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: Michael Goldshteyn (mgoldshteyn_at_[hidden])
Date: 2008-11-20 10:14:10

"Eric Niebler" <eric_at_[hidden]> wrote in message
news:4925187A.6080402_at_boost-consulting.com...
> 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???

Yes, the behavior of the last assert is surprising, so I do see your point. 
"empty" is certainly not the right name for such a function, at least in the 
context of standard container functionality.

>
> 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();
> }

empty() may not a good name for this function, but
agree with me that encapsulating the functionality of testing whether an re
object actually holds a regular-expression should be more intuitive than:

0 == rex.regex_id(); // Test if rex actually contains a regular expression

Perhaps something like unfilled(), bare(), or as you suggested is_invalid() 
should be added to the actual implementation, instead of user code? I would 
further argue that something so trivial and tightly coupled to the 
implementation
(i.e., regex_id() being zero) should be a member function and not stand 
alone.

Opinions on this topic from others would also be greatly appreciated!

Thanks,

Michael Goldshteyn