Hellp,
I agree with the comments made in this email conversation. However, it
may be useful to understand the motivation for implementing a specific
C++ API.
These are essentially that the use of it in a C++ code should appear
simpler and be more robust than the C API. This is achieved by
(i) use of parametric polymorphisms (C++ templates functions)
(ii) encapsulation (existence of private data).
(iii) error handling: the presence of an error automatically "throws"
an error. There is extensive consistency checking within the API.
(iv) inheritance (this relates classes with similiar "traits", eg
NcFile and NcGroup; also NcType, NcVLen, Nc EnumType etc).
The use of complicated user-defined types in the exisiting C-API can be
difficult, particularly if you
do not know the sizes of elements at compile time. This is because you
need to deal with offsets and
alignment issues of the data components. I have implemented a C++ API
that can handle this
for user-defined types of arbitrary complexity. I intended to release
this 15months ago, but unfortunately
it has been delayed due to a a bug, we think in the HDF5 layer.
Lynton Appel
On 03/04/2013 12:18 AM, Lynnes, Christopher S. (GSFC-6102) wrote:
Taylor,
I can't help thinking that the C++ library you are using seems a little more
brittle than the C route at this phase in its evolution. Have you considered
making calls to the C API from your C++ call? THe methods for extracting
subsets of variables are quite clear in the C API...
On Mar 3, 2013, at 4:34 PM, Taylor Binnington<tbinnington@xxxxxxxxx> wrote:
Hello,
I'm attempting to read only certain parts (specific indices of specific
variable arrays), remotely, from a MERRA HDFEOS file.
I've recently upgraded to NetCDF 4.2.1.1, using Lynton's C++ library. At first,
I was using trying to subset the data directly from an OPeNDAP URL supplied to
NcFile, but it's been suggested to me, by an earlier post in the OPeNDAP
forums, that this is not a good way to go. Instead, I should use NetCDF API
calls.
I have carefully read through the C++ interface guide, including this example:
http://www.unidata.ucar.edu/software/netcdf/docs/cxx4/test_var_8cpp-example.html
but am struggling to understand how to do this. A push in the right direction
would be very appreciated. The example (I don't fully understand it, but was
trying to emulate some of the example that I linked above)
#include<iostream>
#include<netcdf>
int main() {
NcFile
dataFile("http://goldsmr2.sci.gsfc.nasa.gov/opendap/hyrax/MERRA/MAT1NXSLV.5.2.0/1991/01/MERRA100.prod.assim.tavg1_2d_slv_Nx.19910101.hdf";,
NcFile::read);
NcGroup grouptest(dataFile.addGroup("Dataset"));
}
gives me the error:
terminate called after throwing an instance of 'netCDF::exceptions::NcNotNc4'
what(): NcNotNc4: Attempting netcdf-4 operation on netcdf-3 file.
file: ncGroup.cpp line:265
Aborted
This is not surprising, since the file is not a NetCDF-3 file.
Thank you in advance.
Taylor
--
Taylor Binnington
e. tbinnington@xxxxxxxxx
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--
Dr. Christopher Lynnes, NASA/GSFC, ph: 301-614-5185
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