Re: [Boost-users] [multi_index] REVISITED: hash_unique with differentlyproduced hash codes

Ovanes Markarian ha escrito:

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?

OK, what's happenning is the following: boost::hashstd::string()(str) is not equivalent to boost()(str.c_str()); the latter hashes the *pointer*, not the contents pointed to. So, when you write your_container.find(str.c_str()) B.MI assumes that what you pass (a const char *) is equivalent hash- and equivalence-wise to the stored keys (std::strings), which is not true (because of the hash, equivalence is indeed interoperable as std::string provides the required overloads for operator== etc.) Returning to your original problem, you said that this works: inline some_type_ptr create_type(std::string const& name)const { types_map::index<hash>::type::const_iterator i =types_.get<hash>().find(name.c_str()); //... } but this does not: inline some_type_ptr create_type(std::string const& name)const { types_map::index<hash>::type::const_iterator i =types_.get<hash>().find(name); //... } This puzzles me a lot: Given that your types_map container is indexed on a std::string, things should be the other way around: it is the "find(name)" version that should work AFAICS. Could you please double-check? Joaquín M López Muñoz Telefónica, Investigación y Desarrollo