C ++ - 如何返回协变量类列表？

Question

我正在处理 Qt + C++ (x11)。

我有一个基类和几个返回指向该子类的新指针（协变）的子类。我也需要返回这些子类的容器（一个 QList）。一个例子：

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*>)
        (...)
        };        
};

这将无法编译，因为 QList 是与 QList 完全不同的类型。但是类似的东西会很好。在现实世界中，B 会比 A 拥有更多的数据成员，并且会有 C、D...，因此需要对列表的返回进行“协变”。我会更好：

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);

比

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);

有什么办法可以做到吗？

Answer 1

我希望你能从下面提到的代码中得到一些关于这个问题的提示。

这里是 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();
}