Changing symbol and color of vector Layer using QGIS 3 C++ APIs

Question

I am trying to change the default symbol for the point/line/ polygon vector layer using QGIS3 C++ APIs.

I was able to render a vector layer with default symbols on QgsMapCanvas. When I try to change the default symbol of the point vector layer, somehow it is not changing. When I am trying to change color, it does not change color as expected, but the symbol color changes in a different color. Here is the piece of code.

layer->startEditing();     
qDebug()<< "is editable " << layer->isEditable();  // returns true
QgsFeatureRenderer * layerRenderer= layer->renderer();
QgsSingleSymbolRenderer *mSingleRenderer = QgsSingleSymbolRenderer::convertFromRenderer(layerRenderer);
QgsMarkerSymbol* symbol = new  QgsMarkerSymbol();
QgsStringMap mp;
mp[QString("name")]= QString("square");
mp[QString("color")]=QString("yellow");
symbol->createSimple(mp);
symbol->setColor(QColor(255,255,255));
mSingleRenderer->setSymbol(symbol);
layer->triggerRepaint();
mLayerTreeView->refreshLayerSymbology(layer->id());
layer->commitChanges();

variable-name layer defines QgsVectorLayer.

Answer 1

A Python correction to get your issues (moreover, was too long to get a minimum C++ project to debug ;) )

layer = iface.activeLayer()

layerRenderer= layer.renderer()
mSingleRenderer = QgsSingleSymbolRenderer.convertFromRenderer(layerRenderer)
# No need to instantiate here
#symbol = QgsMarkerSymbol()

style = {
    "name": "square",
    "color": "yellow"
}
# Issue 1: you create a symbol but do not assign it
# (not void function, it returns a new element)
new_sym = QgsMarkerSymbol.createSimple(style)
# Commented as it was already working with above code
# symbol.setColor(QColor(255,255,255));
mSingleRenderer.setSymbol(new_sym)
# Issue 2: Forgot to reassign the newly created renderer to your layer
layer.setRenderer(mSingleRenderer)
layer.triggerRepaint()

# Did not test, commented so can be tested "standalone" in PyQGIS
# mLayerTreeView.refreshLayerSymbology(layer.id())