expressions: let concat handle NULL like empty string (like in postgres)

python/core/qgsexpression.sip

@@ -194,7 +194,14 @@ class QgsExpression
     class Function
     {
       public:
-        Function( const QString& fnname, int params, const QString& group, const QString& helpText = QString(), bool usesGeometry = false, QStringList referencedColumns = QStringList(), bool lazyEval = false );
+        Function( const QString& fnname,
+                  int params,
+                  QString group,
+                  QString helpText = QString(),
+                  bool usesGeometry = false,
+                  QStringList referencedColumns = QStringList(),
+                  bool lazyEval = false,
+                  bool handlesNull = false );
 
         virtual ~Function();
 
@@ -225,11 +232,11 @@ class QgsExpression
         const QString helptext();
 
         virtual QVariant func( const QVariantList& values, const QgsFeature* f, QgsExpression* parent ) = 0;
-    };
 
+        virtual bool handlesNull() const;
+    };
 
     static const QList<QgsExpression::Function *>& Functions();
-
     static const QStringList& BuiltinFunctions();
 
     static bool registerFunction( Function* function );
@@ -310,6 +317,7 @@ class QgsExpression
       public:
         NodeList();
         ~NodeList();
+        /** Takes ownership of the provided node */
         void append( QgsExpression::Node* node /Transfer/ );
         int count();
         const QList<QgsExpression::Node*>& list();

src/core/qgsexpression.cpp

@@ -1741,7 +1741,7 @@ const QList<QgsExpression::Function*>& QgsExpression::Functions()
     << new StaticFunction( "to_date", 1, fcnToDate, "Conversions", QString(), false, QStringList(), false, QStringList() << "todate" )
     << new StaticFunction( "to_time", 1, fcnToTime, "Conversions", QString(), false, QStringList(), false, QStringList() << "totime" )
     << new StaticFunction( "to_interval", 1, fcnToInterval, "Conversions", QString(), false, QStringList(), false, QStringList() << "tointerval" )
-    << new StaticFunction( "coalesce", -1, fcnCoalesce, "Conditionals" )
+    << new StaticFunction( "coalesce", -1, fcnCoalesce, "Conditionals", QString(), false, QStringList(), false, QStringList(), true )
     << new StaticFunction( "if", 3, fcnIf, "Conditionals", "", False, QStringList(), true )
     << new StaticFunction( "regexp_match", 2, fcnRegexpMatch, "Conditionals" )
     << new StaticFunction( "now", 0, fcnNow, "Date and Time", QString(), false, QStringList(), false, QStringList() << "$now" )
@@ -1763,7 +1763,7 @@ const QList<QgsExpression::Function*>& QgsExpression::Functions()
     << new StaticFunction( "regexp_replace", 3, fcnRegexpReplace, "String" )
     << new StaticFunction( "regexp_substr", 2, fcnRegexpSubstr, "String" )
     << new StaticFunction( "substr", 3, fcnSubstr, "String" )
-    << new StaticFunction( "concat", -1, fcnConcat, "String" )
+    << new StaticFunction( "concat", -1, fcnConcat, "String", QString(), false, QStringList(), false, QStringList(), true )
     << new StaticFunction( "strpos", 2, fcnStrpos, "String" )
     << new StaticFunction( "left", 2, fcnLeft, "String" )
     << new StaticFunction( "right", 2, fcnRight, "String" )
@@ -2638,7 +2638,7 @@ QVariant QgsExpression::NodeFunction::eval( QgsExpression* parent, const QgsFeat
       {
         v = n->eval( parent, f );
         ENSURE_NO_EVAL_ERROR;
-        if ( isNull( v ) && fd->name() != "coalesce" )
+        if ( isNull( v ) && !fd->handlesNull() )
           return QVariant(); // all "normal" functions return NULL, when any parameter is NULL (so coalesce is abnormal)
       }
       argValues.append( v );

src/core/qgsexpression.h

@@ -286,8 +286,23 @@ class CORE_EXPORT QgsExpression
     class CORE_EXPORT Function
     {
       public:
-        Function( const QString& fnname, int params, QString group, QString helpText = QString(), bool usesGeometry = false, QStringList referencedColumns = QStringList(), bool lazyEval = false )
-            : mName( fnname ), mParams( params ), mUsesGeometry( usesGeometry ), mGroup( group ), mHelpText( helpText ), mReferencedColumns( referencedColumns ), mLazyEval( lazyEval ) {}
+        Function( const QString& fnname,
+                  int params,
+                  QString group,
+                  QString helpText = QString(),
+                  bool usesGeometry = false,
+                  QStringList referencedColumns = QStringList(),
+                  bool lazyEval = false,
+                  bool handlesNull = false )
+            : mName( fnname )
+            , mParams( params )
+            , mUsesGeometry( usesGeometry )
+            , mGroup( group )
+            , mHelpText( helpText )
+            , mReferencedColumns( referencedColumns )
+            , mLazyEval( lazyEval )
+            , mHandlesNull( handlesNull )
+        {}
 
         virtual ~Function() {}
 
@@ -327,6 +342,8 @@ class CORE_EXPORT QgsExpression
           return false;
         }
 
+        virtual bool handlesNull() const { return mHandlesNull; }
+
       private:
         QString mName;
         int mParams;
@@ -335,13 +352,26 @@ class CORE_EXPORT QgsExpression
         QString mHelpText;
         QStringList mReferencedColumns;
         bool mLazyEval;
+        bool mHandlesNull;
     };
 
     class StaticFunction : public Function
     {
       public:
-        StaticFunction( QString fnname, int params, FcnEval fcn, QString group, QString helpText = QString(), bool usesGeometry = false, QStringList referencedColumns = QStringList(), bool lazyEval = false, const QStringList& aliases = QStringList() )
-            : Function( fnname, params, group, helpText, usesGeometry, referencedColumns, lazyEval ), mFnc( fcn ), mAliases( aliases ) {}
+        StaticFunction( QString fnname,
+                        int params,
+                        FcnEval fcn,
+                        QString group,
+                        QString helpText = QString(),
+                        bool usesGeometry = false,
+                        QStringList referencedColumns = QStringList(),
+                        bool lazyEval = false,
+                        const QStringList& aliases = QStringList(),
+                        bool handlesNull = false )
+            : Function( fnname, params, group, helpText, usesGeometry, referencedColumns, lazyEval, handlesNull )
+            , mFnc( fcn )
+            , mAliases( aliases )
+        {}
 
         virtual ~StaticFunction() {}
 

tests/src/core/testqgsexpression.cpp

@@ -393,7 +393,8 @@ class TestQgsExpression: public QObject
       QTest::newRow( "wordwrap" ) << "wordwrap('university of qgis\nsupports many multiline',-5,' ')" << false << QVariant( "university\nof qgis\nsupports\nmany multiline" );
       QTest::newRow( "format" ) << "format('%1 %2 %3 %1', 'One', 'Two', 'Three')" << false << QVariant( "One Two Three One" );
       QTest::newRow( "concat" ) << "concat('a', 'b', 'c', 'd')" << false << QVariant( "abcd" );
-      QTest::newRow( "concat single" ) << "concat('a')" << false << QVariant( "a" );
+      QTest::newRow( "concat function single" ) << "concat('a')" << false << QVariant( "a" );
+      QTest::newRow( "concat function with NULL" ) << "concat(NULL,'a','b')" << false << QVariant( "ab" );
 
       // implicit conversions
       QTest::newRow( "implicit int->text" ) << "length(123)" << false << QVariant( 3 );