/*! \page ogr_apitut OGR API Tutorial

This document is intended to document using the OGR C++ classes to read
and write data from a file.  It is strongly advised that the read first
review the <a href="ogr_arch.html">OGR Architecture</a> document describing
the key classes and their roles in OGR.

\section ogr_apitut_read Reading From OGR

For purposes of demonstrating reading with OGR, we will constuct a small 
utility for dumping point layers from an OGR data source to stdout in
comma-delimited format. 

Initially it is necessary to register all the format drivers that are desired.
This is normally accomplished by calling OGRRegisterAll() which registers
all format drivers built into GDAL/OGR.  

\code
#include "ogrsf_frmts.h"

int main()

{
	OGRRegisterAll();
\endcode

Next we need to open the input OGR datasource.  Datasources can be files, 
RDBMSes, directories full of files, or even remote web services depending on
the driver being used.  However, the datasource name is always a single
string.  In this case we are hardcoded to open a particular shapefile.
The second argument (FALSE) tells the OGRSFDriverRegistrar::Open() method 
that we don't require update access.  On failure NULL is returned, and
we report an error. 

\code
    OGRDataSource       *poDS;

    poDS = OGRSFDriverRegistrar::Open( "point.shp", FALSE );
    if( poDS == NULL )
    {
	printf( "Open failed.\n" );
	exit( 1 );
    }
\endcode

An OGRDataSource can potentially have many layers associated with it.  The 
number of layers available can be queried with OGRDataSource::GetLayerCount()
and individual layers fetched by index using OGRDataSource::GetLayer().  
However, we wil just fetch the layer by name. 

\code
    OGRLayer  *poLayer;

    poLayer = poDS->GetLayerByName( "point" );
\endcode

Now we want to start reading features from the layer.  Before we start we 
could assign an attribute or spatial filter to the layer to restrict the set
of feature we get back, but for now we are interested in getting all features.

While it isn't strictly necessary in this 
circumstance since we are starting fresh with the layer, it is often wise
to call OGRLayer::ResetReading() to ensure we are starting at the beginning of
the layer.  We iterate through all the features in the layer using 
OGRLayer::GetNextFeature().  It will return NULL when we run out of features.

\code
    OGRFeature *poFeature;

    poLayer->ResetReading();
    while( (poFeature = poLayer->GetNextFeature()) != NULL )
    {
\endcode

In order to dump all the attribute fields of the feature, it is helpful
to get the OGRFeatureDefn.  This is an object, associated with the layer,
containing the definitions of all the fields.  We loop over all the fields,
and fetch and report the attributes based on their type.  

\code
	OGRFeatureDefn *poFDefn = poLayer->GetLayerDefn();
	int iField;

	for( iField = 0; iField < poFDefn->GetFieldCount(); iField++ )
        {
	    OGRFieldDefn *poFieldDefn = poFDefn->GetFieldDefn( iField );

	    if( poFieldDefn->GetType() == OFTInteger )
		printf( "%d,", poFeature->GetFieldAsInteger( iField ) );
	    else if( poFieldDefn->GetType() == OFTReal )
		printf( "%.3f,", poFeature->GetFieldAsDouble(iField) );
	    else if( poFieldDefn->GetType() == OFTString )
		printf( "%s,", poFeature->GetFieldAsString(iField) );
	    else
		printf( "%s,", poFeature->GetFieldAsString(iField) );
        }
\endcode

There are a few more field types than those explicitly handled above, but
a reasonable representation of them can be fetched with the 
OGRFeature::GetFieldAsString() method.  In fact we could shorten the above
by using OGRFeature::GetFieldAsString() for all the types. 

Next we want to extract the geometry from the feature, and write out the point
geometry x and y.   Geometries are returned as a generic OGRGeometry pointer.
We then determine the specific geometry type, and if it is a point, we
cast it to point and operate on it.  If it is something else we write
placeholders.  

\code
	OGRGeometry *poGeometry;

	poGeometry = poFeature->GetGeometryRef();
	if( poGeometry != NULL 
            && wkbFlatten(poGeometry->getGeometryType()) == wkbPoint )
        {
	    OGRPoint *poPoint = (OGRPoint *) poGeometry;

	    printf( "%.3f,%3.f\n", poPoint->getX(), poPoint->getY() );
        }
	else
	{
	    printf( "no point geometry\n" );
	}	
\endcode

The wkbFlatten() macro is used above to to convert the type for a wkbPoint25D 
(a point with a z coordinate) into the base 2D geometry type code (wkbPoint). 
For each 2D geometry type there is a corresponding 2.5D type code; however, 
there is only a C++ class for both the 2D and 3D cases.   So our code will
handle 2D or 3D cases properly.  

Note that OGRFeature::GetGeometryRef() returns a pointer to
the internal geometry owned by the OGRFeature.  There we don't actually 
deleted the return geometry.  However, the OGRLayer::GetNextFeature() method
returns a copy of the feature that is now owned by us.  So at the end of
use we must free the feature.  We could just "delete" it, but this can cause
problems in windows builds where the GDAL DLL has a different "heap" from the
main program.  To be on the safe side we use a GDAL function to delete the
feature. 

\code
   	OGRFeature::DestroyFeature( poFeature );
    }
\endcode

The OGRLayer returned by OGRDataSource::GetLayerByName() is also a reference
to an internal layer owned by the OGRDataSource so we don't need to delete
it.  But we do need to delete the datasource in order to close the input file.
Once again we do this with a custom delete method to avoid special win32 
heap issus. 

\code
    OGRDataSource::DestroyDataSource( poDS );
}
\endcode

All together our program looks like this.

\code
#include "ogrsf_frmts.h"

int main()

{
    OGRRegisterAll();

    OGRDataSource       *poDS;

    poDS = OGRSFDriverRegistrar::Open( "point.shp", FALSE );
    if( poDS == NULL )
    {
	printf( "Open failed.\n%s" );
	exit( 1 );
    }

    OGRLayer  *poLayer;

    poLayer = poDS->GetLayerByName( "point" );

    OGRFeature *poFeature;

    poLayer->ResetReading();
    while( (poFeature = poLayer->GetNextFeature()) != NULL )
    {
	OGRFeatureDefn *poFDefn = poLayer->GetLayerDefn();
	int iField;

	for( iField = 0; iField < poFDefn->GetFieldCount(); iField++ )
        {
	    OGRFieldDefn *poFieldDefn = poFDefn->GetFieldDefn( iField );

	    if( poFieldDefn->GetType() == OFTInteger )
		printf( "%d,", poFeature->GetFieldAsInteger( iField ) );
	    else if( poFieldDefn->GetType() == OFTReal )
		printf( "%.3f,", poFeature->GetFieldAsDouble(iField) );
	    else if( poFieldDefn->GetType() == OFTString )
		printf( "%s,", poFeature->GetFieldAsString(iField) );
	    else
		printf( "%s,", poFeature->GetFieldAsString(iField) );
        }

	OGRGeometry *poGeometry;

	poGeometry = poFeature->GetGeometryRef();
	if( poGeometry != NULL 
            && wkbFlatten(poGeometry->getGeometryType()) == wkbPoint )
        {
	    OGRPoint *poPoint = (OGRPoint *) poGeometry;

	    printf( "%.3f,%3.f\n", poPoint->getX(), poPoint->getY() );
        }
	else
	{
	    printf( "no point geometry\n" );
	}	
   	OGRFeature::DestroyFeature( poFeature );
    }

    OGRDataSource::DestroyDataSource( poDS );
}
\endcode

\section ogr_apitut_write Writing To OGR

As an example of writing through OGR, we will do roughly the opposite of the
above.  A short program that reads comma seperated values from input text
will be written to a point shapefile via OGR. 

As usual, we start by registering all the drivers, and then fetch the 
Shapefile driver as we will need it to create our output file. 

\code
#include "ogrsf_frmts.h"

int main()
{
    const char *pszDriverName = "ESRI Shapefile";
    OGRSFDriver *poDriver;

    OGRRegisterAll();

    poDriver = OGRSFDriverRegistrar::GetRegistrar()->GetDriverByName(
		pszDriverName );
    if( poDriver == NULL )
    {
	printf( "%s driver not available.\n", pszDriverName );
	exit( 1 );
    }
\endcode

Next we create the datasource.  The ESRI Shapefile driver allows us to create
a directory full of shapefiles, or a single shapefile as a datasource.  In
this case we will explicitly create a single file by including the extension
in the name.  Other drivers behave differently.  The second argument to 
the call is a list of option values, but we will just be using defaults in
this case.  Details of the options supported are also format specific. 

\code
    OGRDataSource *poDS;

    poDS = poDriver->CreateDataSource( "point_out.shp", NULL );
    if( poDS == NULL )
    {
	printf( "Creation of output file failed.\n" );
	exit( 1 );
    }
\endcode

Now we create the output layer.  In this case since the datasource is a 
single file, we can only have one layer.  We pass wkbPoint to specify the
type of geometry supported by this layer.  In this case we aren't passing
any coordinate system information or other special layer creation options.

\code
    OGRLayer *poLayer;

    poLayer = poDS->CreateLayer( "point_out", NULL, wkbPoint, NULL );
    if( poLayer == NULL )
    {
	printf( "Layer creation failed.\n" );
	exit( 1 );
    }
\endcode

Now that the layer exists, we need to create any attribute fields that should
appear on the layer.  Fields must be added to the layer before any features
are written.  To create a field we initialize an OGRField object with the
information about the field.  In the case of Shapefiles, the field width and
precision is significant in the creation of the output .dbf file, so we 
set it specifically, though generally the defaults are OK.  For this example
we will just have one attribute, a name string associated with the x,y point.

Note that the template OGRField we pass to CreateField() is copied internally.
We retain ownership of the object.

\code
    OGRFieldDefn oField( "Name", OFTString );

    oField.SetWidth(32);

    if( poLayer->CreateField( &oField ) != OGRERR_NONE )
    {
	printf( "Creating Name field failed.\n" );
        exit( 1 );
    }
\encode

The following snipping loops reading lines of the form "x,y,name" from stdin, 
and parsing them.  

\code
    double x, y;
    char szName[33];

    while( !feof(stdin) 
           && fscanf( stdin, "%lf,%lf,%32s", &x, &y, szName ) == 3 )
    {
\endcode

To write a feature to disk, we must create a local OGRFeature, set attributes
and attach geometry before trying to write it to the layer.  It is 
imperative that this feature be instantiated from the OGRFeatureDefn 
associated with the layer it will be written to.

\code
	OGRFeature *poFeature;

	poFeature = new OGRFeature( poLayer->GetLayerDefn() );
        poFeature->SetField( "Name", szName );
\endcode

We create a local geometry object, and assign it directly to the feature.
The OGRFeature::SetGeometryDirectly() differs from OGRFeature::SetGeometry()
in that the direct method gives ownership of the geometry to the feature. 
This is generally more efficient as it avoids an extra deep object copy
of the geometry.

\code
	OGRPoint *poPoint = new OGRPoint();
	
  	poPoint->setX( x );
        poPoint->setY( y );
 
        poFeature->SetGeometryDirectly( poPoint ); 
\endcode

Now we create a feature in the file.  The OGRLayer::CreateFeature() does not
take ownership of our feature so we clean it up when done with it. 

\code
	if( poLayer->CreateFeature( poFeature ) != OGRERR_NONE )
        {
	   printf( "Failed to create feature in shapefile.\n" );
           exit( 1 );
        }

        delete poFeature;
   }
\endcode

Finally we need to close down the datasource in order to ensure headers
are written out in an orderly way and all resources are recovered.

\code
    OGRDataSource::DestroyDataSource( poDS );
}
\endcode

The same program all in one block looks like this:

\code
#include "ogrsf_frmts.h"

int main()
{
    const char *pszDriverName = "ESRI Shapefile";
    OGRSFDriver *poDriver;

    OGRRegisterAll();

    poDriver = OGRSFDriverRegistrar::GetRegistrar()->GetDriverByName(
		pszDriverName );
    if( poDriver == NULL )
    {
	printf( "%s driver not available.\n", pszDriverName );
	exit( 1 );
    }

    OGRDataSource *poDS;

    poDS = poDriver->CreateDataSource( "point_out.shp", NULL );
    if( poDS == NULL )
    {
	printf( "Creation of output file failed.\n" );
	exit( 1 );
    }

    OGRLayer *poLayer;

    poLayer = poDS->CreateLayer( "point_out", NULL, wkbPoint, NULL );
    if( poLayer == NULL )
    {
	printf( "Layer creation failed.\n" );
	exit( 1 );
    }

    OGRFieldDefn oField( "Name", OFTString );

    oField.SetWidth(32);

    if( poLayer->CreateField( &oField ) != OGRERR_NONE )
    {
	printf( "Creating Name field failed.\n" );
        exit( 1 );
    }

    double x, y;
    char szName[33];

    while( !feof(stdin) 
           && fscanf( stdin, "%lf,%lf,%32s", &x, &y, szName ) == 3 )
    {
	OGRFeature *poFeature;

	poFeature = new OGRFeature( poLayer->GetLayerDefn() );
        poFeature->SetField( "Name", szName );

	OGRPoint *poPoint = new OGRPoint();
	
  	poPoint->setX( x );
        poPoint->setY( y );
 
        poFeature->SetGeometryDirectly( poPoint ); 

	if( poLayer->CreateFeature( poFeature ) != OGRERR_NONE )
        {
	   printf( "Failed to create feature in shapefile.\n" );
           exit( 1 );
        }

        delete poFeature;
    }

    OGRDataSource::DestroyDataSource( poDS );
}
\endcode

*/
