为什么我不能通过对象调用访问虚函数？

Question

我正在尝试做一个使用银行卡的OOP模拟，只是为了练习，没有什么太复杂的；我收到一个奇怪的错误。这个调用有点傻，但我只是想看看“读取”功能是否正常工作，而不是测试它的真正目的。我不明白为什么会出现以下错误，我知道我可以直接使用对象访问虚方法，而不必使用指针。另外，很抱歉大多数成员不是用英语写的。这是我的代码：

#include <iostream>
#include <string>
using namespace std;

class Card
{

protected:
    string nrCard;
    string NumeDetinator;
    string data_expirare;
    int CVV;
    double credit;

    static unsigned nr_ord;

public:
    Card();
    Card(string nC, string ND, string d_ex, int cvv, double CRD);
    virtual getCredit(); //= 0;
    virtual void print(ostream&) const;//= 0;
    virtual void read(istream&) ;//= 0;
    virtual void print(ostream &) ;//= 0;
};

unsigned Card :: nr_ord = 0;

Card :: Card() : nrCard("XXXX - XXXX - XXXX - XXXX"), NumeDetinator("???? ????"), data_expirare("DD - MM - YY"), CVV(999), credit(0.0) {}
Card :: Card(string nC, string ND, string d_ex, int cvv, double CRD) : nrCard(nC), NumeDetinator(ND), data_expirare(d_ex), CVV(cvv), credit(CRD) {}

void Card::print(ostream& out) const {
    out << "numar card: " << this->nrCard << "\n";
    out << "Nume detinator: " << this->NumeDetinator << "\n";
    out << "data expirare: " << this->data_expirare << "\n";
    out << "CVV " << this->CVV << "\n";
    out << "Credit disponibil: " << this->credit << "\n";
}

void Card::read(istream& in) {
    cout << "Dati datele cardului:\n numar card: ";
    in >> this->nrCard;
    cout << "\nNumele detinatorului: ";
    in >> this->NumeDetinator;
    cout << "\nData expirarii: ";
    in >> this->data_expirare;
    cout << "\nCVV: ";
    in >> this->CVV;
    cout<< "\ncredit: ";
    in >> this->credit;
}

class Card_Standard : Card
{
    int limitaExtragere;
    double comisionDepasireLimita;
};

class Card_Premium : Card_Standard
{
  int m;
};


int main() {

Card C;

C.print(cout);  /// error: undefined reference to vtable for Card , id returned 1 exit status;


return 0;
}

Answer 1

"对 vtable 的未定义引用" 是编译器的说法"并非所有虚拟方法都已实现"。

实施所有 virtual 成员（或将它们标记为纯），错误将消失。

顺便说一句，virtual getCredit(); 缺少 return 类型（可能是 double？）。

为什么我不能通过对象调用访问虚函数？

Why can't I acces the virtual function through an object call?

c++

inheritance

virtual