C++ 使用带有引用和指针的 dynamic_cast 时的行为差异
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Difference in behavior while using dynamic_cast with reference and pointers
提问by Naveen
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:
我正在检查 dynamic_cast 的行为，发现当它失败时，只有当目标是引用类型时才会抛出 std::bad_cast 异常。如果目标是指针类型，则不会从强制转换中抛出异常。这是我的示例代码：
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;
}
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?
输出是“Caught bad cast”和“NULL pointer”。代码使用VS2008编译。这是正确的行为吗？如果是，那么为什么会有差异？
回答by sharptooth
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.
是的，这是正确的行为。原因是你可以有一个空指针，但不能有一个空引用——任何引用都必须绑定到一个对象。
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.
因此，当指针类型的 dynamic_cast 失败时，它返回一个空指针，调用者可以检查它，但是当引用类型失败时，它不能返回空引用，因此异常是唯一合理的发出问题信号的方法.
回答by Naveen
See the C++ Standard, section 5.2.7/9:
请参阅 C++ 标准，第 5.2.7/9 节：
9 The value of a failed cast to pointer type is the null pointer value of the required result type. A failed cast to reference type throws bad_cast (18.5.2).
9 转换为指针类型失败的值是所需结果类型的空指针值。失败的引用类型转换会抛出 bad_cast (18.5.2)。
As to why - these are Stroustrup's words from the D & E book, section 14.2.2:
至于为什么 - 这些是 Stroustrup 在 D & E 书中的话，第 14.2.2 节：
I use a reference cast when I want an assumption about a reference type checked and consider it a failure for my assumption to be wrong. If instead I want to select among plausible alternatives, I use a pointer cast and test the result.
当我想要检查关于引用类型的假设并认为我的假设是错误的失败时，我使用引用强制转换。相反，如果我想在合理的替代方案中进行选择，我会使用指针转换并测试结果。
回答by AProgrammer
Yes, 5.2.7/9
是的，5.2.7/9
The value of a failed cast to pointer type is the null pointer value of the required result type. A failed cast to reference type throws bad_cast (18.5.2).
转换为指针类型失败的值是所需结果类型的空指针值。失败的引用类型转换会抛出 bad_cast (18.5.2)。
回答by Kim Gr?sman
Yes, it is. Because dynamic_castcan't return NULL for a failed reference cast, an exception is the only way out.
是的。因为dynamic_cast不能为失败的引用转换返回 NULL，所以异常是唯一的出路。
That is, a reference can't be NULL, so there is nothing suitable to return.
也就是说，引用不能为 NULL，因此没有什么适合返回。
