Question

如何从指向父项的父类型的指针中调用子函数？

class a{
public:
    virtual void print(){cout << "From a" << endl;}
};

class b: public a{
public:
    void print(){cout << "Form b" << endl;}
    void printFunction(){cout << "Exist in b" << endl;}
};

int main() {

    a* Obj = new b;

    Obj->print();
    Obj->printFunction();
    return 0;
}

在这里，我有一个指向“ b”的类型“ a”的指针。我想调用仅在类b中定义的“ printFunction”。

我收到此错误：

'class a'没有名为'printFunction'的成员

问题是如何使用此指针调用“ printFunction”？或如何在此处进行强制转换？

Answer 1

使用dynamic_cast：

class a{
public:
    virtual ~a(){}
    virtual void print(){ cout << "From a" << endl; }
};

class b: public a{
public:
    void print(){ cout << "Form b" << endl; }
    void printFunction(){ cout << "Exist in b" << endl; }
};

int main() {

    a* Obj = new b;

    Obj->print();

    b* bObj = dynamic_cast<b*>(a);
    if (bObj)
        bObj->printFunction();

    delete a;

    return 0;
}

Answer 2

You could downcast using dynamic_cast conversion

class base {
  public:
    void print() {std::cout << "base class print";}
};

class derived : public base {
  public: 
    void printFunction() {std::cout << "derived class print";}
};

int main() {

  base* ptr_derrived  = new derived();

  if(Derived* d = dynamic_cast<derived*>(ptr_derived) {
    std::cout << "downcast successful\n";
    d->printFunction(); // safe to call
  }

  delete ptr_derrived;

}

调用子类函数

2 个答案: