QGIS » QGIS Application

#include <QDebug>

#include <QList>

#include <QVariant>

#include "qgis.h"

#include "qgsapplication.h"

#include "qgsvectorlayer.h"

#include "qgsvectordataprovider.h"

int main(int argc, char *argv[]) {

  QgsApplication app(argc, argv, false);

  app.setPrefixPath("/usr/local", true);

  app.initQgis();

  // load a layer

  QgsVectorLayer vlayer("polygons.shp", "layer_name", "ogr");

  if ( vlayer.isValid() ) {

    qDebug("layer is valid");

    // get the data provider

    QgsVectorDataProvider *vprovider = vlayer.dataProvider();

    // get index of field 'name' 

    int name_index = vprovider->fieldNameIndex("id");

    // there are NULL values in the field 'name' on sample dataset

    QList<QVariant> uniques;

    vprovider->uniqueValues(name_index, uniques);

    qDebug() << uniques;

  }

  return 0;

}