Exception questions

[Old Wolf]

1. What is the difference between #include <stdexcept>
and #include <exception> ?

2. Is there a list somewhere of what each standard exception is used
for? either to throw them, or throw user-defined exceptions
derived from them? (for example I have been deriving mine from
std::bad_alloc if there was a memory problem, or std::bad_except ion
if there was some other problem)

3. Is it a good idea to make all user-defined exceptions derive from
std::exception?

4. While trying to see why my "catch (std::exception &e)" did not catch
a bad boost::lexical_ cast<>, I discovered that that function throws
an object derived from "std::bad_cast" , but std::bad_cast is NOT
derived from std::exception. Are there any other "special case"
exceptions in the Standard Library that are not derived from
std::exception?

5. If you have a handler like:
catch(std::exce ption &e) { whatever }
and the exception is thrown like:
throw foo_exception(" error occurred");
where foo_exception is derived from std::exception, will it be
caught? (The exception thrown is a temporary, and you can't bind
temporaries to non-const references).

[tom_usenet]

Re: Exception questions

On 25 Feb 2004 17:48:54 -0800, oldwolf@inspire .net.nz (Old Wolf)
wrote:

I'm no expert on exceptions, but:
[color=blue]
>1. What is the difference between #include <stdexcept>
> and #include <exception> ?[/color]

exception defines the fundamental exception handling stuff, including
bad_exception, exception, unexpected, terminate, etc.

stdexcept defines some standard exception types, like
std::runtime_er ror, std::logic_erro r, etc.
[color=blue]
>
>2. Is there a list somewhere of what each standard exception is used
> for? either to throw them, or throw user-defined exceptions
> derived from them? (for example I have been deriving mine from
> std::bad_alloc if there was a memory problem, or std::bad_except ion
> if there was some other problem)[/color]

std::bad_except ion has a special use - you shouldn't really derive
from it. If an exception is thrown that breaks exception
specifications, if the method in question has
throw(std::bad_ exception) the exception will be turned into a
std::bad_except ion instead.

Mostly you should probably derive from one of the exceptions in
<stdexcept>. Or you could add your own exception base class, deriving
from std::exception, that adds the facilities you want (perhaps an
error code, or whatever). This is probably a better idea.
[color=blue]
>3. Is it a good idea to make all user-defined exceptions derive from
> std::exception?[/color]

It is a reasonable idea, although catch(exception const&) doesn't buy
you very much compared to catch(...).
[color=blue]
>4. While trying to see why my "catch (std::exception &e)" did not catch
> a bad boost::lexical_ cast<>, I discovered that that function throws
> an object derived from "std::bad_cast" , but std::bad_cast is NOT
> derived from std::exception. Are there any other "special case"
> exceptions in the Standard Library that are not derived from
> std::exception?[/color]

std::bad_cast does (or should) derive from std::exception. You must
have a non-standard compiler in this respect.
[color=blue]
>5. If you have a handler like:
> catch(std::exce ption &e) { whatever }
> and the exception is thrown like:
> throw foo_exception(" error occurred");
> where foo_exception is derived from std::exception, will it be
> caught? (The exception thrown is a temporary, and you can't bind
> temporaries to non-const references).[/color]

Yes, it will be caught - exception catching is handled differently
from function calling. However, you should catch a std::exception
const& in any case, since std::exception has no non-const methods
anyway.

Tom
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