c++ - dynamic_cast 抛出的 std::bad_cast 异常

Question

在下面的代码中，在将 derived.properties_ 从 BaseProperties 转换为 DerivedProperties 时，我得到了一个 std::bad_cast 异常抛出。查看代码，在我看来，在 Base 类构造函数中引用 DerivedProperties 初始化 BaseProperties 时出现问题.

我想要实现的是一个简单的 UI，其中 Base 本质上是我的组件接口(interface)，Derived 是任何被认为是组件的东西。这似乎是一个合理的假设，即组件可能具有不同的属性但具有一些相似的属性，例如大小和位置。

谁能就如何最好地实现我的目标提供任何建议？

#include <iostream>
#include <string>

// Properties for all objects of Base type
class BaseProperties {
public:
  BaseProperties( std::string baseProperty ):
    baseProperty_( baseProperty ) { }

  virtual ~BaseProperties(  ) {  }
  std::string getBaseProperty(  ) { return baseProperty_; }

protected:
  std::string baseProperty_;
};

// Properties specific to objects of Derived type
class DerivedProperties: public BaseProperties {
public:
  DerivedProperties( std::string baseProperty, std::string derivedProperty ):
    BaseProperties( baseProperty ),
    derivedProperty_( derivedProperty ) {  }

  std::string getDerivedProperty(  ) { return derivedProperty_; }

private:
  std::string derivedProperty_;
};

class Base {
public:
  Base( BaseProperties& properties ):
    properties_( properties ) {  }

  virtual ~Base(  ) {  }

protected:
  BaseProperties& properties_;
};

class Derived : public Base {
public:
  Derived( DerivedProperties properties ):
    Base( properties ) {  }

  friend std::ostream & operator << ( std::ostream& out, const Derived& derived );
};

std::ostream & operator << ( std::ostream& out, const Derived& derived ) {  
  return out << derived.properties_.getBaseProperty(  ) << ", "
             << dynamic_cast< DerivedProperties& >( derived.properties_ ).getDerivedProperty(  );
}

int main(  ) {
  Derived derived( DerivedProperties( "BaseProperty", "DerivedProperty" ) );

  std::cout << derived << std::endl;
  return 0;
}

Answer 1

您的派生类应将 DerivedProperty& 作为参数，类似于您的基类的做法:

class Derived : public Base {
public:
  Derived( DerivedProperties& properties ):
    Base( properties ) {  }

  friend std::ostream & operator << ( std::ostream& out, const Derived& derived );
};

由于您没有将引用作为构造函数参数而是实际对象，因此您实际上存储了对临时对象的引用，一旦构造函数退出，该对象就不再存在。