QgsVectorDataProvider : QgsDataProvider
                                 int index,
                                 const QgsAggregateCalculator::AggregateParameters& parameters,
                                 QgsExpressionContext* context,
-                                bool& ok );
+                                bool& ok ) const;
 
     /**
      * Returns the possible enum values of an attribute. Returns an empty stringlist if a provider does not support enum types
      * or if the given attribute is not an enum type.
      * @param index the index of the attribute
      * @param enumList reference to the list to fill
      */
-    virtual void enumValues( int index, QStringList& enumList /Out/ );
+    virtual void enumValues( int index, QStringList& enumList /Out/ ) const;
 
     /**
      * Adds a list of features
@@ -236,7 +238,7 @@ class QgsVectorDataProvider : QgsDataProvider
     /**
      * Returns the default value for field specified by @c fieldId
      */
-    virtual QVariant defaultValue( int fieldId );
+    virtual QVariant defaultValue( int fieldId ) const;
 
     /**
      * Changes geometries of existing features
@@ -291,12 +293,12 @@ class QgsVectorDataProvider : QgsDataProvider
     /**
      * Return list of indexes to fetch all attributes in nextFeature()
      */
-    virtual QList<int> attributeIndexes();
+    virtual QList<int> attributeIndexes() const;
 
     /**
      * Return list of indexes of fields that make up the primary key
      */
-    virtual QList<int> pkAttributeIndexes();
+    virtual QList<int> pkAttributeIndexes() const;
 
     /**
      * Return list of indexes to names for QgsPalLabeling fix
@@ -333,12 +335,12 @@ class QgsVectorDataProvider : QgsDataProvider
     virtual bool doesStrictFeatureTypeCheck() const;
 
     /** Returns a list of available encodings */
-    static const QStringList &availableEncodings();
+    static QStringList availableEncodings();
 
     /**
      * Provider has errors to report
      */
-    bool hasErrors();
+    bool hasErrors() const;
 
     /**
      * Clear recorded errors
@@ -348,14 +350,14 @@ class QgsVectorDataProvider : QgsDataProvider
     /**
      * Get recorded errors
      */
-    QStringList errors();
+    QStringList errors() const;
 
 
     /**
      * It returns false by default.
      * Must be implemented by providers that support saving and loading styles to db returning true
      */
-    virtual bool isSaveAndLoadStyleToDBSupported();
+    virtual bool isSaveAndLoadStyleToDBSupported() const;
 
     static QVariant convertValue( QVariant::Type type, const QString& value );
 
@@ -384,7 +386,9 @@ class QgsVectorDataProvider : QgsDataProvider
 
   protected:
     void clearMinMaxCache();
-    void fillMinMaxCache();
+
+    //! Populates the cache of minimum and maximum attribute values.
+    void fillMinMaxCache() const;
 
     void pushError( const QString& msg );
 

python/core/raster/qgsrasterdataprovider.sip

@@ -64,7 +64,7 @@ class QgsRasterDataProvider : QgsDataProvider, QgsRasterInterface
     /** Get the extent of the data source.
      * @return QgsRectangle containing the extent of the layer
      */
-    virtual QgsRectangle extent() = 0;
+    virtual QgsRectangle extent() const = 0;
 
     /** Returns data type for the band specified by number */
     virtual QGis::DataType dataType( int bandNo ) const = 0;
@@ -295,7 +295,7 @@ class QgsRasterDataProvider : QgsDataProvider, QgsRasterInterface
     /** Emit a message to be displayed on status bar, usually used by network providers (WMS,WCS)
      * @note added in 2.14
      */
-    void statusChanged( const QString& );
+    void statusChanged( const QString& ) const;
 
   protected:
     /** Read block of data

python/core/raster/qgsrasterinterface.sip

@@ -104,7 +104,7 @@ class QgsRasterInterface
      * Get the extent of the interface.
      * @return QgsRectangle containing the extent of the layer
      */
-    virtual QgsRectangle extent();
+    virtual QgsRectangle extent() const;
 
     int dataTypeSize( int bandNo );
 

src/core/qgsdataprovider.h

@@ -84,12 +84,11 @@ class CORE_EXPORT QgsDataProvider : public QObject
     virtual ~QgsDataProvider() {}
 
 
-    /** Get the QgsCoordinateReferenceSystem for this layer
-     * @note Must be reimplemented by each provider.
-     * If the provider isn't capable of returning
-     * its projection an empty srs will be return, ti will return 0
+    /** Returns the coordinate system for the data source.
+     * If the provider isn't capable of returning its projection then an invalid
+     * QgsCoordinateReferenceSystem will be returned.
      */
-    virtual QgsCoordinateReferenceSystem crs() = 0;
+    virtual QgsCoordinateReferenceSystem crs() const = 0;
 
 
     /**
@@ -126,21 +125,21 @@ class CORE_EXPORT QgsDataProvider : public QObject
 
 
     /**
-     * Get the extent of the layer
+     * Returns the extent of the layer
      * @return QgsRectangle containing the extent of the layer
      */
-    virtual QgsRectangle extent() = 0;
+    virtual QgsRectangle extent() const = 0;
 
 
     /**
      * Returns true if this is a valid layer. It is up to individual providers
-     * to determine what constitutes a valid layer
+     * to determine what constitutes a valid layer.
      */
-    virtual bool isValid() = 0;
+    virtual bool isValid() const = 0;
 
 
     /**
-     * Update the extents of the layer. Not implemented by default
+     * Update the extents of the layer. Not implemented by default.
      */
     virtual void updateExtents()
     {
@@ -163,16 +162,17 @@ class CORE_EXPORT QgsDataProvider : public QObject
     }
 
 
-    /** Provider supports setting of subset strings */
-    virtual bool supportsSubsetString() { return false; }
+    /** Returns true if the provider supports setting of subset strings.
+    */
+    virtual bool supportsSubsetString() const { return false; }
 
     /**
      * Returns the subset definition string (typically sql) currently in
      * use by the layer and used by the provider to limit the feature set.
      * Must be overridden in the dataprovider, otherwise returns a null
      * QString.
      */
-    virtual QString subsetString()
+    virtual QString subsetString() const
     {
       return QString::null;
     }

src/core/qgsvectordataprovider.cpp

@@ -93,7 +93,7 @@ bool QgsVectorDataProvider::changeAttributeValues( const QgsChangedAttributesMap
   return false;
 }
 
-QVariant QgsVectorDataProvider::defaultValue( int fieldId )
+QVariant QgsVectorDataProvider::defaultValue( int fieldId ) const
 {
   Q_UNUSED( fieldId );
   return QVariant();
@@ -275,7 +275,7 @@ QMap<QString, int> QgsVectorDataProvider::fieldNameMap() const
   return resultMap;
 }
 
-QgsAttributeList QgsVectorDataProvider::attributeIndexes()
+QgsAttributeList QgsVectorDataProvider::attributeIndexes() const
 {
   return fields().allAttributesList();
 }
@@ -353,7 +353,7 @@ bool QgsVectorDataProvider::supportedType( const QgsField &field ) const
   return false;
 }
 
-QVariant QgsVectorDataProvider::minimumValue( int index )
+QVariant QgsVectorDataProvider::minimumValue( int index ) const
 {
   if ( index < 0 || index >= fields().count() )
   {
@@ -369,7 +369,7 @@ QVariant QgsVectorDataProvider::minimumValue( int index )
   return mCacheMinValues[index];
 }
 
-QVariant QgsVectorDataProvider::maximumValue( int index )
+QVariant QgsVectorDataProvider::maximumValue( int index ) const
 {
   if ( index < 0 || index >= fields().count() )
   {
@@ -385,7 +385,7 @@ QVariant QgsVectorDataProvider::maximumValue( int index )
   return mCacheMaxValues[index];
 }
 
-void QgsVectorDataProvider::uniqueValues( int index, QList<QVariant> &values, int limit )
+void QgsVectorDataProvider::uniqueValues( int index, QList<QVariant> &values, int limit ) const
 {
   QgsFeature f;
   QgsAttributeList keys;
@@ -409,7 +409,7 @@ void QgsVectorDataProvider::uniqueValues( int index, QList<QVariant> &values, in
 }
 
 QVariant QgsVectorDataProvider::aggregate( QgsAggregateCalculator::Aggregate aggregate, int index,
-    const QgsAggregateCalculator::AggregateParameters& parameters, QgsExpressionContext* context, bool& ok )
+    const QgsAggregateCalculator::AggregateParameters& parameters, QgsExpressionContext* context, bool& ok ) const
 {
   //base implementation does nothing
   Q_UNUSED( aggregate );
@@ -426,7 +426,7 @@ void QgsVectorDataProvider::clearMinMaxCache()
   mCacheMinMaxDirty = true;
 }
 
-void QgsVectorDataProvider::fillMinMaxCache()
+void QgsVectorDataProvider::fillMinMaxCache() const
 {
   if ( !mCacheMinMaxDirty )
     return;
@@ -527,7 +527,7 @@ static bool _compareEncodings( const QString& s1, const QString& s2 )
   return s1.toLower() < s2.toLower();
 }
 
-const QStringList &QgsVectorDataProvider::availableEncodings()
+QStringList QgsVectorDataProvider::availableEncodings()
 {
   if ( smEncodings.isEmpty() )
   {
@@ -595,12 +595,12 @@ void QgsVectorDataProvider::clearErrors()
   mErrors.clear();
 }
 
-bool QgsVectorDataProvider::hasErrors()
+bool QgsVectorDataProvider::hasErrors() const
 {
   return !mErrors.isEmpty();
 }
 
-QStringList QgsVectorDataProvider::errors()
+QStringList QgsVectorDataProvider::errors() const
 {
   return mErrors;
 }