I have identified two principal aims :
- adding support for metadata to OGR driver, datasource and layers (validation of creation options, etc...). That one is easy : make derive the 3 base classes from the GDALMajorObject class
- more difficult and ambitious: making it possible to have a Dataset that contains both raster and vector data. You just open the data container once and can get both raster and vector data. Possible use cases: GeoPackage, PCIDSK, Spatialite/Rasterlite, Postgis/Postgis raster, ...
The class hierarchy of GDAL/OGR 1.X versions is quite simple :
So definitely 2 seperate worlds.
To achieve the first aim of getting metadata into OGR, you just have to do :
And now let's consider where the second aim could lead us :
Another way of presenting it with more details is the following pseudo-code :
/* Interface for major object */
class GDALIMajorObject
{
public:
virtual char **GetMetadataDomainList() = 0;
virtual char **GetMetadata( const char * pszDomain = "" );
[other methods go here]
};
/* Implementation of major object, and base class for dataset, bands, layers, etc.. */
class GDALMajorObject: public GDALIMajorObject
{
/* existing code of GDALMajorObject */
};
/* Interface for raster functions */
class GDALIRasterDataset: public GDALIMajorObject
{
public:
virtual int HandleRasterData() = 0;
virtual int GetRasterXSize( void ) = 0;
virtual int GetRasterYSize( void ) = 0;
virtual int GetRasterCount( void ) = 0;
virtual GDALRasterBand *GetRasterBand( int ) = 0;
[other methods go here]
};
/* Interface vor vector functions*/
class GDALIVectorDataset: public GDALIMajorObject
{
public:
virtual int HandleVectorData() = 0;
virtual int GetLayerCount() = 0;
virtual OGRLayer *GetLayer(int) = 0;
[other methods go here]
};
/* Convenience interface for both raster and vector functions */
class GDALIDataset: public GDALIRasterDataset, public GDALIVectorDataset
{
public:
/* That's all ! */
};
/* Partial implementation of GDALIRasterDataset */
class GDALAbstractRasterDataset : public GDALIRasterDataset, public GDALMajorObject
{
/* Current code of GDALDataset GDAL v1 goes here */
public:
virtual int HandleRasterData() { return TRUE; }
};
/* Convenience class used by vector only drivers */
class GDALEmptyRasterDataset : GDALAbstractRasterDataset
{
public:
virtual int HandleRasterData() { return FALSE; }
};
/* Partial implementation of GDALIVectorDataset */
class GDALAbstractVectorDataset : public GDALIVectorDataset, public GDALMajorObject
{
/* Current code of OGRDatasource GDAL v1 goes here*/
public:
virtual int HandleVectorData() { return TRUE; }
};
/* Convenience class used by raster only drivers */
class GDALEmptyVectorDataset : public GDALIVectorDataset
{
public:
virtual int HandleVectorData() { return FALSE; }
virtual int GetLayerCount() { return 0; }
virtual OGRLayer *GetLayer(int) { return NULL; }
[other methods go here]
};
/* Equivalent of GDALDataset GDAL v1 (plus dummy vector interface). Existing GDAL drivers would derive from it. */
class GDALRasterDataset : public GDALAbstractRasterDataset, public virtual GDALEmptyVectorDataset, public virtual GDALIDataset
{
};
/* Equivalent of OGRDatasource GDAL v1 (plus dummy raster interface). Existing OGR drivers would derive from it. */
class GDALVectorDataset : public GDALAbstractVectorDataset, public virtual GDALEmptyRasterDataset, public virtual GDALIDataset
{
};
/* GDAL v2 base class for new drivers that need both raster and vector data */
class GDALDataset : public GDALAbstractRasterDataset, public virtual GDALAbstractVectorDataset, public virtual GDALIDataset
{
};
The impact on the existing code base is :
- Current GDAL drivers must replace mentions of GDALDataset by GDALRasterDataset (automatic conversion)
- Current OGR drivers must replace mentions of OGRDatasource by GDALVectorDataset (automatic conversion)
- OGRDatasourceH becomes an alias of GDALDatasetH
- C methods cast the opaque dataset pointer GDALDatasetH to GDALIDataset before invoking the C++ methods.
