c ++基类包含派生类的实例 [英] c++ base class contain instance of a derived class

问题描述

我想知道是否有人可以向我解释如何实现类似这样的东西：

I was wondering if someone could explain to me how I might be able to implement something similar to this:

namespace advanced_cpp_oop
{
  class A
  {
    B b;
  };

  class B : public A
  {
  };
}

int main()
{
}

基类的实例可以包含派生类的实例？当上面的代码编译时会产生以下错误：

Where an instance of a base class can contain an instance of a derived class? When the above code is compiled the following error is generated:

g++ advanced_cpp_oop.cpp 
advanced_cpp_oop.cpp:8:5: error: ‘B’ does not name a type

编译是：

public class AdvancedCppOop
{
  public static void main(String[] args)
  {
    A a;
  }
}

class A
{
  B b;
}

class B extends A
{
}

b $ b

感谢

Thanks

推荐答案

您需要一个指针和一个转发声明：

You need to a pointer and a forward declaration:

namespace advanced_cpp_oop
{
  class B;

  class A
  {
    B* b;
  };

  class B : public A
  {
  };
}

在您的C ++代码中，您将创建一个 class B 在里面A类，这是不可能的，因为编译器还不知道任何东西（特别是大小） class B 。

In your C++ code, you are creating an instance of your class B inside class A, which is not possible since the compiler does not yet know anything (especially not the size) of class B.

使用我的答案中的代码，您需要动态分配 B 并将其分配给代码中其他位置的 b 指针。

With the code from my answer, you need to dynamically allocate an instance of class B and assign it to the b pointer somewhere else in your code.

从设计角度来看，这不是真的有意义，因为父类不应该依赖于子类。

On a side note, from design perspective, this does not really make sense since a parent class should not depend on a sub class.

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