Improve efficiency of load old props

src/core/symbology/qgssymbollayer.cpp

@@ -364,7 +364,7 @@ void QgsSymbolLayer::restoreOldDataDefinedProperties( const QgsStringMap &string
   QgsStringMap::const_iterator propIt = stringMap.constBegin();
   for ( ; propIt != stringMap.constEnd(); ++propIt )
   {
-    QgsProperty prop;
+    std::unique_ptr<QgsProperty> prop;
     QString propertyName;
 
     if ( propIt.key().endsWith( QLatin1String( "_dd_expression" ) ) )
@@ -374,7 +374,7 @@ void QgsSymbolLayer::restoreOldDataDefinedProperties( const QgsStringMap &string
       //get data defined property name by stripping "_dd_expression" from property key
       propertyName = propIt.key().left( propIt.key().length() - 14 );
 
-      prop = propertyFromMap( stringMap, propertyName );
+      prop = qgis::make_unique<QgsProperty>( propertyFromMap( stringMap, propertyName ) );
     }
     else if ( propIt.key().endsWith( QLatin1String( "_expression" ) ) )
     {
@@ -383,7 +383,7 @@ void QgsSymbolLayer::restoreOldDataDefinedProperties( const QgsStringMap &string
       //get data defined property name by stripping "_expression" from property key
       propertyName = propIt.key().left( propIt.key().length() - 11 );
 
-      prop = QgsProperty::fromExpression( propIt.value() );
+      prop = qgis::make_unique<QgsProperty>( QgsProperty::fromExpression( propIt.value() ) );
     }
 
     if ( !prop || !OLD_PROPS.contains( propertyName ) )
@@ -400,7 +400,7 @@ void QgsSymbolLayer::restoreOldDataDefinedProperties( const QgsStringMap &string
         key = QgsSymbolLayer::PropertyStrokeColor;
     }
 
-    setDataDefinedProperty( key, prop );
+    setDataDefinedProperty( key, QgsProperty( *prop.get() ) );
   }
 }