Re: [Boost-users] regex and matchresults

From: John Maddock (john_at_[hidden])
Date: 2005-12-02 12:07:11

> this is my my first post to a newsgroup, so i hope it will work...
> I use the boost regex library, and i was wondereing, why the following
> code doesn't work.
> When i output the result of the match, it should output "a", but it
> didn't, the output was something wrong like "yl".
> I figured out, that the reasen for this is, that the string
> sSearchString is just in the scope of the try-block, so it is
> allready destroyed when i do the output. If i put the declaration of
> the string above the try block, all works fine, because the
> searchstring is still existing when i do the output.
> The matchresult seems to save just a reference to the string.
> Am i doing sth wrong? I think this is quite dangerous, or is there a
> way to do this more safe?

It's not a reference: the match_results stores *a pair of iterators* into 
the string being searched.  These are valid only for as long as the string 
from which they originate is both valid *and unmodified*.  This is standard 
iterator behaviour, and I suggest you read up on the STL and iterators if 
you're not familiar with these already.  The alternative - storing a copy of 
the string - is both hugely expensive in comparison, and throws away 
important positional information.  If you want to keep around the strings 
that matched, it's up to you to copy them.

Regards,

John.