[gdal-dev] C++ template functions with raster bands

[Even Rouault]

Le mardi 29 mars 2016 17:35:26, Ari Jolma a écrit :
> I want to write template functions like this:
> 
> template<typename cell_type>
> void map(GDALRasterBand *b, mapper<cell_type> *mapper) {
> ...
> 
> This function would map the cell values of a band to new ones using some
> logic that is coded within the mapper object (and the logic depends on
> the cell_type).
> 
> I would like to catch errors which are due to mismatch between the band
> data type and the template cell_type. So far I've discovered this:
> 
>      band_wrapper band = band_initialize(b);
>      if (band.info.size_of_data_type != sizeof(cell_type))
>          goto fail_because_of_data_type_mismatch;
> 
> but it is not bullet proof since data types can have an equal size but
> be different. The band_initialize contains a switch statement, which
> sets the info element of the band wrapper.
> 
> Are there other ways to solve this problem? I could build long switch
> statements but it is not very appealing. And it gets messy when there is
> more than one band.

Perhaps by using C++ traits like this :

#include <gdal.h>

template <typename T> struct GetGDALDataTypeTraits
{
    static const GDALDataType datatype;
};

template<> struct GetGDALDataTypeTraits<unsigned char>
{
    static const GDALDataType datatype = GDT_Byte;
};

template<> struct GetGDALDataTypeTraits<unsigned short>
{
    static const GDALDataType datatype = GDT_UInt16;
};

template <typename T> static GDALDataType GetGDALDataType()
{
  return GetGDALDataTypeTraits<T>::datatype;
}

#include <iostream>
int main(int argc, char* argv[])
{
  std::cout << "type unsigned char: " << GetGDALDataType<unsigned char>() << std::endl;
  std::cout << "type unsigned short: " << GetGDALDataType<unsigned short>() << std::endl;
  // Link time error
  //std::cout << "type double: " << GetGDALDataType<double>() << std::endl;

  return 0;
}


> 
> Ari
> 
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> gdal-dev at lists.osgeo.org
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