C++ -- dynamic_cast

最新推荐文章于 2025-08-09 16:37:48 发布

maniacdown

最新推荐文章于 2025-08-09 16:37:48 发布

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文章标签： c++ class inheritance

本文链接：https://blog.youkuaiyun.com/maniacdown/article/details/1773201

本文探讨了 C++ 中 dynamic_cast 的工作原理，并通过示例说明了为什么源类型必须是多态的才能进行动态类型转换。通过添加虚函数使类成为多态类后，dynamic_cast 才能正确工作。

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class Widget { };
class SpecialWidget: public Widget { };
void update(SpecialWidget *psw);

int main()
{
    Widget *pw = new SpecialWidget;
    update(dynamic_cast<SpecialWidget*>(pw));
                         //  It seems to work.
}

When compiling, error message shows up as follows:

cannot dynamic_cast `pw' (of type `class Widget*') to type `class SpecialWidget*' (source type is not polymorphic)

why does it say source type is not polymorphic? Dose' t class SpecialWidget extends class Widget?

That's no doubt about that. So why?

Let us add some code to see:

class Widget
{
public:
    virtual void polymorphic( ) { }  // add virtual method
};

class SpecialWidget
{
public:
    virtual void polymorphic( ) { }  // add virtual method
};

void update(SpecialWidget* psw);

int main( )
{
    Widget* pw = new SpecialWidget;
    update( dynamic_cast<SpecialWidget*> (pw) ); // Now it does work.

}

Now, just adding some virtual funciton, no error message shows up. So in C++ it seems polymorphic is not inheritance , but virtual function.