QEP 314: Relax prohibition against "auto" - exception for use of auto for variables initialized using casts

[nyalldawson]

This change relaxes the prohibition against auto to allow use of auto for variable types when the variable type is explicitly stated during its initialization as the result of a cast operation, such as dynamic_cast, static_cast, const_cast, reinterpret_cast, qobject_cast,
qgsgeometry_cast, or qgis::down_cast

Specifically:

// allowed, the pointer types are explicit during initialization:
auto markerSymbol = dynamic_cast< QgsMarkerSymbol* >( symbol );
auto vectorLayer = qobject_cast< QgsVectorLayer* >( layer );
if ( auto lineString = qgsgeometry_cast< const QgsLineString* >( geometry ) ) { ... }

// NOT allowed, the type is not explicit:
auto myLineStringPtr = downcastGeometryAsLineString( geometry );

Refs comment at #319 (comment)

[m-kuhn]

Comparing the examples

case 1

auto markerSymbol = dynamic_cast< QgsMarkerSymbol* >( symbol );

vs.

QgsMarkerSymbol* markerSymbol = dynamic_cast< QgsMarkerSymbol* >( symbol );

case 2

auto vectorLayer = qobject_cast< QgsVectorLayer* >( layer );

vs.

QgsVectorLayer* vectorLayer = qobject_cast< QgsVectorLayer* >( layer );

case 3

if ( auto lineString = qgsgeometry_cast< const QgsLineString* >( geometry ) ) { ... }

vs.

if ( const QgsLineString* lineString = qgsgeometry_cast< const QgsLineString* >( geometry ) ) { ... }

I personally always prefer the explicit version over the auto version, it provides me with more information compared to the "auto" version and I also don't see auto remove a lot of clutter here.

[uclaros]

The class names used in the examples are small enough to not make a big difference between explicit and auto.
When using longer classes though, the difference is also bigger:

const QgsVectorLayerUndoPassthroughCommandChangeGeometry *merge = dynamic_cast<const QgsVectorLayerUndoPassthroughCommandChangeGeometry *>( other );

vs

auto merge = dynamic_cast<const QgsVectorLayerUndoPassthroughCommandChangeGeometry *>( other );