C++ Multiple inheritance with interfaces?

Question

Greetings all,

I come from Java background and I am having difficulty with multiple inheritance.

I have an interface called IView which has init() method.I want to derive a new class called PlaneViewer implementing above interface and extend another class. (QWidget).

My implementation is as:

IViwer.h (only Header file , no CPP file) :

#ifndef IVIEWER_H_
#define IVIEWER_H_

class IViewer
{
public:
  //IViewer();
  ///virtual
  //~IViewer();
  virtual void init()=0;
};

#endif /* IVIEWER_H_ */

My derived class.

PlaneViewer.h

#ifndef PLANEVIEWER_H
#define PLANEVIEWER_H

#include <QtGui/QWidget>
#include "ui_planeviewer.h"
#include "IViewer.h"
class PlaneViewer : public QWidget , public IViewer
{
    Q_OBJECT

public:
    PlaneViewer(QWidget *parent = 0);
    ~PlaneViewer();
    void init(); //do I have to define here also ?

private:
    Ui::PlaneViewerClass ui;
};

#endif // PLANEVIEWER_H

PlaneViewer.cpp

#include "planeviewer.h"

PlaneViewer::PlaneViewer(QWidget *parent)
    : QWidget(parent)
{
    ui.setupUi(this);
}

PlaneViewer::~PlaneViewer()
{

}

void PlaneViewer::init(){

}

My questions are:

1. Is it necessary to declare method init() in PlaneViewer interface also , because it is already defined in IView?

2.I cannot complie above code ,give error :

PlaneViewer]+0x28): undefined reference to `typeinfo for IViewer' collect2: ld returned 1 exit status

Do I have to have implementation for IView in CPP file (because all I want is an interface,not as implementation) ?

Answer 1

Is it necessary to declare method init() in PlaneViewer interface also , because it is already defined in IView?

You do not have to declare init() in PlaneViewer, but if you don't PlaneViewer will be an abstract class, meaning that you cannot instantiate it.

If you mean to ask if you have to have 'void init();' in the header file for PlaneViewer and in the .cpp file. The answer is yes.

I cannot complie above code ,give error : PlaneViewer]+0x28): undefined reference to `typeinfo for IViewer' collect2: ld returned 1 exit status

I think either you are not building the same code or your compile command is incorrect.

I stripped out the QT stuff and was able to build your code just fine with g++.

The error means that the IViewer class was not found by the linker.

I get that error if I remove the '=0' part that makes 'IViewer::init()' a pure virtual function. You could also get that error if you uncommented the constructor and/or destructor in IViewer.

Do I have to have implementation for IView in CPP file?

No. C++ does not care if it is in a .cpp file or a .h file. Unlike Java, the C/C++ preprocessor first resolves all the includes and generates one file containing all the code. It then passes this to the C/C++ compiler. You can actually include a .cpp if you want. Not a good idea though.