Re: [boost] Re: Re: Re: [review] hash functions

From: JOAQUIN LOPEZ MU?Z (joaquin_at_[hidden])
Date: 2005-03-10 17:19:16

----- Mensaje original -----
De: Peter Dimov <pdimov_at_[hidden]>
Fecha: Jueves, Marzo 10, 2005 10:35 pm
Asunto: Re: [boost] Re: Re: Re: [review] hash functions

> JOAQUIN LOPEZ MU?Z wrote:
> >
> > Maybe the following approach could be taken to
> > allow for alternative implementations to be plugged into
> > the framework: [...]
> 
> > unordered_set<my_type,boost::hash<my_type,crypto_impl> > us;
> 
> The proposal is not a framework, it contains two simple tools for 
> constructing hash_value overloads for compound types and a suite 
> of 
> _default_ hash functions.
> 
> Alternative implementations (who may or may not use the supplied 
> tools) are 
> supported by unordered_set by simply using
> 
>    unordered_set< my_type, my_hash > us;
> 
> There's no need to plug them in. 

The idea is that you can alter only some part of the
hashing scheme and benefit of the preexisting definitions:

template<typename It>
std::size_t hash_range(It first,It last,crypto_impl)
{
  ...
}

// third_party_type is an external type with its own
// hash_value overload

unordered_set<
  std::vector<third_party_type>,
  boost::hash<std::vector<third_party_type>, crypto_impl>
> us;

// here, we automatically get an impl of hashing for
// strings using the crypto_impl hash_range

unordered_set<
  std::string,
  boost::hash<std::string, crypto_impl>
> us2;

But I won't defend this to death :) I just felt like
throwing the idea in.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo