Re: exception catch

From:

Abhishek Padmanabh <abhishek.padmanabh@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 8 Dec 2007 22:27:32 -0800 (PST)

Message-ID:

<f799e30e-d0e0-4ee4-bc60-74e805379882@s12g2000prg.googlegroups.com>

On Dec 9, 10:34 am, "Alf P. Steinbach" <al...@start.no> wrote:

* Rahul:

On Dec 8, 4:04 pm, Ian Collins <ian-n...@hotmail.com> wrote:

Rahul wrote:

Hi Everyone,
I have the following exception class,
class E1
{
};
class E2
{
public: E2(const E1&)
           {
                    printf("automatic type conversion\n");
           }
};
int main()
{
try
{
  throw E1();
}
catch(E2 obj)

Don't catch exceptions by value, use a const reference.

--
Ian Collins.

Why do you say that exception's should be caught by value?
is it because of object slicing when a derived class exception is
caught by a base class exception handler?

On reason is to avoid copy construction.

Consider the case where the copy constructor might trow (e.g. allocating
a string).

A copy construction exception at the throw point is bad but at least you
do get an exception -- just not necessarily the one you asked for --
but a copy construction exception at catch point is t r o u b l e.

Aren't exceptions thrown always copied (atleast once) after they are
thrown and before they are caught? To make it persistent during the
stack unwinding? And if copy construction can throw, it is a problem
even before catching the exception. I think the most important reason
for not catching exceptions by value is efficiency i.e. we avoid
atleast one copy creation of the exception object.

The other reason could be writing a generic base exception handler
where all derived types can be caught without slicing.