C++ - How to return list of covariant classes?

Question

I'm dealing with Qt + C++ (x11).

I have a base class, and several subclasses that return a new pointer to that subclass (covariant). I need to return too, an container (a QList) of these subclasses. An example:

class A
{
public:
    int id;
}

class B : public A
{
    int Age;
};

class WorkerA
{
public:
    virtual A *newOne() {return new A()};
    virtual QList<A*> *newOnes {
        QList<A*> list = new QList<A*>;
        //Perform some data search and insert it in list, this is only simple example. In real world it will call a virtual method to fill member data overriden in each subclass.
        A* a = this.newOne();
        a.id = 0;
        list.append(this.newOne()); 
        return list;
        };        
};

class WorkerB
{
public:
    virtual B *newOne() override {return new B()}; //This compiles OK (covariant)
    virtual QList<B*> *newOnes override { //This fails (QList<B*> is not covariant of QList<A*>)
        (...)
        };        
};

This will fail to compile, because QList is a completely different type from QList. But something similar would be nice. In real world, B will have more data members than A, and there will be C, D..., hence the need of "covariantate" the return of list. I will be much nicer:

WorkerB wb;
//some calls to wb...
QList<B*> *bList = wb.newOnes();
B* b = bList.at(0); //please excuse the absence of list size checking
info(b.id);
info(b.age);

than

WorkerB wb;
//some calls to wb...
QList<A*> *bList = wb.newOnes();
B* b = static_cast<B*>(bList.at(0)); //please excuse the absence of list size checking
info(b.id);
info(b.age);

Is there any way to achieve this?

I would always work the the pointers to the base class `A`. In other words, use polymorphism. — vahancho, Mar 09 '20 at 08:21

score 0 · Answer 1 · edited Mar 08 '20 at 19:33

0

I hope from below mention code you will get some hint for this issue.

Here is main.cpp:

#include <QCoreApplication>
#include <QDebug>
#include "myclass.h"

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

    MyClass mClass;
    mClass.name = "Debussy";

    // put a class into QVariant
    QVariant v = QVariant::fromValue(mClass);

    // What's the type?
    // It's MyClass, and it's been registered 
    // by adding macro in "myclass.h"
    MyClass vClass = v.value<MyClass>();

    qDebug() << vClass.name;  

    return a.exec();
}

edited Mar 08 '20 at 19:33

Scheff's Cat

19,528
6
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answered Mar 08 '20 at 19:30

Patel Kalpesh

29
3

I think you have confused "covariant" with "variant". What I really need is to get some quick conversion from QList to QList, without having to reimplement the entire same method in each subclass. This is because I will manage several variables of QList, each one "class*" being a subclass; and having to do static_cast continuously is really tedious. – Bull Mar 08 '20 at 19:50
@Bull _and having to do static_cast continuously is really tedious._ Agree. (I know this from own experience.) With `class B: public class A { };` and knowing that `QList listA;` will contain instances of `B*` only, you could do a `QList &listB = (QList&)listA;` I believe the compiler will "eat" this, and it may even work. **However, it's actually [U.B.](https://stackoverflow.com/a/4105123/1505939)** Your tedious way is the only correct IMHO. [**Demo on coliru**](http://coliru.stacked-crooked.com/a/f3493fd8446de994) (Please, note the compiler complaint.) – Scheff's Cat Mar 09 '20 at 10:06

C++ - How to return list of covariant classes?

1 Answers1