Question

I am designing an exception hierarchy in C++ for my library. The “hierarchy” is 4 classes derived from std::runtime_error. I would like to avoid the slicing problem for the exception classes so made the copy constructors protected. But apparently gcc requires to call the copy constructor when throwing instances of them, so complains about the protected copy constructors. Visual C++ 8.0 compiles the same code fine. Are there any portable way to defuse the slicing problem for exception classes? Does the standard say anything about whether an implementation could/should require copy constructor of a class which is to be thrown?

Answer 1

I would steer clear of designing an exception hierarchy distinct to your library. Use the std::exception hierarchy as much as possible and always derive your exceptions from something within that hierarchy. You might want to read the exceptions portion of Marshall Cline’s C++ FAQ – read FAQ 17.6, 17.9, 17.10, and 17.12 in particular.

As for "forcing users to catch by reference", I don’t know of a good way of doing it. The only way that I have come up with in an hour or so of playing (it is Sunday afternoon) is based on polymorphic throwing:

class foo_exception {
public:
    explicit foo_exception(std::string msg_): m_msg(msg_) {}
    virtual ~foo_exception() {}
    virtual void raise() { throw *this; }
    virtual std::string const& msg() const { return m_msg; }
protected:
    foo_exception(foo_exception const& other): m_msg(other.m_msg) {}
private:
    std::string m_msg;
};

class bar_exception: public foo_exception {
public:
    explicit bar_exception(std::string msg_):
        foo_exception(msg_), m_error_number(errno) {}
    virtual void raise() { throw *this; }
    int error_number() const { return m_error_number; }
protected:
    bar_exception(bar_exception const& other):
        foo_exception(other), m_error_number(other.m_error_number) {}
private:
    int m_error_number;
};

The idea is to make the copy constructor protected and force users to call Class(args).raise() instead of throw Class(args). This lets you throw a polymorphicly bound exception that your users can only catch by reference. Any attempt to catch by value should be greeted with a nice compiler warning. Something like:

foo.cpp:59: error: ‘bar_exception::bar_exception(const bar_exception&)’ is protected

foo.cpp:103: error: within this context

Of course this all comes at a price since you can no longer use throw explicitly or you will be greeted with a similar compiler warning:

foo.cpp: In function ‘void h()’:

foo.cpp:31: error: ‘foo_exception::foo_exception(const foo_exception&)’ is protected

foo.cpp:93: error: within this context

foo.cpp:31: error: ‘foo_exception::foo_exception(const foo_exception&)’ is protected

foo.cpp:93: error: within this context

Overall, I would rely on coding standards and documentation stating the you should always catch by reference. Make sure that your library catches exceptions that it handles by reference and throw fresh objects (e.g., throw Class(constructorArgs) or throw;). I would expect other C++ programmers to have the same knowledge – but add a note to any documentation just to be sure.