Re: [Boost-users] [Exception] How do I inherit custom exception hierarchy from std::runtime_error rather than from std::exception?

Subject: Re: [Boost-users] [Exception] How do I inherit custom exception hierarchy from std::runtime_error rather than from std::exception?
From: Egor Tensin (egor.tensin_at_[hidden])
Date: 2013-11-28 06:29:22

Steven,

Could you please specify, you meant the error_info_injector class
inside the Boost.Exception? I'm just trying to get this straight in my
head. Or maybe you don't know what class exactly derives from the
class passed as the parameter?

Thanks, Egor.

On Wed, Nov 27, 2013 at 10:41 PM, Steven Watanabe <watanabesj_at_[hidden]> wrote:
> AMDG
>
> On 11/27/2013 08:19 AM, Egor Tensin wrote:
>> <snip>
>>
>> #include <iostream>
>> #include <stdexcept>
>>
>> #include <boost/exception/all.hpp>
>>
>> typedef boost::error_info<struct tag_foo_info, unsigned long> foo_info;
>>
>> struct foo_error : virtual boost::exception, virtual std::runtime_error
>> {
>>   explicit foo_error(const char *const what)
>>     : std::runtime_error(what)
>>   { }
>>
>>   explicit foo_error(const std::string& what)
>>     : std::runtime_error(what)
>>   { }
>> };
>> <snip>
>>
>> It says that there's no default constructor available for
>> std::runtime_error. How do I fix this or should I use the idiomatic
>> approach? The interesting thing is that if I erase the "virtual"
>> keyword, the program compiles flawlessly. I wonder what is the reason
>> for that, e.g. why the original code doesn't work and why the new code
>> does, what are the consequences of making boost::exception and
>> std::runtime_error base (non-virtual) classes, etc. I'm confused as
>> the docs clearly recommends I build exception classes hierarchy using
>> virtual inheritance. Could somebody please give me an insight into the
>> inner workings of the library & inheritance mechanisms?
>>
>
> The reason that this causes a problem is that
> BOOST_THROW_EXCEPTION creates a class derived
> from its argument's type.  The constructor of
> a virtual base is always called from the most
> derived class.  Since Boost.Exception doesn't
> know that your exception type derives from
> std::runtime_error, it just calls the default
> constructor of std::runtime_error, which doesn't
> exist.
>
> In Christ,
> Steven Watanabe
>
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