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[LucasDanzinger]

A Basemap object is basically a collection of baseLayers and referenceLayers, so if you wanted to turn off the "Basemap", you would need to iterate through the collections and set the visibility to false - Basemap QML Type | ArcGIS for Developers 

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[LucasDanzinger · Accepted solution]

you would need to create a function that loops through the baseLayers and referenceLayers of your basemap, and set visibility of all of those baseLayers and referenceLayers to false.

[MarcWouters]

With your hint, I managed hide the basemap by simply clearing the baselayers and referencelayers : 

_map->basemap()->baseLayers()->clear();
_map->basemap()->referenceLayers()->clear();

But there is still a gray raster visible, is there a way to get rid of this as well ?

Also when I select a SoilSurveyMap (Specialty/Soil_Survey_Map (MapServer) ), the raster is seen where otherwise water is shown. This is a bit annoying