Re: Exception handling?

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Fri, 20 Feb 2009 20:45:15 +0100

Message-ID:

<gnn16d$82h$1@reader.motzarella.org>

* mlt:

In the constructor of a class I run various test to check that the input
data is valid, else I throw an exception:

...
...
MyTest(
         int const k
       , int const h
       , U const & u
       , U const & v
       , P const & c
       )
     {

   int controlnum = c.size() * c[0].size();
   int basisnum = (u() - k) * (v.size() - h);

       if(k<= 0 || h<=0)
         throw std::invalid_argument("k h must be greater than zero");

       if(uknots.size() <= 0)
         throw std::invalid_argument("u was empty");

       if(vknots.size() <= 0)
         throw std::invalid_argument("v was empty");

       if(controls.size() <= 0)
         throw std::invalid_argument("c was empty");
...
...
     }

The first thing that needs to be done is to catch the exceptions so the
error messages gets printed to the user. But how do I make sure the
right message is printed, do I make some sort of exception switch?

In a small program made just for your own use you simply use the
std::exception::what() method.

But in a larger program it's generally *not a good idea* to transfer UI messages
in exceptions.

There are many reasons for that, including internationalization problems, and
that code that throws generally doesn't know anything about high level goals,
and that it would be in conflict with use of exceptions for programmers, and so on.

Ask yourself, what business has the user knowing about arguments to MyTest?

And by the way, for the specific tests above I'd use 'assert', not exceptions.

Cheers & hth.,

- Alf