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

[Michael Sumner]

Awesome, thanks so much.


Cheers, Mike.

On Sun, May 11, 2025 at 8:29 AM Even Rouault <even.rouault at spatialys.com>
wrote:

>
> 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.
>
>

-- 
Michael Sumner
Research Software Engineer
Australian Antarctic Division
Hobart, Australia
e-mail: mdsumner at gmail.com
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