Andrey Semashev wrote:
I'm not very familiar with xpressive, but having a look from aside it would never come into my mind that "rex.regex_id() == 0" is an emptyness (or whatever) check. Sorry, this looks more like a hack to me.
I think a clear statement is needed: is a default-constructed regex valid?
Yes!
If not, then why does it have a default constructor? If it is, the default-constructed state should be easily detectable,
Why? Does a std::pair tell you whether it was default-constructed? It's just a value. How it was constructed doesn't matter.
since this is a common practice in just about any domain: - Boost.Regex,
We've already covered why regex::empty is confusing.
containers, strings,
These are sequences. empty() tests whether the sequence is empty. A regex is not a sequence. At least not for xpressive.
Boost.Function provide empty() - Boost.Optional, Boost.SmartPtr provide unspecified bool conversions and operator!, or equivalent facilities.
These types all have invalid singular values. Regexes do not.
Personally, I like empty() naming, but if a more precise name can be found, I would be fine. And I agree with Michael, such a function should be a member of the class.
As for application, the empty state of regex may be useful when the expression may, or may not be compiled. It's a sort of implicit optional, but a simpler and more efficient one. However, I'm sure it's not the only valid use case.
A regex is not like an optional. It does not have an invalid state. I'm sorry to disappoint you guys. I'm not adding an empty() member function. -- Eric Niebler BoostPro Computing http://www.boostpro.com