main.cpp:
#include "myclass.h"
#include <QtDeclarative>
int main() {
  // before viewer is setup, add this:
  qmlRegisterType<MyClass>( "MyClass", 1, 0, "MyClass" );
}

myclass.h:
class MyClass: public QObject {
public:
  MyClass( QObject *parent );
public slots:
  buttonPressed( QString msg ) {
    qDebug() << "Button was pressed:" << msg;
    emit blah( "test" );
  }
signals:
  void blah( QString msg );
}

main.qml:
import MyClass 1.0

MyClass {
  id: localObject
  onBlah: console.log( "Received signal: " + msg );
}

Button {
  onClicked: localObject.buttonPressed( "something here" );
}