[Boost-users] Re: Re: Guidelines for effective exception usage

From: Jonathan Turkanis (technews_at_[hidden])
Date: 2004-08-23 14:04:18

"Stephen torri" <storri_at_[hidden]> wrote in message
news:1093236056.8377.60.camel_at_base.torri.org...
> On Sun, 2004-08-22 at 23:41, Jonathan Turkanis wrote:
> > "Stephen torri" <storri_at_[hidden]> wrote in message
> > news:1093230456.8377.44.camel_at_base.torri.org...
> > > Are there any guidelines on where and when to use exceptions?
> >
> > See http://www.boost.org/more/error_handling.html
>
> Thanks for the link. I had already found this link but was confused
on
> some of advice it gave. The question below are some of my confusion
> about using exceptions wisely. What I am seeking for each of them is
an
> example explaining how it implements a solution.
>
> 1)  Use virutal inheritance. I see the example and can understand
why
> its wrong I just do not understand how to fix it.

I believe in the example the fix is to use virtual inheritance in the
definitions of  my_exc1 and my_exc2, so that later someone can derive
from both. Using virtual inheritance in the definition of  your_exc3
doen't help. I see that this advice is not followed consistently
within boost (Robert Ramey's serialization library is the only case I
can find). But it's still good advice, I think.

> 2) Don't embed a std::string ... How can I use the data I have at
the
> point of the error safely? I want to provide a debug message to the
user
> that will lead them to the answer. This affects 6) Expose relevant
> information about the cause of the error.

It's not always possible to satisfy condition (2), but you should do
it whenever possible.

> In my project I have a base class called BaseException that inherits
> from std::exception.

> class DataTypeException : public BaseException
> {
> DataTypeException (std::string msg)
> : Base_Exception(),
>   m_msg(msg)
> {}
>
> // Specify virtual functions?
> virtual const char* what() const throw()
> {
>     try
>       {
>         std::stringstream output;
>         output << m_exception_name << ": " << m_message.c_str()
>                << std::endl;
>          return (output.str()).c_str();
>          }
> catch(std::exception &)
>         {
>         return m_exception_name;
>         }
> }
>   private:
>
> std::string m_msg;
> };

Often you can get away with:

class Data_Exception : public std::exception {
public:
     Data_Exception(int code) : code_(code) { }
     virtual const char* what() const throw()
     {
          [return result of looking up code in a table of error
messages.]
     }
private:
     int code;
};

As I said, this isn't always sufficient, but it often is.

Jonathan