使用dynamic_cast与引用和指针时的行为差异

问题描述

我正在检查dynamic_cast的行为，发现当它失败时，std :: bad_cast异常只会在目标是引用类型时抛出。如果目标是一个指针类型，那么抛出的异常不会抛出。这是我的示例代码：

I was checking the behavior of dynamic_cast and found that when it fails, std::bad_cast exception is thrown only if the destination is a reference type. If the destination is a pointer type then no exception is thrown from the cast. This is my sample code:

class A
{
    public:
        virtual ~A()
        {
        }
};

class B : public A
{
};

int  main()
{
    A* p = new A;

    //Using reference
    try
    {
    B& b = dynamic_cast<B&>(*p);
    }
    catch(std::bad_cast exp)
    {
    std::cout<<"Caught bad cast\n";
    }

    //Using pointer
      try
    {
    B* pB = dynamic_cast<B*>(p);

    if( pB == NULL)
    {
        std::cout<<"NULL Pointer\n";
    }
    }
    catch(std::bad_cast exp)
    {
    std::cout<<"Caught bad cast\n";
    }

    return 0;
}

输出为陷入错误转换和NULL指针。代码是使用VS2008编译的。这是正确的行为吗？如果是，那为什么有区别？

Output is "Caught bad cast" and "NULL pointer". Code is compiled using VS2008. Is this the correct behavior ? If yes, then why there is a difference?

推荐答案

是的，这是正确的行为。原因是你可以有一个空指针，而不是一个空引用 - 任何引用必须绑定到一个对象。

Yes, this is correct behaviour. The reason is that you can have a null pointer, but not a null reference - any reference has to be bound to an object.

所以当dynamic_cast的指针类型失败它返回一个空指针，调用者可以检查它，但是当它失败的引用类型，它不能返回一个空引用，所以一个异常是唯一合理的方式来表示一个问题。

So when dynamic_cast for a pointer type fails it returns a null pointer and the caller can check for that, but when it fails for a reference type it can't return a null reference, so an exception is the only reasonable way to signal a problem.

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