未按预期调用 C++ 虚拟方法

问题描述

我正在摆弄虚拟方法的正确覆盖.我链接了一些构造函数，并在基类构造函数中调用了一个虚方法.

I'm fiddling around with proper overriding of virtual methods. I chain some constructors and in the base class constructor I call a virtual method.

调用链应该如下:

B(p)->A(p)->B.foo(p)

调用链在 C++ 中:

Call chain is in C++:

B(p)->A(p)->A.foo(p)

调用链在 C# 中:

B(p)->A(p)->B.foo(p)

换句话说，在 C# 中的行为就像预期的那样，而在 C++ 中则不然.

So in other words, in C# behaviour is like expected, in C++ it isn't.

C++ 代码:

#include <cstdio>

class A {
public:
  A();
  A(int);

  virtual void foo(int s);
};

class B : public A {
public:
  B();
  B(int s) : A(s) {};

  virtual void foo(int s) override;
};

A::A() {
  std::printf("A\r\n");
};

A::A(int s) : A() {
  std::printf("A: %d\r\n", s);
  foo(s);
};

void A::foo(int s) {
  std::printf("A::foo(%d)\r\n", s);
}

B::B() : A(){
  std::printf("B\r\n");
};

void B::foo(int s) {
  std::printf("B::foo(%d)\r\n", s);
}

int main(int argc, char* argv[]) {
  B b(2);
}

输出:

A
A: 2
A::foo(2)

C# 代码:

using System;

namespace demo {
class A{
  public A() {
    Console.WriteLine("A");
  }

  public A(int s) : this(){
    Console.WriteLine("A: " + s.ToString());
    foo(s);
  }

  public virtual void foo(int s) {
    Console.WriteLine(string.Format("A:foo({0})", s));
  }
}

class B : A{
  public B() : base() {
    Console.WriteLine("B");
  }

  public B(int s) : base(s) {
  }

  public override void foo(int s) {
    Console.WriteLine(string.Format("B:foo({0})", s));
  }
}

  static class Run {
    static void Main() {
      new B(2);
    }
  }
}

输出:

A
A: 2
B::foo(2)

为什么在 C++ 中没有调用 B 类的重写方法?我该如何正确覆盖它?

Why isn't the overriden method of class B called in C++? How do I have to override it correctly?

推荐答案

这就是 C++ 的工作方式.虚函数在对象构造和销毁过程中不会虚拟地运行.

This is just the way C++ works. Virtual functions do not behave virtually during object construction and destruction.

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