Can you catch an exception by the type of a conversion operator?

问题

I don't know how to phrase the question very well in a short subject line, so let me try a longer explanation. Suppose I have these exception classes:

class ExceptionTypeA : public std::runtime_error
{
    // stuff
};

class ExceptionTypeB : public std::runtime_error
{
    // stuff

    operator ExceptionTypeA() const; // conversion operator to ExceptionTypeA
};

Can I then do this, and have it trigger the catch block?

try
{
    throw ExceptionTypeB();
}
catch (ExceptionTypeA& a)
{
    // will this be triggered?
}

I'm going to guess that it will not, which is unfortunate, but I thought I'd ask, since I couldn't find any info on it on the net or on SO. And yes, I realize I could just run the program in my compiler and see what happens, but that wouldn't tell me what the standard says about this behavior, just what my compiler implements (and I don't trust it).

回答1:

You cannot. Standardese at 15.3/3:

A handler is a match for an exception object of type E if

The handler is of type cv T or cv T& and E and T are the same type (ignoring the top-level cv- qualifiers), or

the handler is of type cv T or cv T& and T is an unambiguous public base class of E, or

the handler is of type cv1 T* cv2 and E is a pointer type that can be converted to the type of the handler by either or both of
a standard pointer conversion (4.10) not involving conversions to pointers to private or protected or ambiguous classes

a qualification conversion

Your desired scenario matches none of these. cv means "const and/or volatile combination"

来源：https://stackoverflow.com/questions/2343208/can-you-catch-an-exception-by-the-type-of-a-conversion-operator

标签

c++

exception-handling

conversion-operator