.. _vector_driver_tut:

================================================================================
Vector driver implementation tutorial
================================================================================

.. highlight:: cpp

Overall Approach
----------------

In general new formats are added to OGR by implementing format
specific drivers with instantiating a :cpp:class:`GDALDriver` and subclasses of
:cpp:class:`GDALDataset` and :cpp:class:`OGRLayer`.  The GDALDriver instance is registered with
the :cpp:class:`GDALDriverManager` at runtime.

Before following this tutorial to implement an OGR driver, please review
the :ref:`vector_data_model` document carefully.

The tutorial will be based on implementing a simple ascii point format.

Implementing GDALDriver
-----------------------

The format specific driver class is implemented as a instance of GDALDriver.
One instance of the driver will normally be created, and registered with
the GDALDriverManager.  The instantiation of the driver is normally
handled by a global C callable registration function, similar to the
following placed in the same file as the driver class.

.. code-block::

    void RegisterOGRSPF()
    {
        if( GDALGetDriverByName("SPF") != NULL )
            return;

        GDALDriver *poDriver = new GDALDriver();

        poDriver->SetDescription("SPF");
        poDriver->SetMetadataItem(GDAL_DCAP_VECTOR, "YES");
        poDriver->SetMetadataItem(GDAL_DMD_LONGNAME, "Long name for SPF driver");
        poDriver->SetMetadataItem(GDAL_DMD_EXTENSION, "spf");
        poDriver->SetMetadataItem(GDAL_DMD_HELPTOPIC, "drv_spf.html");

        poDriver->pfnOpen = OGRSPFDriverOpen;
        poDriver->pfnIdentify = OGRSPFDriverIdentify;
        poDriver->pfnCreate = OGRSPFDriverCreate;

        poDriver->SetMetadataItem(GDAL_DCAP_VIRTUALIO, "YES");

        GetGDALDriverManager()->RegisterDriver(poDriver);
    }

The :cpp:func:`GDALDriver::SetDescription` sets the name of the driver.  This name is specified
on the commandline when creating datasources so it is generally good to keep
it short and without any special characters or spaces.

SetMetadataItem( GDAL_DCAP_VECTOR, "YES" ) is specified to indicate that the driver
will handle vector data.

SetMetadataItem( GDAL_DCAP_VIRTUALIO, "YES" ) is specified to indicate that the
driver can deal with files opened with the VSI*L GDAL API.
Otherwise this metadata item should not be defined.

For a driver that can be built as a plugin (that is to say a standalone shared object, loaded at runtime by GDAL), since GDAL 3.9 and :ref:`rfc-96`, there is a way to implement the driver in a way where the plugin will be loaded only when necessary, and not immediately at :cpp:func:`GDALAllRegister` time. Consult :ref:`rfc96_example_driver` for the changes needed to make the driver compatible of deferred plugin loading.

The driver declaration generally looks something like this for a
format with read or read and update access (the Open() method) and creation
support (the Create() method).


.. code-block::

    static GDALDataset* OGRSPFDriverOpen(GDALOpenInfo* poOpenInfo);
    static int          OGRSPFDriverIdentify(GDALOpenInfo* poOpenInfo);
    static GDALDataset* OGRSPFDriverCreate(const char* pszName, int nXSize, int nYSize,
                                        int nBands, GDALDataType eDT, char** papszOptions);


The Open() method is called by :cpp:func:`GDALOpenEx`. It should quietly return NULL if
the passed filename is not of the format supported by the driver.  If it is the
target format, then a new GDALDataset object for the dataset should be returned.

It is common for the Open() method to be delegated to an Open() method on
the actual format's GDALDataset class.

.. code-block::

    static GDALDataset *OGRSPFDriverOpen( GDALOpenInfo* poOpenInfo )
    {
        if( !OGRSPFDriverIdentify(poOpenInfo) )
            return NULL;

        OGRSPFDataSource *poDS = new OGRSPFDataSource();
        if( !poDS->Open(poOpenInfo->pszFilename, poOpenInfo->eAccess == GA_Update) )
        {
            delete poDS;
            return NULL;
        }

        return poDS;
    }


The Identify() method is implemented as such :


.. code-block::

    static int OGRSPFDriverIdentify( GDALOpenInfo* poOpenInfo )
    {
        // Does this appear to be an .spf file?
        return EQUAL(CPLGetExtension(poOpenInfo->pszFilename), "spf");
    }


Examples of the Create() method is left for the section on creation and update.

Basic Read Only Data Source
---------------------------

We will start implementing a minimal read-only datasource.  No attempt is
made to optimize operations, and default implementations of many methods
inherited from GDALDataset are used.

The primary responsibility of the datasource is to manage the list of layers.
In the case of the SPF format a datasource is a single file representing one
layer so there is at most one layer.  The "name" of a datasource should
generally be the name passed to the Open() method.

The Open() method below is not overriding a base class method, but we have
it to implement the open operation delegated by the driver class.

For this simple case we provide a stub :cpp:func:`GDALDataset::TestCapability` that returns FALSE
for all extended capabilities.  The TestCapability() method is pure virtual,
so it does need to be implemented.

.. code-block::

    class OGRSPFDataSource : public GDALDataset
    {
        OGRSPFLayer       **papoLayers;
        int                 nLayers;

    public:
                            OGRSPFDataSource();
                            ~OGRSPFDataSource();

        int                 Open( const char *pszFilename, int bUpdate );

        int                 GetLayerCount() { return nLayers; }
        OGRLayer            *GetLayer( int );

        int                 TestCapability( const char * ) { return FALSE; }
    };


The constructor is a simple initializer to a default state.  The Open() will
take care of actually attaching it to a file.  The destructor is responsible
for orderly cleanup of layers.


.. code-block::

    OGRSPFDataSource::OGRSPFDataSource()
    {
        papoLayers = NULL;
        nLayers = 0;
    }

    OGRSPFDataSource::~OGRSPFDataSource()
    {
        for( int i = 0; i < nLayers; i++ )
            delete papoLayers[i];
        CPLFree(papoLayers);
    }


The Open() method is the most important one on the datasource, though
in this particular instance it passes most of its work off to the
OGRSPFLayer constructor if it believes the file is of the desired format.

Note that Open() methods should try and determine that a file isn't of the
identified format as efficiently as possible, since many drivers may be
invoked with files of the wrong format before the correct driver is
reached.  In this particular Open() we just test the file extension but this
is generally a poor way of identifying a file format.  If available, checking
"magic header values" or something similar is preferable.

In the case of the SPF format, update in place is not supported,
so we always fail if bUpdate is FALSE.


.. code-block::

    int  OGRSPFDataSource::Open( const char *pszFilename, int bUpdate )
    {
        if( bUpdate )
        {
            CPLError(CE_Failure, CPLE_OpenFailed,
                    "Update access not supported by the SPF driver.");
            return FALSE;
        }

        // Create a corresponding layer.
        nLayers = 1;
        papoLayers = static_cast<OGRSPFLayer **>(CPLMalloc(sizeof(void *)));

        papoLayers[0] = new OGRSPFLayer(pszFilename);

        pszName = CPLStrdup(pszFilename);

        return TRUE;
    }


A GetLayer() method also needs to be implemented.  Since the layer list
is created in the Open() this is just a lookup with some safety testing.


.. code-block::

    OGRLayer *OGRSPFDataSource::GetLayer( int iLayer )
    {
        if( iLayer < 0 || iLayer >= nLayers )
            return NULL;

        return papoLayers[iLayer];
    }


Read Only Layer
---------------

The OGRSPFLayer is implements layer semantics for an .spf file.  It provides
access to a set of feature objects in a consistent coordinate system
with a particular set of attribute columns.  Our class definition looks like
this:


.. code-block::

    class OGRSPFLayer : public OGRLayer
    {
        OGRFeatureDefn     *poFeatureDefn;
        FILE               *fp;
        int                 nNextFID;

    public:
        OGRSPFLayer( const char *pszFilename );
    ~OGRSPFLayer();

        void                ResetReading();
        OGRFeature *        GetNextFeature();

        OGRFeatureDefn *    GetLayerDefn() { return poFeatureDefn; }

        int                 TestCapability( const char * ) { return FALSE; }
    };


The layer constructor is responsible for initialization.  The most important
initialization is setting up the :cpp:class:`OGRFeatureDefn` for the layer.  This defines
the list of fields and their types, the geometry type and the coordinate
system for the layer.  In the SPF format the set of fields is fixed - a
single string field and we have no coordinate system info to set.

Pay particular attention to the reference counting of the OGRFeatureDefn.
As OGRFeature's for this layer will also take a reference to this definition,
it is important that we also establish a reference on behalf of the layer
itself.


.. code-block::

    OGRSPFLayer::OGRSPFLayer( const char *pszFilename )
    {
        nNextFID = 0;

        poFeatureDefn = new OGRFeatureDefn(CPLGetBasename(pszFilename));
        SetDescription(poFeatureDefn->GetName());
        poFeatureDefn->Reference();
        poFeatureDefn->SetGeomType(wkbPoint);

        OGRFieldDefn oFieldTemplate("Name", OFTString);

        poFeatureDefn->AddFieldDefn(&oFieldTemplate);

        fp = VSIFOpenL(pszFilename, "r");
        if( fp == NULL )
            return;
    }

Note that the destructor uses :cpp:func:`OGRFeatureDefn::Release` on the OGRFeatureDefn.  This will
destroy the feature definition if the reference count drops to zero, but if
the application is still holding onto a feature from this layer, then that
feature will hold a reference to the feature definition and it will not
be destroyed here (which is good!).


.. code-block::

    OGRSPFLayer::~OGRSPFLayer()
    {
        poFeatureDefn->Release();
        if( fp != NULL )
            VSIFCloseL(fp);
    }

The :cpp:func:`OGRLayer::GetNextFeature` method is usually the work horse of OGRLayer
implementations.  It is responsible for reading the next feature according
to the current spatial and attribute filters installed.

The while() loop is present to loop until we find a satisfactory
feature.  The first section of code is for parsing a single line of
the SPF text file and establishing the x, y and name for the line.


.. code-block::

    OGRFeature *OGRSPFLayer::GetNextFeature()
    {
        // Loop till we find a feature matching our requirements.
        while( true )
        {
            const char *pszLine = CPLReadLineL(fp);

            // Are we at end of file (out of features)?
            if( pszLine == NULL )
                return NULL;

            const double dfX = atof(pszLine);

            pszLine = strstr(pszLine,"|");
            if( pszLine == NULL )
                continue; // we should issue an error!
            else
                pszLine++;

            const double dfY = atof(pszLine);

            pszLine = strstr(pszLine,"|");

            const char *pszName = NULL;
            if( pszLine == NULL )
                continue; // we should issue an error!
            else
                pszName = pszLine + 1;

The next section turns the x, y and name into a feature.  Also note that
we assign a linearly incremented feature id.  In our case we started at
zero for the first feature, though some drivers start at 1.


.. code-block::

        OGRFeature *poFeature = new OGRFeature(poFeatureDefn);

        poFeature->SetGeometryDirectly(new OGRPoint(dfX, dfY));
        poFeature->SetField(0, pszName);
        poFeature->SetFID(nNextFID++);

Next we check if the feature matches our current attribute or
spatial filter if we have them.  Methods on the OGRLayer base class
support maintain filters in the OGRLayer member fields :cpp:member:`OGRLayer::m_poFilterGeom`
(spatial filter) and :cpp:member:`OGRLayer::m_poAttrQuery` (attribute filter) so we can just use
these values here if they are non-NULL.  The following test is essentially
"stock" and done the same in all formats.  Some formats also do some
spatial filtering ahead of time using a spatial index.

If the feature meets our criteria we return it.  Otherwise we destroy it,
and return to the top of the loop to fetch another to try.

.. code-block::

            if( (m_poFilterGeom == NULL ||
                FilterGeometry(poFeature->GetGeometryRef())) &&
                (m_poAttrQuery == NULL ||
                m_poAttrQuery->Evaluate(poFeature)) )
                return poFeature;

            delete poFeature;
        }
    }

While in the middle of reading a feature set from a layer, or at any other
time the application can call :cpp:func:`OGRLayer::ResetReading` which is intended to restart
reading at the beginning of the feature set.  We implement this by seeking
back to the beginning of the file, and resetting our feature id counter.

.. code-block::

    void OGRSPFLayer::ResetReading()
    {
        VSIFSeekL(fp, 0, SEEK_SET);
        nNextFID = 0;
    }

In this implementation we do not provide a custom implementation for the
GetFeature() method.  This means an attempt to read a particular feature
by its feature id will result in many calls to GetNextFeature() until the
desired feature is found.  However, in a sequential text format like spf
there is little else we could do anyway.

There! We have completed a simple read-only feature file format driver.
