Qgis C++ hello world example

Question

i am new in developpement with C++, i try to realise an application with C++ which can read and display a *.qgs file from Qgis, i tried the code bellow but it doesn't work, it always show me errors "no such file or directory"

#include <QtCore/QString>
#include <QtGui/QApplication>

#include <qgsapplication.h>
#include <qgsproviderregistry.h>
#include <qgssinglesymbolrenderer.h>
#include <qgsmaplayerregistry.h>
#include <qgsvectorlayer.h>
#include <qgsmapcanvas.h>

#include <iostream>

int main(int argc, char *argv[])
{
QgsApplication app(argc, argv, true);

QString myPluginsDir        = "/usr/lib64/qgis";
QString myLayerPath         = "pathtoashapefile";
QString myLayerBaseName     = "italy";
QString myProviderName      = "ogr";

// Instantiate Provider Registry
QgsProviderRegistry * preg = QgsProviderRegistry::instance(myPluginsDir);

QgsVectorLayer * mypLayer =  new QgsVectorLayer(myLayerPath, myLayerBaseName, myProviderName);

QgsSingleSymbolRendererV2 *mypRenderer = new QgsSingleSymbolRendererV2(QgsSymbolV2::defaultSymbol(mypLayer->geometryType()));


QList <QgsMapCanvasLayer> myLayerSet;

mypLayer->setRendererV2(mypRenderer);
if (mypLayer->isValid())
{
    qDebug("Layer is valid");
}
else
{
    qDebug("Layer is NOT valid");
}

// Add the Vector Layer to the Layer Registry
QgsMapLayerRegistry::instance()->addMapLayer(mypLayer, TRUE);
// Add the Layer to the Layer Set
myLayerSet.append(QgsMapCanvasLayer(mypLayer, TRUE));

// Create the Map Canvas
QgsMapCanvas * mypMapCanvas = new QgsMapCanvas(0, 0);
mypMapCanvas->setExtent(mypLayer->extent());
mypMapCanvas->enableAntiAliasing(true);
mypMapCanvas->setCanvasColor(QColor(255, 255, 255));
mypMapCanvas->freeze(false);
// Set the Map Canvas Layer Set
mypMapCanvas->setLayerSet(myLayerSet);
mypMapCanvas->setVisible(true);
mypMapCanvas->refresh();

return app.exec();
}

Answer 1

I was at that point a few hours ago; I was following this guide, nice although outdated.

It seems your code are missing some headers. Try adding these:

#include <QString>
#include <QApplication>
#include <QWidget>

Your makefile may also need these modifications (in Windows, check the corresponding configurations in the tools you're using):

INCPATH       = -I/usr/share/qt4/mkspecs/linux-g++-64 -I. -I/usr/include/qt4/QtCore -I/usr/include/qt4/QtGui -I/usr/include/qt4/QtXml -I/usr/include/qt4 -I/usr/include/qgis -I. -I.

LIBS          = $(SUBLIBS)  -L/usr/lib/x86_64-linux-gnu -lQtXml -lQtGui -lQtCore -lpthread -lqgis_core