[gdal-dev] replacing a dataset in C++

[Even Rouault]

Le 11/05/2025 à 00:20, Michael Sumner via gdal-dev a écrit :
> Hello, I have a question about safely re-casting a dataset to another, 
> I want to use Multidim to re-orient an MDArray as a classic 2D raster. 
> I was a bit surprised that this works (I just overwrite poSrcDS with 
> the classic form):
>
> I'm passing in 'osFilename', 'osArrayName' and two dimension indexes 
> 'index_x' and 'index_y', the first "Open()" line is a literal copy 
> from vrtdataset.cpp:
>
>
> // I've removed all the nullptr checks from this code
> auto poSrcDS = std::unique_ptr<GDALDataset, GDALDatasetUniquePtrReleaser>(
>          GDALDataset::Open(osFilename, GDAL_OF_RASTER | 
> GDAL_OF_VERBOSE_ERROR,
>                           aosAllowedDrivers.List(), aosOpenOptions.List(),
>                           nullptr));
> // now open in mdim mode, and get the specified MDArray in Classic 
> form with index_x,index_y
> auto poMDimDS = 
> std::unique_ptr<GDALDataset>(GDALDataset::Open(osFilename, 
> GDAL_OF_MULTIDIM_RASTER));
> auto poMdimGroup = poMDimDS->GetRootGroup();
> auto poArrayDS = poMdimGroup->OpenMDArrayFromFullname(osFilename);
> auto poClassicDS = poArrayDS->AsClassicDataset(index_x, index_y);
>
> // FIXME how to safely swap poSrcDS with poClassicDS
> poSrcDS = std::unique_ptr<GDALDataset, 
> GDALDatasetUniquePtrReleaser>(poClassicDS);
>
> My question is about the last line where poClassic "replaces" poSrcDS: 
> what am I responsible for to make that safe?

That's perfectly fine. In the mdim API, objects returned by 
GetRootGroup(), OpenMDArray(), AsClassicDataset(), etc. , keep the 
necessary parent objects alive through shared_ptr mechanism. Furthermore 
in your example, there is no relationship between poSrcDS et 
poClassicDS, so even if that wasn't the case, your final assignment 
would be still safe.

-- 
http://www.spatialys.com
My software is free, but my time generally not.
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