類、繼承和虛函數

Question

class Musician {
public: virtual void play() { std::cout << "Play an instrument"; }
 };

class Guitarist : public Musician {
public: void play() { std::cout << "Play a guitar"<<endl; }
};

int main(){
Musician armstrong;
Guitarist steve;
Musician* m = &armstrong;
m->play();
m = &steve; //type of (m)=Musician type of (*m) = Guitarist???
}

為什么 (*m) 的類型 = Guitarist？ 為什么如果我刪除音樂家 class 的 function 中的虛擬關鍵字，那么 (*m) 的類型 = 音樂家？

class Musician {
public: void play() { std::cout << "Play an instrument"; } //this makes type of(*m) = Musician
 };

如果我添加armstrong = static_cast<Musician>(steve); 最后它不會給出錯誤，但不會導致 armstrong （地址和類型）發生任何變化。 那條線是什么意思？ 而steve = static_cast<Guitarist>(armstrong); 給出錯誤：

 error: no matching function for call to 'Guitarist::Guitarist(Musician&)'
   27 | steve=static_cast<Guitarist>(armstrong);
      |                                       ^

Answer 1

在我在這里學習之前嘗試過這樣做，我提供了一個解決方案，您可以用 Musician 替換 A，用 Guitarist 替換 B。

#include <iostream>

class A {
public:
  void Same(){
      std::cout << "From A" << std::endl;
  }
  virtual A& operator= (const A& a) {
    assign(a);
    return *this;
  }

protected:
  void assign(const A& a) {
  }
};

class B : public A {
public:
  void Same(){
      std::cout << "From B" << std::endl;
  }
  virtual B& operator= (const A& a) {
    if (const B* b = dynamic_cast<const B*>(&a))
      assign(*b);
    return *this;
  }

protected:
  void assign(const B& b) {
    A::assign(b); 
  }
};

int main(){
    A a;
    B b;
    a.Same();
    a = b;
    a.Same();
    return 0;
}