[gdal-dev] Number of fields when creating layer?

[Even Rouault]

Abel Pau,

no you can't know the number of fields at the moment of creating the 
layer. A number of drivers only accept the CreateField() method to be 
called until the first call to ICreateFeature(). Cf the 
"bFeaturesWritten" variable in ogr/ogrsf_frmts/jml/ogrjmlwriterlayer.cpp

Typically ogr2ogr will never call CreateField() after ICreateFeature()

Even

Le 02/10/2023 à 15:54, Abel Pau via gdal-dev a écrit :
>
> Hi everybody,
>
> do anyone know if a driver can know the number of fields (and it’s 
> names, etc) of a layer BEFORE starting fetch features? So in the 
> moment of create the layer.
>
> I mean, when a driver is creating the own layer it could probably be 
> interested in creating all fields before starting and not in the 
> middle of translation. It could be caused because the output format 
> requires that in terms of efficiency.
>
> For instance, first feature have 3 fields, second feature have four, 
> etc... Until the last feature driver is not sure of all fields are 
> definitive.
>
> And every time a feature is added the driver have to verify in which 
> destination field corresponds to the new one.
>
> So, I accepted that it happens with the number of features (Even can 
> remember my concerns that time) and I am wondering if I have to accept 
> that for the number of fields.
>
> Thanks a lot.
>
> *Abel Pau Garcia*
>
> *GIS developer*
>
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