[Boost-users] [multi_index] REVISITED: hash_unique with differently produced hash codes

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2007-04-26 03:50:40

Hello Joaquín,

now I found the failure, to the problem described in message
http://lists.boost.org/boost-users/2007/04/27211.php with std::string as a key not being found.
The problem in my case was that I use Xalan to parse XML and have to convert string to single byte
characters. Since std::string has some build in caching to improve performance and I decode double
byte string (XalanDOMString) as follows:

std::string get_string(XalanDOMString const& str)
{
   XalanDOMString::CharVectorType v;
   str.transcode(v);

   return std::string(v.begin(), v.end());
}

this code produces a string which still points to the double byte buffer and therefore this
results in a different hash value for the returned string as the value would be for the
basic_string<char> or C-String:

An example:

XalanDOMString str("Sint8");
std::string char_str(get_string(str));

Calling find in the multi-index results in a hash value 40.

Calling find in the multi-index for
const char* str="Sint8";
results in a hash value 13

The same result (13) can be achieved when calling on char_str.c_str(); c_str() cause the strings
internal buffer to be rebuild.

Since boost::hash is on the way to become a part of STL it should ensure to work with the
correctly intended copy of string. What is your opinion on this issue?

With Kind Regards,

Ovanes