c++ - Boost::polymorphic_downcast 示例错误

Question

昨天我开始阅读 boost 文档，我被复制了示例代码，程序返回错误，怎么了？

我的代码:

#include <iostream>
#include <boost/cast.hpp>


using namespace std;

class Fruit { public: virtual ~Fruit(){}; };
class Banana : public Fruit { };

int main()
{
    Fruit * fruit = new Fruit();
    Banana * banana = boost::polymorphic_downcast<Banana*>(fruit);
    system("pause");
    return 0;
}

错误:

Assertion failed: dynamic_cast<target>(x) = x, file [...] boost\cast.hpp line 97

Documentation

Answer 1

您不只是复制示例代码。您替换了以下评论:

// ... logic which leads us to believe it is a Banana

用一些不让我们相信的东西:

Fruit * fruit = new Fruit();

断言是预期的，来自您链接到的文档:

template <class Derived, class Base>
inline Derived polymorphic_downcast(Base* x);
// Effects: assert( dynamic_cast<Derived>(x) == x );
// Returns: static_cast<Derived>(x)

失败是因为fruit 没有指向动态类型Banana 的对象。

如果您要用创建香蕉的内容替换该行:

Fruit * fruit = new Banana;

然后断言(和转换)应该成功，给出一个指向该 Banana 的正确类型的指针。