Re: [boost] Exception Visitor [needs Gennadiy Rozental attention]

From: Oleg Abrosimov (beholder_at_[hidden])
Date: 2006-07-28 00:50:34

Alexander Nasonov wrote:
>>
>> try {
>>     throw std::logic_error("testing");
>> } BOOST_CATCH(my_exception_handler());
> 
> I'd changed BOOST_CATCH to something less generic.

Why? boost doesn't have such a macro till now. For me it means that 
there was no need in such a generic facility and it can be concluded 
that a chance for name clash in future is only theoretical here.
 From other point actual meaning of this macro is "catch and handle" but 
BOOST_CATCH_AND_HANDLE is too verbose for me. I believe that BOOST_CATCH 
is good enough. It is a compromise between user-friendliness and 
brevity. Fill free to suggest something if you are not convinced yet.

> 
>> try {
>>     throw std::logic_error("testing");
>> } BOOST_CATCH_EX(exceptions, my_exception_handler());
> 
> Well, if it's macro, why not just generate handle-seq (a list of catch clauses) rather then catch(...) { handle(); throw; } ?
> 
> For example:
> 
> try {
>      throw std::logic_error("testing");
> } BOOST_CATCH_PP_SEQ((ex1)(ex2)(ex3), my_exception_handler());
> 
> 
> You could even pass mpl sequence to a macro if you know a size of the sequence:
> 
> try {
>      throw std::logic_error("testing");
> } BOOST_CATCH_MPL_SEQ(3, mpl::vector<ex1,ex2,ex3>, func);
> 
> This could be expanded to
> 
> try {
>      throw std::logic_error("testing");
> }
> catch(mpl::at<mpl::vector<ex1,ex2,ex3>,0>::type const& ex)
> {
>     BOOST_STATIC_ASSERT((
>         3 == mpl::size<mpl::vector<ex1,ex2,ex3> >::type::value));
>     func(ex);
> }
> catch(mpl::at<mpl::vector<ex1,ex2,ex3>,1>::type const& ex)
> {
>     func(ex);
> }
> catch(mpl::at<mpl::vector<ex1,ex2,ex3>,2>::type const& ex)
> {
>     func(ex);
> }
>

Interesting, but complicates usage. Note that in BOOST_CATCH macro 
exceptions_handler() defines
typedef ... exceptions;

but in you version of BOOST_CATCH_EX macro it is defined as a PP 
sequence. Why two ways to do the same thing? There should be a very good 
reason to add such a functionality. Yes, only add, not replace current. 
average user like me ;-) should be isolated from PP sequences. They are 
good for library internals, not for interface IMO.

Alexander Nasonov wrote:
 >
 > Two reasons why I would consider PP version:
 >
 > 1. catch(...) followed by rethrow might be less efficient than 
handle-seq. I never measured this, though.

boost::filesystem has two versions of all functions that could generate 
exceptions. reason is clear - in performance critical applications one 
can not use exception machinery because the cost of stack unwinding is 
not acceptable. My contra-point here is that one shouldn't use 
exceptions if performance is too important for this particular piece of 
code.

 > 2. catch(...) catches too many things on some compilers. I know only 
one (very popular) compiler that also catches system exceptions (aka 
SEH). Though, I don't remember how it rethrows those exceptions.
 >

Seems that we are both not very knowledgeable in this area. I remember 
that SEH handling is done in boost::test library. So, maybe Gennadiy 
Rozental can shed some light here?

Best,
Oleg Abrosimov.