Re: unordered_map

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Tue, 8 Apr 2008 08:18:00 CST
On 8 Apr., 00:37, Daniel Kr?gler <>

On 7 Apr., 18:13, 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 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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