c++ passing subclass as parameter

Question

I got a superclass named Obje

#ifndef OBJE_H
#define OBJE_H

#include <QString>

class Obje
{
    public:
    Obje();
    QString name="";
    int id;
    int itemid=0;
    void setItemid(int i);
    virtual void update(){};
};

#endif // OBJE_H

and I got a subclass named Npc

#ifndef NPC_H
#define NPC_H

#include "obje.h"
#include "QString"
#include "QPixmap"
class Npc : public Obje
{
public:

    Npc(QString dir="left");
    QString direction="left";

    QPixmap* spriteSheet = nullptr;
    QPixmap face;
    QPixmap sprites[3];
    void update(int nid);
    QPixmap* getCurrentSprite();
    int currentFrame;
    QString frameDirection;

};

#endif // NPC_H

I also got a class to manage listWidgets

class ListUpdate
{
public:
    ListUpdate();
    static void update(QListWidget *q, vector<Obje>& i);

    private:
    static struct item{
        QString id;
        QString name;
        QPixmap px;
    };

    static QListWidgetItem* newItem(ListUpdate::item &i, int c);
};

#endif // LISTUPDATE_H

When I try to pass object named "npcs" which is type vector"Npc" to ListUpdate::update , it gives this error Passing the vector:

ListUpdate::update(ui->listWidget,npcs);

error:

C:\Qt Projects\RpgEditor\npcscreen.cpp:64: error: C2665: 'ListUpdate::update': none of the 3 overloads could convert all the argument types

Note: I also got 2 other overload functions and I didn't write them here to prevent unnecessary confusion(that's why it says , "none of the 3 overloads")

Why doesn't it accept my subclass ?

yeah, It didn't show < > , I was trying to fix that by editing — Ozan Deniz, Jan 15 '17 at 14:33
yep , I know but I'm not planning to make it opensource or a group project. I'm just trying to build this editor so I can use it for my actual project. I just didn't want to extend the coding process — Ozan Deniz, Jan 15 '17 at 14:53
@OzanDeniz See [this question](http://stackoverflow.com/questions/461203/when-to-use-virtual-destructors) for information on virtual destructors. — François Andrieux, Jan 15 '17 at 14:54

François Andrieux · Accepted Answer · 2017-01-15T14:46:48.557

0

The relationship between a base class and a derived class does not extend to containers (except for a few special cases). std::vector<Npc> is not convertible to std::vector<Obje>. Perhaps you can template the update method like this :

template<class T>
static void update(QListWidget *q, vector<T>& i);

This solution will work for vectors of any type that matches the expected interface of Obje including, but not limited to, types that publicly derive from Obje.

edited Jan 15 '17 at 14:46

answered Jan 15 '17 at 14:37

François Andrieux

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Ohh I see, I suspected using a container was the problem. Unfortunately I don't know using templates but perhaps I should study that. Thanks – Ozan Deniz Jan 15 '17 at 14:54

c++ passing subclass as parameter

1 Answers1