Re: [Boost-users] [Xpressive] Baffled about results of search

From: Eric Niebler (eric_at_[hidden])
Date: 2006-04-12 17:27:40

Lynn Allan wrote:
> Eric Niebler wrote:
> 
>>Lynn Allan wrote:
>>
>>>The subsequent searches give valid information about the length and
>>>position, but other fields in the smatch:what variable seem 
>>>invalid.
>>>regex_search( altDow.substr(32), what, rex)
>>>regex_search( altDow.substr(45), what, rex)
>>
>>Watch out! altDow.substr(32) is returning a temporary string object.
>>After regex_search returns, the temporary is destroyed and the 
>>results
>>object is left holding iterators into a destroyed string.
> 
> 
> Arggggg. Trying to get "up to speed" with Boost is probably a flawed 
> thing to try to do at the same time as learning rudimentary aspects of 
> STL. "We're not in MFC-land any more, Toto." <g>
> 
> std::string altDow32 = altDow.substr(32);
> if( regex_search( altDow32, what, rex ) )  { unpack }

A better approach would be to use iterators.

// Begin searching 32 characters into altDow
regex_search( altDow.begin() + 32, altDow.end(), what, rex )

Note #1: the match results will now contain iterators into altDow, but 
position offsets will be relative to the start of the search, not the 
start of altDow.

Note #2: this code is only valid if altDow has at least 32 characters. 
Error checking is left as an exercise. :-)

Incidentally, your earlier mistake was distressingly easy to make. I can 
think of a way I might be able to catch this mistake at compile time in 
a future versions of xpressive.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com