从BaseClass*到DerivedClass的转换无效*

Question

我正在尝试使用工厂方法返回派生类,但返回类型是基类类型.根据我的理解,我认为继承会让我这样做,显然我错了.

WeightExercise和CardioExercise都来自练习.

我可以投射物体,但我认为我的设计意味着我不必这样做.有人可以指出我的错误吗？

主要

ExerciseFactory ExerciseFactoryObj;
WeightExercise *WeightExerciseObj = ExerciseFactoryObj.createExercise(menuselection);

工厂类

class ExerciseFactory
{
public:
ExerciseFactory();
~ExerciseFactory();
Exercise* createExercise(int exercisetype);


private:
static WeightExercise* createWeightExercise() { return new WeightExercise(); }
static CardioExercise* createCardioExercise() { return new CardioExercise(); }
};

工厂实施

Exercise* ExerciseFactory::createExercise(int exercisetype)
{
if ( 1 == exercisetype )
{
    return this->createWeightExercise();
}
else if ( 2 == exercisetype )
{
    return this->createCardioExercise();
}
else
{
    cout << "Error: No exercise type match" << endl;
}
}

Answer 1

您可以将从工厂返回的派生类分配给基类1:

ExerciseFactory ExerciseFactoryObj;
Exercice *WeightExerciseObj = ExerciseFactoryObj.createExercise(menuselection);

编辑:

如果您确实需要访问WeightExerciceObject元素,请使用:

WeightExerciceObject * weight = dynamic_cast<WeightExerciceObject *>(ExerciseFactoryObj.createExercise(menuselection));

如果类不是确切的类,则返回NULL.您需要检查NULL.