Re: Catching exceptions

From:

Pete Becker <pete@versatilecoding.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 12 May 2009 08:08:51 -0400

Message-ID:

<T6WdnVsn6tFO-5TXnZ2dnUVZ_uednZ2d@giganews.com>

Andy Champ wrote:

Pete Becker wrote:

magnus.moraberg@gmail.com wrote:

Hi,

I have the following incomplete code -

File file;
file.open(filePath);

try
{
    someOneElsesApi::startComplexProcessingOfFile(file);
    std::cout << "The file is now processed" << std::endl;
}
catch exception(/* all exceptions */)
{
    std::cout << "The file didn't process because: " << std::endl;
}

startComplexProcessingOfFile() can throw a whole range of different
exceptions.

How do I catch them and inform the user in a useful manner. In python,
there is a base exception class which can be used for this purpose. C#
has inner exceptions which I also found quite useful.

C++ has a standard exception class named, oddly enough, exception. If
you use catch(const std::exception& ex) you can display ex.what() to
show whatever the exception object holds as the reason.

If startComplexProcessingOfFile() throws exceptions derived from
std::exception that's all you need to do. If it throws exceptions
derived from some other type, then you need to catch that type and
deal with it. If it's as random as your wording suggests, and the
types of its exceptions have no inherent connection with one another,
then you have to catch each type individually.

Yes... but... what guarantee is there that the API function returns an
exception inherited from std::exception?

At the language level, none.

In C# (and I assume form his

comment, Python) _all_ exceptions inherit from the same class. That's
not the case for C++.

I _hope_ his API class is defined to throw exceptions from some "family"
with a common base class, but I can't guarantee it; in that case,
catching the base would be enough. But he may need _lots_ of catch
statements, possibly ending with catch(...) { cout <<
"startComplexProcessingOfFile blew up in an unexpected manner"; throw;}
to log where this odd thing happened.

Hmm, I thought that that was what I said.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of
"The Standard C++ Library Extensions: a Tutorial and Reference"
(www.petebecker.com/tr1book)