Don't crash when iterating empty QgsFields

Author: nyalldawson

src/core/qgsfield.cpp

@@ -441,32 +441,50 @@ bool QgsFields::operator==( const QgsFields &other ) const
 
 QgsFields::const_iterator QgsFields::constBegin() const noexcept
 {
+  if ( d->fields.isEmpty() )
+    return const_iterator();
+
   return const_iterator( &d->fields.first() );
 }
 
 QgsFields::const_iterator QgsFields::constEnd() const noexcept
 {
+  if ( d->fields.isEmpty() )
+    return const_iterator();
+
   return const_iterator( &d->fields.last() + 1 );
 }
 
 QgsFields::const_iterator QgsFields::begin() const noexcept
 {
+  if ( d->fields.isEmpty() )
+    return const_iterator();
+
   return const_iterator( &d->fields.first() );
 }
 
 QgsFields::const_iterator QgsFields::end() const noexcept
 {
+  if ( d->fields.isEmpty() )
+    return const_iterator();
+
   return const_iterator( &d->fields.last() + 1 );
 }
 
 QgsFields::iterator QgsFields::begin()
 {
+  if ( d->fields.isEmpty() )
+    return iterator();
+
   d.detach();
   return iterator( &d->fields.first() );
 }
 
 QgsFields::iterator QgsFields::end()
 {
+  if ( d->fields.isEmpty() )
+    return iterator();
+
   d.detach();
   return iterator( &d->fields.last() + 1 );
 }

tests/src/core/testqgsfields.cpp

@@ -494,6 +494,10 @@ void TestQgsFields::qforeach()
 void TestQgsFields::iterator()
 {
   QgsFields fields;
+
+  //test with empty fields
+  QCOMPARE( fields.begin(), fields.end() );
+
   QgsField field( QString( "1" ) );
   fields.append( field );
   QgsField field2( QString( "2" ) );
@@ -531,6 +535,17 @@ void TestQgsFields::iterator()
 void TestQgsFields::constIterator()
 {
   QgsFields fields;
+
+  //test with empty fields
+  QCOMPARE( fields.constBegin(), fields.constEnd() );
+  QCOMPARE( const_cast< const QgsFields* >( &fields )->begin(), const_cast< const QgsFields* >( &fields )->end() );
+  Q_FOREACH ( const QgsField& f, fields )
+  {
+    Q_UNUSED( f );
+    //should not be called!
+    QVERIFY( false );
+  }
+
   QgsField field( QString( QString( "1" ) ) );
   fields.append( field );
   QgsField field2( QString( QString( "2" ) ) );