[netCDF #ROZ-206338]: Use of NcGroup::addVar() in C++ API

Hi Mike,

> I am converting some code from the old C++ API to the new one.
> 
> I notice that there is no NcGroup::addVar() method specifically for scalars.
> 
> Therefore I need to use the following call:
> 
> NcVar NcGroup::addVar(const string& name, const NcType& ncType, const
> vector<NcDim>& ncDimVector)
> 
> and pass in an ncDimVector of 0 length.
> 
> I notice that in the code, the following call is made:
> 
> ncCheck(nc_def_var(myId,name.c_str(),tmpType.getId(),dimIds.size(),&dimIds[0],&varId),__FILE__,__LINE__);
> 
> and this specifically takes the address:
> 
> &dimIds[0]
> 
> So I have 2 questions:
> 
> (1) Am I correct in using this method to add a scalar variable?

Yes. but I think it would also work OK if you just gave a 0 argument
(a null pointer) instead of &dimIds[0].

> (b) Is &dimIds[0] guaranteed to be safe if the length of dimIds is 0,
> i.e. dimIds[0] does not exist? I have done some tests and it seems that
> this operation returns 0 for a zero-length vector, but this may just be
> luck.

Yes that's the way it's supposed to work in the C API, as documented here:

  
http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-c.html#nc_005fdef_005fvar

where it says

   dimids
     Vector of ndims dimension IDs corresponding to the variable dimensions.
     For classic model netCDF files, if the ID of the unlimited dimension is
     included, it must be first. This argument is ignored if ndims is 0. For 
     expanded model netCDF4/HDF5 files, there may be any number of unlimited 
     dimensions, and they may be used in any element of the dimids array. 

--Russ

> 
> Thanks for any advice.
> 
> Mike Dixon, EOL
> 
> 
Russ Rew                                         UCAR Unidata Program
address@hidden                      http://www.unidata.ucar.edu



Ticket Details
===================
Ticket ID: ROZ-206338
Department: Support netCDF
Priority: Normal
Status: Closed