Re: hash_map

"kanze" <>
29 Sep 2006 11:44:56 -0400
AlbertSSj wrote:

BTW, you need specialized hask<std::string> to
work with g++, IIRC.

A related question...

To specialize hash std::string this works...
namespace __gnu_cxx {
template<> struct hash<std::string>
    size_t operator()(const std::string& __s) const {
           return __stl_hash_string(__s.c_str());

And it actually uses the hash for const char * in

My question is...

It is ok in to write such code?

No. At least, not if you consider __gnu_cxx to obey the rules
of the standard library.

It compiles and works... But everywere is said to not add
something in the standard library (for a lot of good reasons).

Well, you're explicitly allowed to add specializations of
standard functions and classes for types you define. And of
course, it's inconceivable that the standard won't define a
specialization for std::string. So when you switch to the
standard version, suddenly, you'll find yourself with two
specializations of the same thing.

It is better to use something like this?
struct str_hash {
    size_t operator()(const std::string& __s) const {
           return ::__gnu_cxx::__stl_hash_string(__s.c_str());

There's less risk.

Or maybe __stl_hash_string should not be called at all?

Given that the hash function it uses has some noticeably
weaknesses, that might be the best idea of all:-). (Seriously,
the g++ uses the same hashing function as Java, and is almost
certainly sufficient for most use.)

James Kanze GABI Software
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