• Gold Dragon <dragonwing dragonu.net> Jul 05 2004
• Arcane Jill <Arcane_member pathlink.com> Jul 06 2004
• Sam McCall <tunah.d tunah.net> Jul 06 2004

Gold Dragon <dragonwing dragonu.net> writes:

Can I have a clear tutorial on how to create exceptions. You throw the 
exception of course and I guess inherit from Error. Which module is that 
and exactly how does D want you program the throwing?

Arcane Jill <Arcane_member pathlink.com> writes:

In article <cccfah$700$1 digitaldaemon.com>, Gold Dragon says...
Can I have a clear tutorial on how to create exceptions. You throw the 
exception of course and I guess inherit from Error. Which module is that 
and exactly how does D want you program the throwing?


If I'm in a hurry, I might just do:

#    throw new Error("Bad parameter 3 to function f()");

If I'm writing something a bit more enduring, I might first define:

#    class MyError : Error
#    {
#        this(char[] s)
#        {
#            super(s);
#        }
#    }

and then I can do:

#    throw new MyError("Cannot divide by zero");

which is pretty similar, all told, but of course a MyError can be caught in
isolation from other kinds of errors.

There's a localization problem, of course. If you don't speak English, the
display of my error message won't help you - but adding localization to error
handling may be going a wee bit too over-the-top.

But there is also such a thing as an exception with is /programmatically/ useful
- that is, useful to software, not just an error message to humans. You can
stuff information into a class derived from Error or Exception, such as the
exact parameters which caused things to go wrong, the offset into a stream gone
bad, etc. This is only useful if you catch the exception, but it's something you
might want to do if you thing there's a chance that some calling function MIGHT
want catch it and examine it.

(Others please chime in here - I'm not an expert on this, and D's exception base
classes are not that clear to me).

Arcane Jill

Sam McCall <tunah.d tunah.net> writes:

If you do want to catch a MyError, I think the convention is to subclass 
it from Exception, with Errors being uncaught in almost all cases.

Arcane Jill wrote:
 There's a localization problem, of course. If you don't speak English, the
 display of my error message won't help you - but adding localization to error
 handling may be going a wee bit too over-the-top.


exception base class that looked up message format strings from a table 
or something. Doesn't seem to be a need for it to be in the base 
classes, since a lot of exceptions will be invisible (or at least 
opaque) to the user.

Sam