Re: [boost] [xpressive] Is there a way to test for an empty regex?

Subject: Re: [boost] [xpressive] Is there a way to test for an empty regex?
From: Eric Niebler (eric_at_[hidden])
Date: 2008-11-20 02:57:46

Michael Goldshteyn wrote:
> "Eric Niebler" <eric_at_[hidden]> wrote in message 
>>
>> if( 0 != re.regex_id() )
>>
> 
> Thanks for the speedy reply. I did notice the postcondition on the 
> constructor and the fact that the regex_id() will return 0 for a 
> non-initialized regex, but was hoping that there was a more intuitive way to 
> go about this test, since readers of the code may get confused without a 
> comment. How hard would it be to add an empty() function with a signature 
> that is similar to the one in boost::regex before Boost 1.38.0 comes out, 
> since this functionality, at least in my humble opinion is very useful.
> 
> The signature for the function in boost::regex is:
> 
> bool empty() const;

boost::regex goes to some length to present itself as a souped up 
container of characters. You can assign a character range to it, get 
begin() and end() iterators for stepping through the characters with 
which the regex was initialized, etc. Given that, see if you can answer 
this without checking the docs:

   assert(std::string().empty());    // OK
   assert(std::string("").empty());  // OK
   assert(boost::regex().empty());   // Is this true???
   assert(boost::regex("").empty()); // How about this???

Unsurprisingly, regex::empty() is not part of the standard regex 
interface in C++0x. I don't like regex::empty() and I'm not inclined to 
add it. Sorry. You already have a way to get the information you're 
interested in. If you would like to give it a pretty name, by all means...

template<class Iter>
bool is_invalid(xpressive::basic_regex<Iter> const &rex)
{
   return 0 == rex.regex_id();
}

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com