Re: [boost] [c++1] BOOST_NOEXCEPT macros?

Subject: Re: [boost] [c++1] BOOST_NOEXCEPT macros?
From: Dave Abrahams (dave_at_[hidden])
Date: 2011-10-28 19:31:58

on Fri Oct 28 2011, "Stewart, Robert" <Robert.Stewart-AT-sig.com> wrote:

> Domagoj Saricj wrote:
>> On 20.10.2011. 15:15, Stewart, Robert wrote:
>
>> > Domagoj Saric wrote:
>> >> On 19.10.2011. 17:47, Stewart, Robert wrote:
>> >>
>> >>> Using (1) on existing code could change behavior.
>> >>
>> >> Again, how could it change behaviour?
>> >> Unless, again,: "...there is a real world person/problem
>> >> that depends on the defined std::unexpected/std::terminate
>> >> behaviour of noexcept as opposed to undefined or
>> >> implementation defined behaviour of a compiler specific in
>> >> case "someone violates a promise given about the particular
>> >> function"..."
>> >
>> > State it however you might like, but telling a compiler that
>> > a function no longer emits any exceptions can mean changes
>> > in optimization, insertion of validation logic, etc., which
>> > can *change behavior*.
>>
>> AFAIK optimization is not allowed to manifest any change in
>> behaviour, and the addition of noexcept already by itself
>> changes/inserts "validation logic" so I still don't see what
>> difference does that make?
>
> Let me try it this way.  With (1), we get the following before and after for C++03:
>
>    before: void f();
>    after:  void f() compiler-specific-nothrow;
>
> For C++11, (1) inserts noexcept, which was the reason for introducing it.
>
> Because the addition of the compiler-specific-nothrow decoration
> implies lost optimization opportunities and the addition of validation
> logic, it can affect client code in undesirable ways.

Rob, I don't know what you mean by "lost optimization opportunities and
the addition of validation logic."  The only compiler-specific behaviors
I know of do the exact opposite: they add optimization opportunities and
remove validation logic.

Code can use the noexcept operator to detect noexcept-ness of
expressions at compile-time and do something semantically
different... but it shouldn't.  Normally, noexcept should only be used
for optimization.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com